BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF ficnrg M. Sage 1891 / ^.?> wj 7-Sc^ O.-S/z./.l^bJ-. 7673-2 Cornell University Library arV18668 ABC of the telephone: ,. 3 1924 031 291 945 olin.anx Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031291945 *'C/| The Calling Current. — The cur- rent passes from the generator through a wire attached to one of the hinges; thence to the bell-magnet; thence back again through a wire attached to the other hinge, and out upon the line wire to the central station, where it moves a signal to inform the operator that your station is calling. The line wires are attached to two of the binding posts shown at the top of the box in Fig. 2. Fig. 4.— One form of bind- ing post. The screw at its end is intended for contact with the ^electrical conductor, and the line wire is inserted in the hole through the centre, where it is held tight by the thumb screw. Fig. 5. — Single Pole Receiver, Shell made of hard rubber. Wires attached to the binding posts at the tail end. The Hook Switch. — Directly in front of the magneto- generator is a lever, which passes through the side of the box land ends in the forked hook supporting the receiving instru- ment, as shown in Fig. i. It is known as the "hook-switch," and its use, as will be explained in a later chapter, is to switch the talking instruments into circuit, when it is in the raised position, and to break that circuit and switch in the call-bell generator again, when the receiver is restored to its hook. The Receiver. — Fig. 5 represents the receiver. At one end we see two binding-posts to receive wires which connect with the two binding-posts at the left of the row of six that XXlt A B COF THE TELEPHONE: are p-aced directly beneath the generator-box. Tlie receiver is a magnet telephone of the kind invented and patented by Bell. Although many variations of the original instrument have since been devised by experimenters and manufacturers, none have improved materially on the original. The other speaking instrument, the transmitter, is made on a different principle. ^o. 6.— The Transmitter mounted on Rocker Arm, attached to the Induction Coil Box at its base. being a carbon microphone (or " sound-magnifier ") of the kind first devised by Mr. Edison. Both instruments, together with the many variations and improvements, will be fully explained in the proper place. We are at present concerned with their outer appearance, and the functions they serve in the practical operation of the apparatus. The Transmitter and Induction Coil.— The usual form of the transmitter, enclosed in a semi-circular, or a cylindrical case, surmounted by a bell-shaped mouthpiece, and supported at the end of a movable arm, is shown in Fig. 6. Wires run from it to the iron box at the base of the arm, and from this again other wires run from the four binding-posts shown in the figure, through the back of the mounting-board, to the remain- ing four binding-posts of the six beneath the generator-box. THE TELEPHONE APPARATUS. XXlll The iron box contains an induction coil, a very useful and wonderful piece of apparatus, whose principles would require a book for discussion. Fig. 7 shows the form iasual to tele- phones. The connections of the four wires just mentioned may Fig. 7. — Induction coil mounted in the base of transmitter arm, showing wiriug connections. be seen in Fig. 8, two at either end. Two of them are con- nected, through the transmitting instrument, with a voitaic, or chemical, battery contained in the box' at the base of the mounting-board, as shown in Fig. i. The Other two are directly Fl&. S.^Section of transmitter arm and coil box, showing connections between the transmitter and induction coil. connected to the line wires and carry the current used in t:ans- mitting the sounds spoken into the mouthpiece of the instru- ment. This brief description will give a good general idea ol XXIV A B C OF THE TELEPHONE. various parts of an ordinary telephone apparatus. To fully understand the use of each one, and why all are " assembled " into one piece of machinery will involve a careful study of the whole theory of the telephone. This study, as we shall see, will require us to cover a wide range of scientific facts. For it is true that a complete understanding of telephony demands a good working knowledge of nearly every other branch of electrical industry. Pio. 9.— Form of Telephone Apparatus, manufactured in Sweden The mazneto- generator and iaduction coU are enclosed in the box below the call bills; the bitterv cells, in the cupboard at the base of the backboard. The transmitter is mounted at the top. CHAPTER TWO. 4 BRIEF SURVEY OF THE THEORY OF SOUND, NECES- SARY TO AN UNDERSTANDING OF THE TELEPHONE. The Transmutation of Energy. — Among the many mar- velous discoveries of modern science is what is known as the "transmutation " of energy or force, that is, the fact that one force may under the proper conditions be changed into any other. "Transmutation" means " changing about. " Thus heat may be transformed into electrical activity and chemical energies may become either the one or the other. Heat may result from friction (rubbing), or from chemical action, as wp see in fire, where the process by which a substance is reduced to ashes is called "combustion." Again heat, and light also, result from electricity. In short, scientists tell us that heat, light and sound are all one and the same thing, differing only in degree. Thus, when we feel that a body is hot, warm or cold ; or when we see that light is brilliant, bright or dim ; or when we hear a sound, and know that it is loud or indistinct, or differs from other sounds in representing a different musical note we are only perceiving one fact, under different forms, and by different senses. The Solar Spectrunn. — A good illustration of this fact is to be found by comparing the octave of a musical instrument with the solar spectrum, as seen in a rainbow, or when sunlight shines through a prism. Very different things Indeed these may seem to niost observers, but science compares them. The solar spectrum shows seven colors — violet, indigo, blue, green, yellow, orange and red — in a row in the order given. But these colors are not sharply Separated from one another so that, for example, we have blue on one side of a line and green on the other; they seem to melt into one another, passing through all the possible shades and tints of each color. Further, we have violet at I 2 A B C OF THE TELEPHONE. one end of the line and red at the other. Now we know that in the arts violet is produced by a proper blending of shades of red with shades of blue. Thus violet and indigo stand in the series between red and blue, jugt as green stands between, and is to be made by a blending of, blue and -yellow, and orange stands between yellow and red. From these facts, we may understand what scientists mean when they say that the solar or light, spectrum is only one of a series of spectrums, each beginning with violet or what corresponds to it in some other form of force, and ending with red. The Musical Scale'. — Now, turning to the musical scale, we have a fact precisely similar to the spectrum. Here we have another series of seven; sounds, which musicians name A,' By C, D, Ey F, G, so that whenever we run over the notes in order, we find that every eighth note is the same sound as the one We began with, only higher or lower, nearer the bass or tt-eble, as the case may be. Now, we have beside the seven " naturals, " as musicians call them, five other notes called half tones or "sharps" and "flats" — thus, "B" sharp is "A'' flat^and, furthermore, certain delicate • instruments used by experimenters have demonstrated the fact that each simple note, as we suppose it to be, is in reality a bundle or collection of many separate tones or sounds, differing in pitch and also iq loudness. The Qualities of Sounds. — The pitch of each separate note, as it affects the ear, is, in' reality, only the pitch of the gravest and loudest of the collection, and it is called the " fundamental. " The other sounds are called the " overtones " or "harmonics" and these,' mingling with the fundamental-, give the timbre or quality of the note, although, when we speak of timbre, we carinot by the ear separate the individual notes, which combine to produce its character, pleasing or displeasing. These overtones we mdy compare to the succession of shades by which one color of the spectrum passes almost imperceptibly iiito another, even while not interfering with the eye's sensation THE THEORY OF SOUND. 3 of seven colors. In one kind of instrument, such as the piano, we have one set intensified, and in another, such as the violin, we have another set ; and it is this fact that gives the character- istic difference between a piano and a violin note, although the " fundamentals " may be the same in both. Thus in a musical note the ear finds three things — the loudness of the note, its pitch or tone, and its timbre or quality. Octaves of Force. — Having thus demonstrated that light and sound both follow one and the same law, scientists conclude that both are manifestations of the one force in nature. In other words they find that both are forces, manifestations of vibration in the air, in solid bodies, or in the finer "ether" which is believed to pervade both. As may be familiar to many, the difference between any two notes in one octave, and between the same notes in two different octaves, is one of vibration, slow or rapid. The ear can perceive as " one tone," a sound consisting of less than 32 vibrations per second, and can perceive a sound consisting of as many as 32,000 vibrations per second. This represents a difference of fifteen octaves. On the basis of the computed number of vibrations of the faintest ray of light, scientists have agreed to call the light area the " fiftieth octave," thus giving the difference between what the ear perceives and what the eye perceives, as thirty-five octaves of vibrations in regular scale of increasing intensity in multiples of 8. Moreover, we have no senses to perceive the effects to be derived from the vibrations in all this wide range of the unknown. Such marvelous modern discoveries as the Roentgen, or "X," rays may give us some idea of the facts arising under conditions we cannot "sense." For, here we have ^ power that can impress images on the sensitized film of the photographic plate, although we cannot perceive it as we can perceive light. Yet, as if it were in a way a stronger or subtler form of light, it can pass its rays through the living body of an animal, showing the outlines of the bones, or of foreign substances, and impress these on the sensitized film. 4 A B C OF THE TELEPHOME. or make them visible to the eye, when viewed through the ' ' fluorescent " screen of calcium tungstate, devised by that great genius, Thomas A. Edison. Reflected Sound ', Resonance. — Accepting the scien- tinc theory that sound is a "mode" or form of force and ./ibration, capable of producing on the ear a sensation we describe as ' ' hearing, " we can readily understand that when we speak or make other sounds with the vocal organs we do some- thing more than expel air from the lungs. The air then used is merely the motive power which works the vocal cords and carries the vibrations they produce, high or low, loud or soft, into the atmosphere, where they appear as sounds to affect the ear. This process has a complete analogy in the experiments of striking a tight string or a bell, having a given tone, and observing that any> other string, bell, or vessel capable of giving forth the same note when struck, will respond, being set in vibration by the sound waves that reach it. Thus, if we have a room full of bells, all of the same tone, and strike one, all will be set in vibration and begin to sound out the note belong- ing to them. If, however, there be bells in this collection that give forth a different note from the one struck, they will not respond. Such experiments prove that the true power of sound is not in the mere motion of the air but in the kind or quality of such motions; it is, in short, a question of the particular note sounded in a given case, and of its characteristic number of vibrations per second. Sound Waves. — The way in which sound waves are set in motion may be understood from what occurs when we throw a stone into the water. Then we see, first a commotion in the spot where the stone falls which at once becomes the center of a series of rings that move from it, growing larger and larger, as they are further away from the common center, and finally ceasing altogether. But sound waves originate in an object entirely surrounded with the atmosphere — we have heat and light waves from outside, from the sun, but no sounds — hence THE THEOR V OF SOUND. 5 we must conclude that they are not "on one plane," like the waves on the surface of the water, but producing a series of spheres of force in the air, consisting of alternate moving layers and quiet layers, moving out from one center, which is the instrument making the sound. In these alternate active and Fig. 10. — An illustration of the movements of sound "waves." or vibrations, as recorded by the phonautograph. These represent musical sounds ; the lines produced by the human voice are extremely irregular. inactive layers of air, we have a condition like the waves on water; first a " hump " and then a " trough." Thus we have the fact of "sound waves," and their exist- ence is due to the fact that air, unlike water, may be compressed. That is to say, we may by use of a .pump put a mass of air 6 A B C OF THE TELEPHONE. many times the density of the atmosphere into a closed receptacle or "condenser." This is what is done when a bicycle tire is " blown up " with a hand pump, so that it becomes hard and firm. But air has also "resistance; " thus it is that one must use considerable strength in inflating a tire. Movements of Sound Waves. — These two qualities of air unite in making sound waves possible. Take a closed tube, as shown in Fig. ji, irl which a piston mai-ked P can be worked back and forth.;; Suppose it, is worked rapidly between the Fig. II,— Illustrating the production of sound waves. lines marked, aa and AA. When it moves in the direction of the arrow it slightly condenses the air in front of it, but because the air is at once compressible and resisting, this condensation does not affect the air in the whole length of the tube, but only that between a and H, which is then called the "condensed wave." It has been demonstrated that when the 'first " layer" of air between a and H comes to rest its motion is imparted to the first "layer" of air between ^and H^, and so on until the force exhausts itself. Now, when the piston, P, is drawn in the opposite direction, that is from a to ^, a vacuum, or air-vacancy, is produced be- hind it, causing the air to expand there to fill it up. Thus, we have, as it were, a back movement, which, like the forward movement, is transmitted from a to H, from H, to H^, and so on, giving us the alternate active and inactive (or reactive) "layers." The forward movement of the piston f rom ^ to a shows exactly what is done when a sound of a note, a letter or a syllable comes out of the mouth in speaking. The back move- ment from a to A shows exactly what happens at the end of THE THEORY OF SOUND. m that particular sound. Every sound is such a short and abrupt forward movement, due to the passage of a vocal sound through the mouth, when the tongue, palate and teeth are for a moment in some particular relative position, as will be explained later. Vowels and Consonants. — AH the sounds called " con- sonants " such as, B, C, D, F, G, etc. , are short and explosive as any one can find out; that is a consonant sound may not be "prolonged," or sounded for more than an instant. It is this abrupt ending that is like the back movement of the piston. But there is another class of letters, called "vowels," such as A, E, O, U, which may be made to sound as long as we wish, also certain "sonants," like M, N, L, R, S, Z, and Th. But, if we will notice, we will find that each of these makes a " buz- zing" or vibration that may be felt in the teeth, or perceived by the ear. This "buzzing" is due to the fact that each vowel or sonant sound is made by a rapid series of "moves and stops" like the forward and back movements in the figure just described. Thus in all sounds we have " hump " and " trough" movements, like waves on water. Waves of a Moving String. — This fact may be beautifully illustrated by the experiment of striking a string drawn tight, so as to cause it to give out the note belonging to it. While it is vibrating its sharp outlines are lost in the tremor that results, and in the middle it seems to swell, "belly out" as they say, into a hazy spindle of agitated matter. The eye is incapable of following all the swift changes of the string's movements, and the result is a blur. If now, we make "riders," by bending short strips of light paper in the middle, and place them on the vibrating string, one at a point one-third of the string's length from one end, and another midway between the two points, we will find that the first " rider" will remain on the string, being only slightly. agitated, while the second will be thrown off like a stone out of a sling. Or, if we place sharpened blocks beneath at points one-third and two-thirds the length of the string, we will find that they do not materially interfere with 8 A B C OF THE TELEPHONE the vibration or pitch of the string These facts prove that there are alternate series of activity and inactivity in any moving string, the former being called the "nodes" and the latter, 'loops;" thus we have a regular wave motion, humps and troughs. Fig. 12. — Diagram of I^on Scott's Phonautograph. How Sounds May be "Seen." — The wave motions ot sound have been still further analyzed and recorded by delicate and ingenious instruments, such as Leon Scott's "phonauto- graph" (/. e., "sound-self-writer") and Koenig's apparatus for producing what are known as manometric (/. «., "thinness- measuring ") flames. The former consists of a sounding-cham- ber or barrel, open at one end to admit the sound and closed at the other by a brass tube with a flexible membrane stretched at its extremity. To this membrane is attached a hog's bristle which is able to trace the lines of vibration affecting the mem- brane on soot-covered paper wound about a cylinder revolved by a hand-crank. The tracings shown in Fig. lo illustrate some of the common forms of sound-autographs, thus produced. Some of them resemble the lines traced by the sphygmograph of the pulsations of the blood. Manometric Flames. — Koenig's apparatus is equally in- genious and effective. It consists of a metal " capsule" or box. THE THEORY OF SOUND. divided into two chambers by an elastic diaphragns. At one' end is an orifice for the admission of sounds from the voice or an instrument, and at the other is an ordinary gas-burner, gas for which is admitted to the chamber in front of the diaphragm. If the gas be lighted and its supply be agitated by the sound waves striking against the diaphragm the flame will undergo perceptible changes in strength and steadiness. These may be seen to advantage if we take a four- sided box with mirrors on SiCTION Of CAPSULE Fig. 13. — Koenig's Apparatus for producing Mauometric Flames. every side and turn it rapidly in front of the flame. The result- ing " image " will, differ according to the strength, timbre and othpr characteristics of the sound, giving such figures as may be seen in Fig. 14. lO A B C OF THE TELEPHONE. The Human Voice.— Wonderful as are the instruments for producing such effects the vocal organs and ear of a human being are even more remarkable in their complexity, adjust- ment and range of powers. The mechanism for producing articulate speech consists of two important parts ; the sound- producer and the sound-shaper. Both are peculiar to man in the range of sound possibilities and in the power of shaping. Other animals can make their distinctive calls and cries, ex- pressing their emotions and needs, and such noises are to a certain extent shaped and definite sounds, besides partaking of musical qualities — timbre and volume — but in none of them is there anything approaching to the human power of articulation. 1*16. 14. — Manometric Flames produced by Koenig's apparatus. These illustrate a blending of two distinct musical notes. The Vocal Cords. — The sound-producer in man is the larynx, and its action consists in forcing air from the lungs through a slit between two membranes, known as the "vocal cords." By means of a number of delicate muscles these may be tightened or relaxed, so as to produce high or low notes as the case may demand. In the full, deep sounds, known to musicians as "chest notes," the entire membrane is found to vibrate, but in the high notes and in the "falsetto" only the edges are used. This fact will be seen to depend on the same principle as that by which the strings of a guitar give forth differ- ing notes, according as they are shortened by the pressure of the finger on this or that fret. The characteristic difference between the voice of a man and of a woman is due to the fact that in the THE THEORY OF SOUND. \ \ former the vocal cords are larger and thicker, and the larynx longer, than in the latter; hence able to vibrate less rapidly. The action of the vocal cords, may be readily compared to the action of the lips in whistling. An attentive study with a look- ing glass will reveal the fact that the lips open more in the pro- duction of low notes and contract, sometimes almost to closing, in the production of the high ones. Powers of the Voice. — The vocal chords, together with the mouth and the several resonant cavities opening into it, are able to produce a wide range and variety of sounds. The average reach of the human voice is about two octaves — some voices reach as far as three, but few beyond that — and the power of varying the timbre, to permit the imitation of various musical instruments, animal calls and other sounds and noises seems almost unlimited. That the ear is capable of perceiving the wide variety of tones and shapes argues that it is a most delicate instrument. i The Organs of Hearing. — Fig. 15 gives a good general view of the series of organs, by which sound waves entering the "meatus," or opening, of the outer ear are transfprmed from atmospheric vibrations to nerve pulses that can affect the brain with definite and finely separated impressions of sound. The process by which one form of force is thus changed to another may be compared to the transformation of heat to motion, as through. the steam engine, or of motion to electricity, as in the dynamo electric generator. The mechanism of the ear thus combines the functions of both separator and transformer, -as those terms are used in mechanical science. The essential parts, as shown, are: i, the tympanum, or "drum," a mem- braneous diaphragm stretched across the inner end of the external canal; 2, a series or jointure of three minute bones, or "ossicles," called respectively the malleus, or hammer, the incus, or anvil, and the stapes, or stirrup, the first pressing against the inner side of the drum, and the third, against another mem- brane closing the passage to the inner ear ; 3, the inner ear, con- 12 A B C OF THE TELEPHONE. sisting of a marvelous series of canals in the bony substance of the skull, known as the " labyrinth " and the " cochlea " (snail- shell) ; 4, the auditory nerve attached to the nerve fibers filling the former and also running into the latter through its turns, or " convolutions." Fig. 15. — Section of the ear. The Inner Ear." — The inner membrane of the cochlea is lined with some 3,000 minute fibres, which are the ends of the auditory nerve. They are known as " Corti's fibres," and each seems to be tuned to some particular note or tone, which, conveyed into the ear by the sound waves from the outer air, can cause it to vibrate in unison, just as the bells tuned to the same note. Each fibre is unresponsive to other notes, just as is a string or a bell, and thus we may understand how that some disease among them will make a person "tone-deaf," or unable to distinguish one note or tune from another. Some persons seem to be so affected that they cannot hear certain notes, which are then said to be "out of their register." THE THEORY OF SOUND. How do We Hear? — Exactly how the nerve fibres of the ear are able to respond not only to notes of different tone, but also to sounds of different "shape" — articulate sounds — we have no satisfactory idea. No more is it plain how the minute dent^ traced by vocal stress in the tin-foil or wax on the revolving cylinder of the phonograph are capable of reproducing the same sounds, words, timbre and tones when the process is reversed. We can derive some idea of the process by whirh an electric current, alternately interrupted and closed, by a series of "dots "and "dashes" — shorter and longer connections, a- in the Morse telegraphic alphabet — can convey intelligible mes Fig. i6.— The Ossicles. Fig. 17.— Cast of the Id ":_ — "S7SI~~~~——Z zz Fig. 2r. — ^Diagram of Grotius' theory of the galvanic cell. Row I shows position of molecules w^hen circuit is open ; 2, at the moment ' of closing; 3, when current is flowing. PRINCIPLES OF ELECTRICITY. 21 thus liberated, unites with the oxygen of the molecule just ahead; that, in turn, giving its oxygen to the molecule in front of it, and so on, until the copper electrode is reached. There we find the hydrogen given off to unite again with the liquid and perform the same rotation, until all the oxygen hais united with and consumed the zinc plate. The Galvanic Current: Dynamic Effects. — The funda- mental fact about the galvanic current is that it moves in a definite direction, like a stream of water, between the terminals of the circuit. In so doing it exerts certain effects in the line of its travel, which fact is utilized in electro-plating, electrotyping, etc., when the substance of metal sheets, copper, silver, nickel, etc., is transferred, a particle at a time, from the anode or positive terminal, in the direction of the cur- rent, and is deposited on the cathode or negative plate to any desired thickness. Although, so far as can be determined, the passage of an electric current along a conducting wire involves no transfer of material substance, unless it may be the ether, as some hold, its movement and the effects it may be used to accomplish are in very many ways analo- gous to currents of water. Thus, its practical utility depends upon several considerations, among which may be mentioned the pressure at which it is generated in the galvanic cell or dynamo, the resistance it meets in the process of transmission, and the available strength or intensity to be obtained for operating machinery or producing other effects at any given point on the line. To apply the analogy of water currents, we have again an initial head pressure, which is efficient in producing mechanical effects, according to the amount of fluid passed and to the size of the conductor. In water-power systems the head pressure depends upon the height of the source above the working level, while the amount of work that can be done is determined spleljf by the size of the pipes. Thus, a large head-pressure nr^ifig a srnaft" amount of water cannot accomplish as much work as a lower n'ead urging a large amount of water, because the smallness of the conductors reduces the active energy with the amount of water passed. A'larger head of water urging a large amount of water can accomplish more work in a given time, because it can give a higher speed to the water, hence, also, to the machinery affected by it. 22 A B C OF THE TELEPHONE. Resistance, Electrical and Mechanical, — With water cur- rents the only practically significant element in calculating resistance is the size of the tube through which it flon-s, although the friction encountered, as the walls are rougher or smoother, undoubtedly oper- ates to retard its progress, consequently reducing its energy. With electric currents the size of the conductor is also of prime importance in facilitating or retarding the electrical energy flowing over or about the wire. But an essential difference to be found here is that wires of the same size, made of different substances, exert different degrees of resistance. Thus we have not only the resistance belonging to a given size of wire of a given metal, but the resistivity, which is the specific resisting power or action, as between two wires of like size but of dif- ferent metals. As the reciprocals, respectively, of these quantities, we Fig. 22. — Diagram illustrating the difference of resistance between two water circuits. have also the conductance and the conductivity ; the one defining the rate and amount of current conduction to be had with a given size of wire of a given metal, the other furnishing the basis of comparison between the same sizes of different metals. The resistivity of the vari- ous metals and other substances will be given later, also the resistance and conductance of the several sizes of wire, according to gauge stand- ards adopted in this country and abroad. In the phenomena of electrical resistance there is considerable analogy between the action of a current of water and a current of elec- trical energy. Fig. 22 shows two pipes, one large and one small, opening into the fi-ont and back of a cylinder. If, now, the pipes be filled with water, and the piston be moved backward and forward, considerable resistance will be experienced in moving the masses in the two pipes. That there is a difference on this point between the PRINCIPLES OF ELECTRrCITY. 23 two may be found by closing a stop-cock, attached to either pipe, and merely moving the water in the other. By this means we will find that we experience less resistance in the large pipe than in the narrow one, although both may be equally smooth in bore. The same fact will be noticed if we have a short wide pipe and a long narrow one, of such dimensions as to contain precisely the same amount of water. The same is true of two electrical circuits, one shorter and of thick wire and one longer and of thin wire. A given strength of cur- rent passed through both will show a greater resistance in the longer and thinner wire than in the shorter and thicker wire, although both wires may weigh exactly the same and contain the same amount of metal. Laws of Electrical Resistance. — These facts give us the following rules, applicable as between th,e several sizes of wire of a given metal, although,, of course failing as between wires of different metals. ' r. Doubling the cross-section of a wire, halves the resistance. Halving the cross-section doubles the resistance. 2. Doublirig the length of a wire doubles the resistance. Halv- ing the length of the wire halves the resistance. 3. To sum up : The resistance is measured in inverse ratio to the cross-section of the wire, and in direct ratio to the length of the wife. A thorough understanding of this principle enables the con- struction of electrical instruments of great delicacy and' exactitude of effect. " False Resistance." — In addition to the true metallic resist- ance, which varies, as we have already seen, between the several vai-ie- ties of metal and the several sizes of wire of each, there are other ele- ments that act to resist the flow of electric current and to cut down the pressure on the line. These are generally summed under the head of "false resistance," and include the magnetic impedance of the line, which tends to the creation of a counter-electromotive force (CEMF), and the capacity, which describes the tendency of the line to act as a charging surface under the influence of the current. 24 A B C OF THE TELEPHONE It is more pronounced with iron wires than with copper. With the direct current the combined action of the true resistance and the CEMF speedily limits the possible distance of transmission; so that the attempt to double the length of the line involves either thai: the area of the wire be doubled or that the initial pressure be squared. Similarly, if the length of the line is so trebled, the original area must also be trebled, or the initial pressure cubed. Either alternative speedily reaches the practical limit of construction. With alternating, currents, although the situations involved in electrical transmission are ; considerably more complicated,, the possibility of using step-up trans- formers enables the pressure to be raised -.to the highest figure required for effective operation of the line. 1 ^ , Wi^^ F " ^ •*-^ V ===:=-. - C d' '/ f' r A L»J y y y ? <■_ ^ Fig. 23. — Diagram illustrating the effect of" back force " in water. CEMF and Capacity. — The counter-electromotive force, which, in general, increases in ratio to the true metallic resistance, behaves, in many respects, like a current moving in an opposite direc- tion. It may be compared directly to the resisting agencies at work in the flow of water under head pressure, as is shown in Fig. 23. Here Cis a tank containing water, wWch flows^out through the pipe, AB. If the vent at B is shu^ the wafer will stand, at the same level in the tank and in the upright pipes, a, b^ c,di.e,f,g. But if it is open, the?e immediately begins a conflict between the natural tendency of wafer^ to seek its own level and, theforce of gravity, which causes th? watier to flow out of the vent. The result^ is that the water in the up- right tubes stands at the levels a'^. bf ^ ^^ 4', etc. ^ On precisely similar principles, the electromotive force, or prepare, of an el^trig current PRINCIPLES Ofi ELECTRICITY. 25 is so reduced by resistance, true and " false," over a long circuit that U is difficult to transmit the telephonic current, or any other of low pressure over very great distances. Another current-opposing element of equal importance in both telegraphy and telephony is the capacity, which operates, as we have already seen, to make the line a very true kind of condenser. The situation may be briefly summed in the words of a well-known tele- PiG. 24.— Illustration of the whirl of magnetic force around a current-bearing wire. phone authority, as follows: " Every line is practically a condenser, the surface of the wire being one plate, the earth the other, and the air or covering of the wire the insulating layer or dielectric, between the two. Before a current can reach its full strength at the distant end of the line this condenser must be fully charged ; and when re- versed currents are used it must be discharged and charged again at each reversal. This explains why a high capacity is so detrimental to telephone working, as the rapid changes and reversals of the telephonic current are deadened and flattened out if they have to charge much capacity before reaching the receiving instrument." Elements of Magnetism. — ^The principal effects to be ob- tained from direct application of the electrical current — that is, l)y allowing it to flow through a given path — are electrolytic, or electro- chemical, productive of changes in substances, resolving them into their constituent elements, or causing new combinations, and the gen- eration of heat by passing the current through a highly resistant wire. The passage of the current, however, is always accompanied by other phenomena, upon which depend many of its most important industrial applications. Thus, while a wire is conveying current in one direc- tion, it is found to be always surrounded by a continuous ring of mag- 26 A B C OF THE TELEPHONE. netic force, which acts at right angles to its length. As judged by the effects produced, this magnetic influence increases directly with the current intensity and pressure on the line. The existence of the magnetic flux around a live wire may be demonstrated, as follows ; If we run a wire through a hole in a card, or piece of paper, and then connect it in circuit with a galvanic bat- tery or dynamo, causing a current to move through its length, we will Fig. 35.— Concentric circles 01 magnetic force around a " live," or current-bearing, wire. find that iron filings scattered on the card or paper will arrange themselves into nearly concentric circles about the wire, as shown in Fig. 25. The most important effects of the magnetic flux on a live wire are found in the phenomena of induction and electro-magnetism, which, although dependent on the same fundamental principles, are extremely diverse in their practical and commercial applications. Thus, if two similarly wound coils of wire, carrying currents, are so suspended as to swing freely, it will be found that they will assume such relative posi- tions that the end ^% which the current enters on the one will be ap» PRINCIPLES OF ELECTRICITY. 2y proached to the end at which the current leaves on the other. Such coils of wire are known as solenoids, and the experiment just men- tioned proves that, when carrying a current, they show the polarity of magnets. Electrical Induction. — As the effects of a current carried from one end to another of a wire are said to be due to "conduction" (a leading-along-with), so the effects- that are thrown off at every point as it moves along — thes(? are the magnetic effects — are attributed to a quantity called "induction" (a leading-into). B Fig. 26.— Diagrain'illustrating induction between two circuits ; one energized by a battery, the other bearing a galvanometer. That it is proper to attribute the effects of induction to magnetic influence is evidenced by the fact that there are no magnetic insulators or cut-offs. A magnet will attract iron, as we have seen, through a pane of glass, which would effectually cut off a current of electricity. Consequently, it is able to produce its effects in spite of the insulating silk-windiiigs on a magnetizing wire. Laws of Current Induction. — The fact of current and mag- netic induction is exceedingly significant in general electrical indus- tries. Thus, if a circuit, as shown in Fig. 26, formed by connecting the two terminals of a galvanometer by a wire, without including a 28 A B C OF THE TELEPHONE. battery, be placed near to another circuit, formed by connecting the poles of a battery of galvanic cells by a length of wire including a suitable hand-key or switch, it will be found that whenever the sec- ond circuit, A, is closed by the key or switch, allowing a current to pass in a given direction, a momentary current will be induced in the first circuit, B, as shown by the galvanometer. A similar result will follow on the opening of the battery circuit, the difference being that the momentary induced current occurring at closure moves in a direc- tion opposite to that in the battery circuit, while the momentary cur- rent at opening moves in the same direction. The following rules govern the phenomena of current induction : 1. Increasing the strength of the current in A increases the strength of the current in B. 2. Decreasing the strength of the current in A, or opening the circuit, decreases the strength of the current in B, also causing it to flow in the same direction as the current in A. 3. If we move the current-carrying wire. A, nearer to B, we pro- duce a strong current in the opposite direction ; if we move it farther from B, we induce a weaker current in the same direction. 4. If the wire used in the circuit A is thick, and that used in B is thinner, the current induced in B will show a greater electromotive force than that in A. Conversely, if the wire used in A be thinner than that used in B, the induced current will show a lower electromo- tive force than that in A. In precisely similar manner, if an ordinary steel permanent mag- net be approached to the closed circuit B, the galvanometer will in- dicate a momentary current in one direction, and on the withdrawal of the magnet, a momentary current in the opposite direction ; thus showing that the force existing in the magnet is the same in effect as that surrounding a current-carrying wire. The Principles of Electro-Magnetism.— The effects of current and magnetic induction are practically applied in electrical industries in such devices as the electro-magnet, the induction coil, repeating coils, static transformers, and in the dynamo electrical gen- erator. In the production of magnets, the experiment last described PRINCIPLES OF ELECTRICITY. 29 is reversed : a bar of iron or steel being wound with a leijgth of insu- lated wire, through which a current is allowed to pass. The effect of the current in this case is to give the bar magnetic properties, which are practically permanent if it be steel, and enduring no longer than the current if it be soft iron. In other words, as shown by Fig. 27, the magnetic lines of force are at right angles to the path of the cur- rent. The bar becomes polarized, according to the direction of the winding ; in the right-handed winding, as shown in the figure, the north, or positive, pole is at the end where the current leaves, but- with a left-handed winding it would always be at the end where the J>trtcUmvof Urtts offvrau Fig. 27. — Diagram illustrating induction between a wire and a magnet. current enters, showing that the theoretical magnetic rotation of the molecules is in a direction opposite to the movement of the hands of a clock, could it be noted by a person looking toward the positive pole. Magnetic Lines of Force. — The current from an electrical cell, as it passes steadily from the point of positive, or high, potential to that of negative, or low, potential, finds analogy in what are called the "lines of forces" in a magnetic "field." If we take a perma- nent bar magnet and place it near a quantity of iron filings we will find that these will be attracted to both poles, and will adhere there with considerable force. But toward the center of the bar we will see very few. This indicates that there is some system in the magnetic power of attraction, and that it is not the same at all points. If now we place over the bar magnet a plate of glass or a sheet of stiff paper, and on it dust a quantity of iron filings, we will find that 30 A B C OF THK TELEPHONE. they arrange themselves as shown in Fig. 28 ; lines of filings starting out from either pole, and at points midway between, which seem to describe arcs (arches) or parts of circles, over the bar, meeting in the middle of the "field." The lines thus described by the filings are supposed to represent the lines of force, or the direction of the current, as it leaves the north pole and re-enters at the south. This fact is illustrated in Fig. 36, where the lines of force are indicated by arrows supposed to enter and 'leave the magnet as lines of force. Fig. 28.— Lines of force in a bar magnet. Shown by dusting iron filings over the *' field." Fig. 29 further illustrates the theory of the magnet, showing how that filings arrange themselves as rays, or straight lines starting from a common center, when scattered on a sheet of paper or glass placed over either end, or pole, of a magnet. In this figure we have the end view of the semicircles of force shown in Fig. 28. Magnets and Magneto-Electricity.— On the fact of these lines of force in a magnetic field is based the theory of the dynamo- electric generator, commonly called the " dynamo." In this machine the current is obtained by producing mechanical excitation across the PRINC.PLhS OF ELECTRICITY. 31 lines of force in an electro-magnet. In may be well to explain here that there are two kinds of magnets. The first is the electro-magnet, consisting of a soft-iron bar, around which a silk-covered Avire is wound a large number of times, like thread on a spool. When an electric current is passed through the wire the iron bar receives all the qualities of a magnet — power to attract iron, etc.- — which continue so long as the current is passed through the wire, and cease when it is with- drawn. The second kind of magnet is the perraaneht magnet, con- PlG. 29. — Lines of magnetic force at the pole ofa bar magnet. sisting of a bar of hardened steel. The magnetic qualities are given to it in exactly the same manner — passing a current through the coil or "helix" of wire: — but are retained by it for an indefinite time after the current ceases. The horseshoe magnets used in telephone genera- tors and receivers are usually made in another way — stroking the hard- ened steel bar across the poles of a very powerful electro-magnet. Either form of magnet may be used as the basis of a magneto-electrical generator, but it has been found most suitable to employ electro-mag- nets in commercial dynamos of large power. Although the construction and use of dynamos form a distinct branch of electrical industry, the principles involved are applied not 32 A B C OF THE TELEPHONE. only in the construction of the telephone and the magneto-generator, but are becoming increasingly important in other branches. Fig. 32 shows the simplest possible form of dynamo. Here, iVand ».9 represent the two poles of a " horseshoe," or curved-bar, magnet, between which lines of magnetic force flow, as shown by the dotted lines. Between these poles the wire loop, C C , re- volves on the bar or spindle, A A, thus cutting across the lines of force. This process makes a cur- rent which flows out and back Fig. 30.— DiaBrara illustrating the theory again On the cirCuit wire, E, which of magnetic "circulation "along the liues of . i j ^ ..i_ ,, i, i. ji r>i force. IS attached to the " brushes," .5* and £^, so-called because th&y brush against the revolving "drums," as shown in the cut. How the Dynamo Works. — In this figure the loop of wire would be called the "armature," and the greatest difference between this simple machine and the powerful dynamos used to produce cur- rents to move street cars and supply electric lights is in the construc- tion of the armature. In the latter, instead of a simple loop of wire, there are a great number of such loops or of coils, wound on a " drum," or " ring," and running at right angles to the magnetic lines of force. The rule is that, electromo- tive force (EMF) generated by any dynamo, is in pro- portion to the number of turns of wire about the "core," or frame-piece, of the rotating armature; and within certain limits, to the speed with which the armature is revolved. Owing to this arrangement, the lines of force are greatly deflected, as shown in the dotted lines in Fig. 33, and by the turning of the armature a very intense excitement of the " field " is created. From this excitement a current results, whose strength for work is in proportion to terms of the rule quoted above. Fig. 31.— Diagram of the usual method of producing magnets. PRINCIPLES OF ELECTRICITY. zz The effect produced by the revolving of the armature of a dynamo is that the armature itself is transformed into an electro-magnet, hav- ing two north poles and two south poles, points for the exit and entrance of the current, respectively. These poles alternately exert an attract- ing and repelling force on the magnetic field, and hence continually distort the magnetic lines of force, as is shown in Fig. 34. The result is a constant shifting of the neutral points, and a continual series of alternate attractions and repulsions at the active points. These rapid changes, demanding a constant readjustment of the magnetic Fig. 32.— Simplest possible form of dynamo, ^and ,9 are the poles ; Cis the armature ; B B the brushes and E the line wire. lines of force, transform the magnetic movements among the molecules into electrical energy, which emerges from the machine in the form of a current. The Induction Coil. — In the induction coil both varieties of induction are utilized to raise the EMF. The core is a bundle of fine iron- wire, lengths. The primary winding is in circuit with the source of electricity, and the secondary winding connected to line for conveying an induced current to the destined terminals. On these general lines are constructed all forms of inductorium and static trans- former, the proportions between the sizes of wire used for primary and secondary, and of the number of turns for each being regulated accord- 34 A B C OF THE TELEPHONE. \ ing to the desire to "step up," or increase the EMF on the line, of to "step down," or decrease it. As already stated in Rule 4 for current induction, a smaller size of wire in the secondary circuit raises the EMF of the induced current. It is also true that, by increasing the length of the wire used in the secondary circuit, the EMF is raised. Consequently, when the two wires are wound upon a common core the general rule is that the ratio of the EMF in the secondary to that in the primary is as the number of turns in the secondary are to the number of turns in the primary. Since, therefore, the rule followed in the construction of all magnetic coils is that the diameter of the core is at least one-third of the total Fig. 33.— Diagram illustrating the polarization of the armature of a two-pole dynamo. outside diameter, it follows that, for the end of obtaining a given increase of EMF in the induced current, the wire of the secondary winding is madis of such diameter, that the two windings together shall form at most the other two-thirds of the diameter. Thus, if we wish to raise an EMF" of 10 volts in the primary to 50 volts in the secondary, we wind with a length and size of wire capable of carrying this pressure, and wind the secondary with five times the number of turns. Of course, in order to operate the induction coil, it is necessary that the current in the primary winding be periodically interrupted ; PRINCIPLES OF ELECTRICITY. 35 that the pressure or resistance be periodically varied ; or that the polar- ity be periodically changed, making a true alternating current. These conditions obviously follow from the rules of induction already given, which involve, as we have seen, that no effect is produced on the sec- ondary circuit, so long as the current moves constantly in the primary. Since in the induction coil we have a combination of both magnetic and voltaic induction, the effect produced partakes of both. The principal difference in effect is that the former continues as long as the current endures, while the latter is momentary, indicating the moments of closing and opening the circuit of the primary coil. It is possible that we have an explanation of the difference in the fact that the mo- mentary currents in the secondary coil indicate the successive adjust- ments and readjustments of the lines of magnetic force, as the core is alternately magnetized and demagnetized. The effects of simple vol- FiG. 34. — Diagram illustrating the distortion of the magnetic field in a two-pole dynamo by the revolution of the armature. taic induction, seen between two wires, as previously described, is probably due to the influence of the magnetic area of the "live," or current-bearing, wire on a closed, but currentless circuit, in which it produces magnetic properties; that is, effects a molecular readjust- ment in the "dead " wire. Dimensions of Transformers, — In calculating the exact pro- portions of an induction coil or transformer, with the object of produc- ing a given transformation in EMF, there are a number of considera- 3^ A B C OF THE TELEPHONE. tions involved, also a number of formulae of general value. The matter depends, however, very largely on the rule of "cut and try," as has been frequently stated, and cannot be precisely determined, on account of several uncertain elements. In other words, according to the best practice, coils for given types and grades of telephonic apparatus are constructed on data gained only from elaborate experiments, touching the number of convolutions, the sizes of wire, the resistance of both primary and secondary windings, and by tests with instruments thus produced on lines of different lengths. It is important, in the first place, to neutralize the induced EMF in the core, which tends to oppose the EMF of the primary coil, hence cutting down the magnetic flux through the secondary winding and diminishing the induced secondary EMF. This is accomplished by " laminating " the core, or constructing it of a number of parallel iron wires or strips, which act to increase the internal resistance, hence to reduce the EMF of the core. The amount of magnetic flux to be calculated in a transformer is approximately determined by experiment, and is stated in terms of magnetic densities in C. G. S. lines of force per square inch, with reference to the cycular frequency of the current used, or the number of complete alternations per second. Thus, to calculate the flux of a core, the following formula holds : „, EMF X io» 4.44 X N X C in which N indicates the number of turns in the secondary winding and C the cycular frequency. Thus, to find the proper size of core in square inches cross-section, we have : ^ . EMFxtqS Section = j^^ — — , 4.44 X N X C X F in which i^indicates the number of lines of force per square inch, which is determined with reference to the length of the coil inducing mag- netism and 'the permeability of the iron. Thus, with a frequency of 125 cycles, the number of magnetic lines is, given by authorities as between 20,000 and 40,000 per square inch : higher frequencies requir- ing a much smaller number. PRINCIPLES OF ELECTRICITY. 37 Windings of Transformers. — In the matter of arranging the windings the fundamental formula makes the EMF's of the two equiva- lent to the ratio of turns between the secondary and primary windings, while the current in the secondary winding is equivalent to the current in the primary multiplied by the ratio between the primary and sec- ondary pressures. Thus: primary EMF Secondary current t= primary current x -^ ~ — ^^.^ •' r J. secondary EMF, Knowing then the size of wire that will safely carry a given primary current at the given EMF, we can readily determine the proper size for the secondary, after calculating the number of turns for each. In this process we may employ such forrnulae as the following ; ^ EMF X io8 Turns = 5 -=- -, 4.44 X flux X frequency as indicating the number for the primary winding. For the secondary, the following is approximately correct : secondary EMF Secondary turns = primary turns x — ; „^,_. -. ■' tr J pnmary EMF Because the induction coil of a telephone apparatus is effective in raising the pressure of the talking current, enabling it to traverse a longer line than would be possible without transformation, it follows that the longer the line the larger and more powerful the coil required to step up the voltage to enable the through transmission of the current. The same rule applies in the transformers used on power-transmission circuits. The Transformation of Telephone Currents. — The prac- tical situations involved in the use of induction coils in telephony vary somewhat from those found in other connections, owing principally to the necessity of transforming a speech-bearing current from a low press- ure to a higher, without impairment of its essential qualities of phase, frequency, etc. ; in other words, maintaining the distinctness of speech. In ordinary local-battery apparatus, the EMF varies from 1.5 volt, with one Leclanche cell, to 4 volts, with two or three cells. The resistance of the transmitter and primary circuit vary between 5 ohms, with the Blake; between 10 and 20 ohms with the White, and as 38 A B C OF THE TELEPHONE. high as 40 ohms with some special long-distance instruments. The volume ranges, therefore, between .3 and .075 ampere, and the power, between .225 and .8 watt. Within limits an increase in the internal resistance of the transmitter involves two advantages : economy of bat- tery power and such reduction of the current intensity as will give sharp and distinct transmission of speech over short lines. With cen- tral energy S3?stems, in which the current in the primary circuit often has a power as high as 3 watts, the transmitter is sometimes adjusted to a resistance as high as 100 ohms, or more, with the result of cutting down the current, at the expense of distinctness in articulation. Briefly expressed, then, it is desirable to reduce the current below the point at which saturation of the induction coil core will produce a degree of retardation sufficient to displace the waves and cause an indescribable jumble of sounds, instead of distinct articulation. Since, however, it is impracticable to increase the voltage of the battery or the resistance of the primary circuit, beyond definite limits, without risking the safety of the transmitter by overheating, it is evident that the proportions of the induction coil must be carefully calculated, or found by a series of elaborate experiments. In practical work, it has frequently occurred that better results may be obtained with a coil of high resistance winding than with one of larger conductance. Even this rule cannot be taken as absolute. CHAPTER FOUR. ELECTRICAL QUANTITIES. EFectrical Measurements. — Electrical measurements are sim- ple in theory, readily computed and of extreme exactitude. Starting with a few known quantities, we can get very exact figures for the strength of a given current, the degree of electromotive force any source can emit, and the resistance of any line of wire, as well as the amount of work it is capable of accomplishing in a given time, under any assumed conditions. Considering the need of exact instruments in such an industry as telephony, and the minute quantities of force and volume to be dealt with in them, it is indispensable that the practical telephonist have standards of measure that will permit him to deal with things and conditions almost unimaginably small. Names of Electrical Units. — In addition to the need of units that may be expressed as readily and with as few figures as possible, the science of electricity further demands distinct names for its units in order that there may be no uncertainty as to what particular quantities are being measured. The names of electrical vmits of measure are based on the names of several great scientists and electricians of history, who thus have a lasting fame in the terminology of the science they labored to perfect. The electrical units are, accordingly, the "volt," named from Alessandro Volta, Professor in the University of Pavia, and discoverer of the electric current; the "ohm," from Professor Georg Simon Ohm, of Berlin, a profound investigator in electrical matters, and the formulator of Ohm's Law ; the " ampere," from Andre Marie Ampere, Professor of Mathematics in the Polytechnic School of Paris, author of a celebrated work on dynamic electricity and one of the earliest experimenters in electric telegraphy; the "coulomb," from Charles Auguste Coulomb, a brilliant investigator of electricity, and the author of a number of books on magnetism, the mariner's compass 30 40 A B C OF THE TELEPHONE. and general mechanics ; the " farad," from Michael Faraday, the cele- brated English physicist and Professor in the Royal Institution of London; the "watt," from James Watt, the inventor of the practical steam-engine; the "joule," from James P. Joule, a noted English physicist who determined the mechanical equivalent of heat and form- ulated Joule's Law. Other electrical and magnetic units are the henry, the gauss, the maxwell, the gilbert, and the oersted, all of which are as evidently derived from the names of great scientists. The Use of Electrical Units. — Electrical units, while com- puted on the basis of the metric, or decimal system, are often expressed in fractional forms, with the prefix, micro (Greek, micros, small), in- dicating the one-millionth part of a given unit, as micro-farad and microhm ; or, similarly, with the prefix mega (Greek, megas, great), as indicating one million times that unit, as megohm, etc. The com- mon metric prefixes are also used, when necessary. Thus, we have milliampere, for i-i,oooth ampere; kilovolt for i,ooo volts and kilo- watt for i,ooo watts, etc. The Ohm, the Unit of Resistance. — ^The unit of electrical resistance is the "ohm," which, by the Electrical Congress at the ('olumbian Exposition, at Chicago, in 1893, was fixed as the equiva- lent of the resistance to the electric current of a column of liquid mercury, 106.3 centimeters (about 41.3 inches) long, and one square millimeter (.00155 square inch) cross-sectional area at the temperature of melting ice. Mercury is chosen for the test, because, as we shall see later, its conducting power is low (1.6 in a scale of 100 for silver), and its specific resistance is comparatively high (99. 74 in a scale with i . 5 2 1 for silver). It is, therefore, about one on the scale when measured for conduction ; and about one hundred on the same scale when measured for resistance. The standard specifies the temperature of melting ice, since, as is known, a column of mercury is shortened or lengthened with the fall or rise of temperature, and on this fact the thermometer is constructed. Electrical Pressure.— The fact that there is such a thing in the electrical conductor as resistance, which may be measured and ELECTRICAL QUANTITIES. 4t must be met by a force which is able to do work, establishes the further fact that there is such a thing as pressure. The steam-engine works on this principle : if there is a sufficient pressure of steam generated in. the boiler the machinery may be moved ; otherwise, not. Similarly, if we have a sufficient " head " of water, we may move the wheel of a mill. In both cases there is a power or force in the steam or the water that can overcome a resistance in matter, which is known as " inertia," or the tendency to remain motionless unless compelled to move. Now, the electrical element that can set up an active condition in a circuit of metal, possessing the electrical inertia, called resistance, is the electro- motive force (EMF) that we have previously mentioned. It is for electricity what the steam pressure is for the engine, and what the head of water is to the mill-wheel. The Volt, the Unit of Pressure. — The unit of electrical press- ure is called the " volt," and it is about equal to the average strength of an ordinary galvanic cell. Thus, the usual type of Daniell (or "blue-stone") cell used in telegraphy has a capacity of about one (correctly, 1.08) volt. The Leclanch^ cell, which is the kind most often used in telephony, has a capacity of one and one-half (1.50) volts. The Clark cell, adopted as the standard for EMF, has an out- put capacity of 1.434 volts at 15° Centigrade. One volt of EMF passed through a circuit having a resistance of one ohm yields one unit of working electrical energy. Such a unit of resistance may be found in one foot of No. 40, A. W. G. (American Wire Gauge) copper wire, which has a diameter' of three one-thousandths of an inch (.003145 inch, or 3.145 mils.). The same unit of resistance is to be found in about four miles of ordinary copper wire used in electric trolley lines (No. 0000, which has a resistance of about .25 ohm to the mile). . In both cases we have just about the equivalent of the column of mercury above mentioned if we have also a temperature of 45 degrees, Fahrenheit. The Ampere, the Unit of Current. — ^The unit of current intensity given forth after one volt of electromotive force has passed through a resistance of one ohm, is called an " ampere." Its strength 42 A B C OF THE TELEPHONE. has been found to be equal to that of a current which can deposit .00033 gram of pure copper or .001118 grain of pure silver in one second of time by the process of electro-plating, when the solution of the electrolyte is mixed according to standard specifications. C. G. S. Units in Electricity. — Electrical measurements are based upon the metric units of length (centimeter), of mass (gram) and of time (second), and are, for this reason, designated by the initial letters of these three; being called C. G. S. units. Since electrical effects and quantities are dynamic, or capable of being reduced to dynamic terms, the significance of these units is evident : all dynamic effects being expressed in terms of weight, length, and time. Thus, one horse-power is the force that can move one pound through 550 feet in each second of time, or the equivalent. One C. G. S. unit is similarly such a force as can move one gram weight (.0022 pound) through one centimeter (-3937079 inch) in each second of time. According to accepted standards, 7,460,000,000 C. G. S. units make one horse-power, and one C. G. S. unit is .000000000134 horse-power. ..< in order, therefore, to reduce the units of electrical measurement to ttrms of C. G. S. units, their " absolute values " are given as powers of 10. Thus, the volt is made equivalent to 100,000,000 (or lo*) ; the ohm, to 1,000,000,000 (or 10'), and the ampere, to o. i (or 10"') absolute units. On this basis the unit of resistance is treated as a counter-force, which, in dynamic terms, is to the unit of pressure as 10' is to 10', as may be understood from the practical applications of Ohm's Law. ' Ohm's Law. — ^Because, as we have seen, the resistance of any wire of given length and cross-section is known, because the current may be measured by efficient instruments, and because there is a known standard of voltage for most common battery cells and gener- ators, we have in every circuit at least two known quantities, which will enable us to determine the other. The relation of all these quan- tities to a given standard — since one volt, after meeting a resistance of one ohm, will yield an active current of one ampere intensity estab- ELECTRICAL QUANTITIES. 43 lishes certain proportions among them. This may be expressed in the formula, known as Ohm's Law, which is as follows : 1 I. — The current is in direct proportion to tlie electro- motive force (EMF), and in inverse proportion to the resistance. This is evident, because it is by the resistance of the conductor that the initial electrical pressure of the source or battery is modified, or divided; one percentage being, supposedly, "dissipated," or "ab- sorbed," as is steam power or water pressure, and the other given forth as working current. With an understanding of these facts, we may see that the rule given above means, in simple language, that : The greater the elec- tromotive force, or the smaller the resistance, the greater the current ; and the smaller the electromotive force, or the greater the resistance, the smaller the current. Moreover, because, as we have seen, there is a definite proportion between the three units in any circuit, the ratio of the tliree is precise and not general. Thus, we may derive the rule : II. — The current (amperage) is equal to the electro- motive force (voltage) divided by the resistance (ohmage). This rule may also be expressed as follows : EMF , Volt Current = _ — -. or Ampere = _, -. Resistance Ohm lo' Thus, —^ = lo* or .1 e. G. S. unit. Therefore, knowing the voltage of the source and the ohmage of the circuit, the current strength may be determined by dividing the former by the latter. Ill, — The resistance varies directly with the electro- motive force, and inversely with the current. This is a law of proportions precisely similar to the first rule given, and, like it, may be reduced to a more exact form, as follows : 44 A B C OF THE J&LEPHONE. IV. — The resistance (ohmage) is equal to the electro- motive force (voltage) divided by the current (amperage). This is equivalent to saying : The ohm is the quotient of the volt divided by the ampere, or : ^ . EMF ^, Volt Resistance = 7^ -^ -r or Ohm = Current Strength Ampere Thus, — 5 =10° C. G. S. units. ip» 10' v.— The electromotive force (EMF) varies directly with the current and with the resistance. This rule may be readily understood by reference to the first and third of the series, wherein it is stated that both the current and the resistance vary directly, or just as does the electromotive force. Con- sequently, the formula is readily understood : VI. — The electromotive force is equal to the current multiplied by the resistance. In tabulated form, this statement may be expressed as follows : EMF = Current x Resistance, or Volt = Ampere x Ohm, Thus, 10*' X io'= 10* C. G. S. units. Water Currents and Electrical Currents, — In many points the behavior of water currents is comparable to that of electrical currents. The three elements — pressure, resistance, efficient force per second — ^are also present. Thus, if we have a body of water confined in a convenient tank, or other receptacle, so that a constant stream may escape through an orifice one inch square, under the pressure of six inches — this is measured from the bottom of the tank to the surface of the water — ^we have the equivalent of what is known as the "min- er's inch," an exact analogy to the ampere. Taking the pressure of the column of water in the tank, which increases directly with its height and the dimensions of the orifice through which it escapes, we find that the miner's inch is, in the first place, a measure of rate or velocity, a time measure, giving inch-seconds. ELECTRICAL QUANTITIES. 45 Thus the water flows at the rate of so many miner's inches, just as electricity flows at the rate of so many amperes, both in direct propor- tion to the initial pressure. In the water current the size of the orifice of escape measures the resistance ; in the electrical current it is the size, or diameter, of the wire, as we have seen, provided it be com- pared with another wire of the same metal. Thus it is that in neither case do we have a reference to time. An " ampere per second " is a simple repetition, and means no more than an "ampere." Thus, in speaking of a current of, say, ten amperes, we do not refer to the cur- rent passing in ten seconds, but to that passing in one second. The Coulomb, the Unit of Electrical Quantity. — This brings us to another unit of electrical measurement, the measure of electrical quantity. This is the "coulomb," and it is equivalent to the efficient quantity of electrical energy passed by a current of one ampere intensity in one second. It is, in fact, the " ampere-second." Experimentally it has been found to equal the decomposition of .0936 milligrams of water, by electrolysis. Expressed in C. G. S. units its equivalent is 10"', and its numerical value is the same as that of the amperage. Thus, it is correct to speak of so many coulombs per hour, although in speaking of the discharge of a battery as 10 ampere-hours, we mean simply that it can give forth a current of one ampere for 10 hours, or its equivalent. This may mean 10 amperes for one hour, in which case the number of coulombs is 10 x 60 x 60 = 36,000. Ohm's Law Applied. — As may be readily understood by care- ful study, the foregoing rules are merely so many variations of one statement of proportions between the three factors of any active elec- trical circuit. They are, in fact, corollaries of the first rule, which is a sufficiently complete statement of Ohm's law for general purposes. A few illustrations will suffice to make the applications of the law perfectly clear. If, for example, we have two active electrical circuits with precisely the same current strength, we know that one of three things must be true ; either, I. They are the same in all respects, as regards voltage, ohmage and amperage ; or, 4^ A B C OF THE TELEPHONE. 2. The original voltage is greater, involving that the resistance, both internal and external, must be greater also ; or, 3. The original voltage is smaller, involving that the resistance, internal and external, must be smaller also. If, in any circuit, the original pressure is 8 volts and the resistance 4 ohms, it follows that the current strength is z amperes. Now taking these figures as the basis of equations, instead of the unit-names, as above, we have : 8 8 (current) 2 = -; (ohmage) 4 = -; (voltage) 8 = 4x2. 4 2 In another circuit, however, we may have an initial pressure of 16 volts and a resistance of 8 ohms, which will also give a current of 2 amperes. In each case we would have 120 coulombs per minute and 7, zoo per hour. The Watt: the Unit of Power. — ^The mechanical effective power of an electrical circuit is estimated in terms of neither amperes nor coulombs, since these quantities give no suggestion of the pressure at which the electrical energy is delivered. The case is very much the same as in the steam-engine. Thus one engine of large dimensions may run at low initial pressure — say, 60 or 80 pounds — while a smaller engine may run at a pressure of 150 or 200 pounds per square inch. If there is considerable disparity in size between the two, the larger engine will, of course, give the greater horse-power, even though the pressure is smaller. With two engines of smaller disparity in size a large piston surface and a low pressure may be balanced by a small piston surface and a high pressure, as is done in compound engines. Thus, we have two elements in the steam-engine corresponding to the voltage and available curr«it energy in an electrical circuit — the initial pressure in pounds per square inch and the piston area in square inches. The total working pressure before the cut-off is, of course, indicated by the product of the pressure and piston area, indicating the amount of steam actually used. In practical operation, the mean effective pressure, which is the average of pressures at a number of points before and after cut-off, is the figure to be multiplied by the area in finding the total effective pressure. The figure expressing the piston ELECTRICAL QUANTITIES. 47 area is thus virtually the reciprocal of the figures indicating the ele- ments tending to confine or restrict the action of the steam pressure. In precisely similar manner we discover the unit of power in an electrical circuit, taking the product of the voltage and amperage. This unit is called the "watt," and represents the equivalent of one ampere of current urged by one volt EMF, or the equivalent of lo' C. G. S. units. Thus : (voltage) lo' X (current) lo"* = (watt) lo'; Applying the principles of Ohm's law, given above, we find that it is, further, the product of resistance multiplied by the square of the current, and the quotient of the square of the voltage divided by the resistance. If, then, we have a circuit with a voltage of 8, a resistance of 4, and a current of 2, we may compute the number of watts as follows : (voltage) 8 x (current) 2 = (watts) i6. (resistance) 4 x (square of current) 4 = (watts) 16. (square of the voltage) 64 -j- (resistance) 4 = (watts) 16. The Equivalent of the Watt. — The wattage of a given active circuit is equivalent to the product of the voltage and the amperage, because, as is evident on reflection, the efficiency of any current for the accomplishment of work depends upon the pressure that impels it. For example, the energy furnished per second by a current of 10 am- peres supplied at a pressure of 2,000 volts is exactly the same as that furnished per second by a current of 400 amperes supplied at a pressure of 50 volts. In each case the product is 20,000 watts. Watts and Horse-Power. — ^Approximately 746 watts is equal to one electrical horse-power. A watt-hour is the expression for a power of one watt exerted by a current during the space of one hour. The watt-minute represents the same power exerted during one min- ute. The watt-second is equivalent to the product of the volts and the coulombs of a given circuit, and is also known as the volt-coulomb. The Joule, the Unit of Wori<. — In estimating the work done by the electric current, the watt-second is taken as the unit under the name of the "joule." This quantity, as defined by the International 48 A B C OF THE TELEPHONE. Congress of Electricians in 1893, "is represented sufficiently well for practical use by the energy expended in one second by an international ampere in an international ohm." The joule is, accordingly, equiva- lent to 10' C. G. S. units, and is found by multiplying the expression for watts (EMF x current) by the time expressed in seconds. Thus, a current yielding work at the rate of one joule in each second would accomplish a total effect in one hour equal to the number of coulombs, which is to say i x 60 x 60 = 3,600 joules, or one watt-hour. Resistance : Specific and Comparative. — From what has already been said, it is evident that the resistance of the conductor in an electric circuit is nearly the most important element in calculating a line for the highest efficiency in any kind of service. The current will always move along the " line of least resistance," as the term is ; that is to say, it always avoids the path that is most filled with obsta- cles to its flow, and chooses the one that is most easy. From the observations on this fact, scientists have found that all substances are divided into two classes : conductors, along which the electrical cur- rent flows with ease, transmitting its effects from one end of the line to the other ; and non-conductors, or insulators, in which it meets with difficulties, or is not able to flow at all. - The following substances are good conductors, the best being first named and the less good following in order : Substance. ^^l^trl'c'^r'Z''- ^^^^^^^nT'"" ^^'^^ At Zero, Centigrade. Cubic Cm. gravity. Pure silver 100.00 1.521 10.530 Refined copper 99-90 1. 603 8.820 Pure gold 86.65 2.077 19.32° 99. 5^ aluminum 63.09 2.889 2.660 Zinc 29.07 7.150 Cadmium 23.70 8.600 Brass 22.00 8.568 Swedish iron 16.00 15.000 7.774 Tin 15.45 7.290 Platinum 10.60 8.982 21.500 Lead 8.88 19.630 li.37o Nickel 7.89 g.800 German silver 7.70 20.760 Antimony 3.88 6.710' Mercury 1. 60 99.74 13^590 Bismuth 1.20 9.800 Graphite .... 0.07 2,0001042,000................ ELECTRICAL QUANTITIES. 49 The conductivity of all these substances having been computed at a common temperature, we have as correct an idea of their relative qualities as is possible, on a common standard. It has been found, however, that a number of conducting substances, including water and most acids, increase in power to conduct electricity with aft increase in temperature. In addition to the above list there are a few substances, which, while they are capable of conducting electrical currents, vary in the capacity according to conditions. They are, briefly : Animal bodies, cotton, dry wood, marble, paper. The substances that will not conduct the electrical current, and will interfere with its transmission are : Oils, porcelain, wool, silk, resin, gutta-percha, shellac, ebonite, paraffine, glass. They hold the same relation to the electrical current that any solid body does to the flow of water — they check or dam it. On account of this property they are extensively used in all the branches of electrical industry where it is desirable to confine the current to definite limits,- as to a wire, or to prevent it from spreading beyond the particular apparatus it is desired to energize. They are thus called "insulators" (Latin, insula an island; insulatus, made into an island). For purposes of thus confining, or insulating, an electri- cal current, telegraph and telephone wires are attached to poles by the familiar glass caps, and the wires used about electrical machinery, or where likely to come in contact with wood or other combustible materials, are covered with silk-windings or india-rubber. The insulation of the electric current is, of course, not absolute : rather is it correct to say that the resistance of insulators is so im- mense that the leakage through them is practically negligible. Thus, while certain poor conductors offer a high resistance to current — the human body opposing about 300,000 ohms at the closure of the cir- cuit, falling to about 3,000 after the passage of current has raised the temperature sufficiently — the resistance of true insulators is vast in comparison. Thus, glass, at 20° Centigrade, offers a resistance of 9r,- ooo,ooo,ooo,ooo ohms, or 91,000,000 megohms (millions of ohms) per cubic centimeter; the gutta-percha insulation used on submarine '5© A B C OF THE TELEPHONE cables offers a resistance of 450,000,000,000,000 ohms, or 450,000,- 000 megohms per cubic centimeter ; hard rubber, or ebonite, 28,- 000,000,000 megohms, and paraffine, 34,000,000,000 megohms. The Resistance Unit Wire.— In calculating the size of wires for a given circuit the mil-foot is used as the standard unit, which is to say a wire of one circular mil cross-section and one foot in length ; a circular mil being the square of a diameter of i mil or i-i,oooth inch. Although, for all conducting wires, iron, copper and aluminum, the value in ohms of the mil-foot varies, according to t'he specific resistance of the metal, the rule already noted on the point of doubling the cross- section to halve the resistance or doubling the length to double the re- sistance, holds good universally. Thus the specific resistance of the commercial qualities of the various wires is. given, as follows : Resistance in ohms per mil-foot at zero Cen- Metal. tigrade. Copper 9.59 Aluminum i5-zo Iron • 58.00 Steel 82.00 German Silver 125.91 In telegraph and telephone line work the sizes of wire used are usually determined by established standards. However, for special cases, where it is desirable to calculate the proper wire for a given cir- cuit, the following simple formulae may be used to determine the unit resistance at usual temperatures : R = r (CT -f 1), in which R is the required resistance ; r, the given unit resistance ; C, the temperature coefficient per degree, and T, the desired tempera- ture in degrees. On the Centigrade scale the coefficient for copper is .00388 ; for iron, .00453 ; for aluminum, .00390 ; for German silver, .00044. Hence, to find the unit resistance of copper wire at 75° Fahrenheit, we reduce to Centigrade scale, finding the equivalent as •.23.8°, and proceed as follows to find the value of R : R = 9.59 (.00388 X 23.8 -)- i) = 10.475 ohms. ELECTRICAL QUANTITIES. 5 1 Determining the Size from the Unit. — Such a wire would have a resistance of 10,475 ohms per thousand feet, or of 55,308 ohms per mile, at 75° Fahrenheit. If, however, we have a circuit of 1,000 feet in length and must find the size of wire that will give a resistance of only 10 ohms, we divide the unit resistance per 1,000 feet by the required resistance to find the number of circular mils in the cross- section of the proper conductor. Thus : r -T- R = cm. or 10,475 -^ 10 = 1,047.5 cm. The nearest size is No. 20 B. & S., which has a cross-sectional area of 1,024 circular mils, and a resistance of 10.26 ohms per thousand feet at 75" Fahrenheit. Most of these proportions are given in the 00 Fig. 35. — Electrical circuits, i : Diagram illustrating the connection of three gal* vanic cells in series. The wire connecting the dissimilar poles of the several cells passes through each in turn, making a negative terminal at one end of the battery of cells, and a positive at the other. regular wiring tables, but it frequently happens that the proper resist- ance for a conductor must be found by calculation, in which case the unit resistance for the required metal wire must invariably be used as an element in figuring. Another formula frequently used for copper wire makes the cross- sectional area in circular mils equivalent to the product of the required current, by the average distance of transmission one way, by the con- stant 21, divided by the percentage of loss. Thus : I X D X 21 cm. = ^ , in which I is the current required in amperes ; D, the distance trav- 52 A B C OF THE TELEPHONE. ersed by one-half the circuit, and, L, the percentage of loss in voltage to be allowed. Circuits : Series and Parallel. — The total resistance on a circuit is a matter depending largely upon the manner in which it is arranged. Thus, if a number of apparatus are connected in series, the total resistance is the sum of the individual resistances, plus the resist- ance of the conductor. If, however, the same number of apparatus be connected in parallel, the total resistance is as their conductances, or as the reciprocals of their individual resistances. Thus, in arrang- ing circuits of incandescent electric lamps in parallel, the rule is that the total lamp resistance is found by dividing the resistance of one lamp by the number of lamps. An ordinary i6-candle-power incan- FiG. 36.— Electrical circuits, 2 : Diagram illustrating; three cells joined in mutiplk or PARALLEL. The circuit is made by two wires, by which the like poles of all the cells are connected throughout. descent lamp for a iio-volt circuit has a resistance of about 244 ohms. If, therefore, on a given circuit 20 such lamps are arranged in parallel, we find that 244 R = 12.2 ohms. A similar economy of power and higher degree of efficiency is secured by arranging motors, etc., in parallel, although generators — chemical cells and dynamos — are arranged in series, in order to obtain the cumulative effect of their individual EMF's. With motors arranged in series a CEMF, in addition to resistance, acts to cut down the effec- tive pressure. ELECTRICAL QUANTITIES. 53 The Farad, the Unit of Electrical Capacity.— The theo- retical unit of electrical capacity, as applied to condensers and con- ductors, is the farad, which represents the capacity to retain one cou- lomb of electrical energy at a potential of one volt. The quantity of electricity charged upon a conducting surface, however, raises its potential, so that a condenser of one farad capacity could hold two cou- lombs at two volts, or three coulombs at three volts. According to the established proportions among electrical quan- tities, the capacity of a circuit is equivalent to the quotient between the coulombs and the EMF. Thus : ^ . Current x time , , Capacity = 5 = farads, voltage 10"* XI .1 » /-i /-. c. -.^ or , — = — — = .000,000,001 or 10 " C. G. S. unit. 10 100,000,000 In practical computations the microfarad, or one-millionth of a farad, is used. Since, therefore, one farad is equivalent to io"° C. G. S. unit the microfarad is to be represented as lo"'*, which represents the real standard for all computations. The average limit of condenser capacity, as used in practical work, is one-third microfarad. The faradical capacity of the earth is estimated as ^^ of a farad, or 636 microfarads. In practical work the capacity of a current-carrying wire, in terms of the charge actually received, depends upon the arrangements of the circuit — whether it is a lead-sheathed cable, a ground return line or a full metallic — and, upon the distance between the conductors. As given by several authorities, the capacity of conductors may be determined by the following formulae : For pole-suspended bare aerial wires, the capacity of each per mile may be found thus : .01942 Microfarads per mile = — - log — Here, D represents the distance apart from center to center of the wires, and r, the radius of each wire, both in inch terms. In order 54 A B C OF THE TELEPHONE. to find'the capacity for fractions of a mile the numerator .01942 is divided by the figure representing the proportionate size of that frac- , .01042 . tion to the mile. Thus, for 1,000 feet, we have - „. ; for one .01042 foot, — ^, etc. 5280 In order to find the capacity per mile" of copper wires of size No. 8, B. & S., which has a diameter of 128 mils, or a radius of 64 mils (.064 inch), hung twelve inches apart on a cross-arm, the following operation is necessary : .01942 -01942 Microfarads = ^=105187.5 In calculating the capacity per mile of single pole-suspended wires with earth return, the following formula holds good for mile lengths : .03883 Microfarads = 1 4H Here, H is the height of the conductor above the ground in inches, and, d, its diameter expressed in thousandths of an inch. Figures for Capacity in Pole Lines. — According to practi- cal data furnished by several writers on telephone subjects, the follow- ing figures hold good for the capacity per mile of wires used in such work: Size of Wire B. &S.. 6 7 8 9 10 12 14 16 Capacity to earth ; mi- •01537 .01510 .0150 .0148 .0144 .0142 .0141 Capacity between wires ; microfarad . . .00936 .00918 .oogio .0090 .0088 .0086 .0084 .0083 In calculating the total capacity for a metallic telephonic circuit the factor here given is multiplied by one-half the total length of the ELECTRICAL QUANTITIES. 55 circuit and by, the total resistance. Thus, with a line of No. 14 B. & S. wire 100 miles between terminals, we have : K = 13.3 X 200 X .0142 X 100 = 3777.2 mf. Such figures may give approximate results, but cannot be de- pended on when exact calculations are required. The Henry, the Unit of Inductance. — Another quantity important in alternating current circuits, is inductance, indicated by the coefficient of self-induction, whose theoretical unit is the henry. This unit is defined by the International Electrical Congress as meas- uring the "induction in the circuit, when the EMF induced in the circuit is one international volt, while the inducing current varies at the rate of one international ampere per second." It is, therefore, the " coefficient by which the time rate of change of the current in the circuit must be multiplied, in order to give the EMF of self-induction in the circuit. ' ' In terms of C. G. S. units, the henry is equivalent to 10' abso- lute, and the accepted formula for determining the quantity is as follows : Henrys = ■ amperes. However, as in the case of the farad, the theoretical unit is too grejt for practical computations, which involves that the millihenry, or i-i, 000th henry, is the accepted unit. In pole-suspended lines the inductance varies as the metallic resistance, the distance between the wires on the cross-arm and the number of cycles per second, as indi- cated by accepted tables. Thus, for one mile of No. 8 B. & S. cop- per wire, with a resistance of 3.406 ohms, the coefficient of self- induction with 6 inches between centres is .00153, ^'^^t ^ith 12 inches, .00175. As a general rule, the elements of alternating current circuits may be represented by a right-angled triangle, whose base represents the metallic resistance \ whose right side is the reactance or inductive resistance, and whose hypothenuse is the impedance of the line. The same figure may be taken to represent the proportions of the EMF's in the circuit; the impressed EMF being represented by the hypoth- 56 A B C OF THE TELEPHONE. enuse of the triangle, the induced EMF, by the right side, and the effective EMF, by the base. Thus, this figure graphically represents the conditions actually existing in an alternating circuit ; since, be- cause the CEMF is highest when the impressed EMF is increasing or decreasing, it differs in phase from it by 90°- This condition, pro- ducing an impedance to the current, involves that the power-equiva- lent of an alternating-current circuit in watts is always less than the product of the volts and amperes, as set forth by Ohm's Law, since the EMF and the current are never in phase. Neutralizing Self-induction by Capacity. — High self- induction in a circuit tends to reduce the possible distance of trans- mission as quickly as a high capacity. The two may be arranged, however, to neutralize one another. Thus, if in a given circuit a con- denser be bridged between the leads, it will be charged as the EMF rises and discharged as it falls, or whenever the inductive EMF is opposing the line EMF. Practical Methods of Neutralizing Capacity. — The elec- trostatic capacity of a line, which proves a source of annoyance in telephone work, by enabling the reproduction of noises from tele- graphic and power lines, may also be neutralized by self-induction. Thus, in noisy sections of ground-return lines, repeating coils are used in transferring the line to a full metallic; and in metallic lines, simi- .larly situated, the two wires are transposed regularly, as will be explained later. Could the two wires of a line be insulated and twisted together, as in cables, the disturbances due to electrostatic capacity would be entirely neutralized. In one case reported by seve- ral writers, where a grounded telephone line was disturbed by a neighboring telegraphic circuit, the trouble was allayed by winding the series ringer magnet coils to a resistance equal to that of the tele- graphic relay coils. Effects of High Capacity, — The troubles due to electrostatic induction are more numerous and prominent than those resulting from current and self-induction in both telephony and telegraphy. The ELECTRICAL QUANTITIES. 57 most important ill effect of general inductive disturbances is the flat- tening-out or displacement of the transmitted waves, so that the mes- sages sent over a line of more than a very definite length are liable to be rendered largely unintelligible at the receiving station. In this respect, the alternating telephonic current suffers about the same deadening influence as is found in the telegraphic messages transmitted through great lengths of ocean cable. In the latter, the well-marked positive and negative impulses — above and below the neutral line, as traced by the siphon recorder — are so distorted as to be nearly beyond decipherment, except by expert operators. While, with the use of delicate galvanometer receivers, transoceanic messages may be deci- phered, in spite of the wave displacements due to the immense static \^.JJ^X,J^^ Fig. 37. — Diagram of electrical telegraphic wave impulses as traced by the siphon re- corder. The upper row represents the impulse waves as they should appear on a short line ; the lower, the message as received after passing through a long cable. capacity of the cable, it is obviously impossible that telephonic mes- sages could be received at all under similar conditions. It has been demonstrated that the EMF due to the self-induction of a circuit lags 90° behind the active EMF, while that due to the capacity is 90° in advance. Experiments looking toward a practical method of making the one neutralize the other have been largely unsuc- cessful, until Professor Pupin perfected his cable. The situation as ap- plied to long-distance telephony is thus expressed by Kempster Miller : "Unfortunately for long-distance telephony such a balancing of self-induction against capacity can be obtained only for one frequency at a time. To thus iune a circuit for one particular frequency would render that circuit capable of transmitting efficiently one particular fre- quency of vibration, while the requirements of telephony are that all frequencies within the range of the human voice shall be transmitted with equal facility. Again, and unfortunately, it has been found im- 58 A B C OF THE TELEPHONE. possible to neutralize distributed capacity with anything but distritiited self-induction, and this has not yet been accomplished in practice." Choking Coils in the Circuit. — After an elaborate series of experiments Professor Pupin has demonstrated that the static capacity of a conductor may be offset by distributing "sources of inductance," in the form of " choking coils " at intervals of every eighth of a mile. By the use of these coils the circuit may be completely "tuned," and all ' ' blurring ' ' of the speaking current by ' ' distortion ' ' and "attenuation" effectually neutralized. His coils are of the ordinary pattern used with electrical circuits, consisting briefly of a coiled wire of high self-induction, containing a laminated core. Both wires of a circuit are passed through such coils, the best arrangetaent being, probably, to superpose the windings, as in ordinary transformers. A well-known electrical authority, in an elaborate article on the Pupin and other inductance cables, explains the neutralizing of distor- tion, involving the reproduction of " currents of all frequencies at the distant end of the line in proper proportion, attenuated, as nearly as may be, to the same degree," and says: "We know, however, that in general the attenuation increases with the frequency. . . . This is prevented by making the inductance high. In the extreme case in which the inductance per unit length of line is large in comparison with the resistance per unit length, the velocity of propagation and the attenuation become independent of the frequency. That is, all fre- quencies are transmitted with the same velocity of propagation and attenuated to the same degree." The general situation may then be summarized by saying that a " properly constructed long-distance tel- ephone line must have a high inductance," and that, in order to reduce attenuation and distortion at the same time, the sources of inductance must be distributed at equal intervals on the line. "Phe practical results obtained in such a circuit demonstrate that " the loss in the line — and, therefore, the ' attenuation ' — is just as effectively diminished by in- creasing the inductance as by diminishing the resistance." CHAPTER FIVE. HISTORY OF THE SPEAKING TELEPHONE. When we are told that the steam engine was invented by James Watt, or the steamboat by Robert Fulton, or the steam locomotive by George Stephenson, or the telegraph by S. F. B. Morse, or the telephone by Alexander Graham Bell, we are very apt to suppose that each of them originated both the idea and the contrivances embodied in their several inventions, and that no one ever thought of any such things before. As a matter of fact, however, these and all the other great inven- tions of the Nineteenth Century are merely the perfected product of many minds working during many years, although the man who finally completed the labor gets all the credit. Almost from the time when electricity began to be of prac- tical worth in the world there were some people who were at work on the idea of perfecting contrivances for the electrical transmission of speech and other sounds. And in this depart- ment of endeavor, as in others, we find that the final and perfect instrument is the simplest. Reis's Telephone. — One of the earliest experimenters to realize successful results was Philip Reis, Instructoi- in Natural Sciences at Prof. Garnier's Institute, a select school for boys, at Friedrichsdorf, near Homburg, Germany. His principle has frequently been described as "making and breaking" an elec- tric circuit, in somewhat the same fashion as the electric telegraph transmits messages ; but Prof. Sylvanus P. Thompson, in a paper on Reis's invention, read before the Naturalists' Society of Bristol, England, in 1883, declares that it was rather the "1 employment of a loose or imperfect contact between two parts of a conducting circuit," so that the pressure and resist- ance might be varied by differing stress. 59 6o A B C OF THE TELEPHONE. On the occasion of reading this paper he exhibited a model of Reis's original transmitter, which was shaped like the human ear, and was designed to work on the same lines. Fig. 38 shows a section of this instrument. Here we have an opening, A^ corresponding to the meatus of the ear, and closed at the "inner" end with a membrane, B, corresponding to the tym- panum, or drum. Immediately against this membrane rested Fig. 38. — Reis's first transmitter. Fici. 39. — Detail of lever, Reis's transmitter. the lower end of a curved lever of platinum wire, C, which represented the "hammer bone." It was attached to the membrane with a drop of sealing wax, and was left free to move, as the membrane vibrated, by being soldered to a short cross-wire, serving as an axis, as shown in Fig. 39. The upper end of the lever, D, rested against the end of a vertical spring, E, which bore on its extremity a slender and resilient strip of platinum foil, G. The contact of the spring could be varied, as occasion demanded, by the screw, H. The electrical connec- tions were made by the wires, F and K, the former bearing a current to the lower end of the lever, through its connection at the center of the diaphragm, and the latter transferring to HISTORY OF THE TELEPHONE. gl the line the sound-impulses caused by varying the pressure of the lever on the strip of platinum, G, as it moved with greater or less force in obedience to the vibrations of the diaphragm, according to the motions of sound waves. A later form of the Reis transmitter was a box having two openings, one at the side for the mouthpiece, and the other at the top, closed by a diaphragm made from the smaller intestine of a pig. At the center of this memb-rane was cemented a strip of platinum in loose contact — some say not touching — with the point of a platinum wire held in position above it by a leaf spring, as in the former instrument. Reis's Receiver. — The receiver of the Rei? system was a steel knitting-needle wrapped around with a coil of insulated wire. It was, in fact, a form of electro-magnet, and was mounted on a violin or resonant box, which served as a sound- ing-board, and to this was later added a cover against which the ear was pressed to receive the sounds transferred along the wire from the transmitter. By the Reis system musical sounds could be transmitted with all the variations of pitch and loud- ness, although without timbre — probably resembling somewhat the sound of a xylophone, or wooden piano — and less perfectly, also, the sounds of the human voice; the consonants being readily represented, but the vowels less distinctly, if at all. Prof. Reis anticipated many of the later features of the practical telephone, particularly the transmission of sound by the variation of pressure in an electrical circuit, and also made his receiver on the plan at first adopted by Prof. Bell — an electro-magnet and a resonant diaphragm. His instruments attracted wide attention in that day, but no one thought of applying them to any field of commercial usefulness, most probably because the transmission of speech was too imperfect, even if the social and business conditions had created a demanci for such a contrivance. During the next sixteen years little was heard of the subject ;f transmitting speech by electricity, and when, in 1876, Prof. Bell exhibited his imperfect instru- 62 A B C OF THE TELEPHONE. ments at the Centennial Exposition in Philadelphia, tTiere was another sensation among scientists. House's Telephone. — Meantime, however, the musical tele- phone was much improved, and several independent experi-- menters actually made some progress toward a practical speak- ing apparatus. Among the latter was Royal E. House, of Binghamton, N. Y., who in 1868 invented and patented a device which he called the "electro-phonetic telegraph." Its general construction included a sounding-chamber, open at the front end and closed at the rear by a diaphragm of varnished ' ^////////m////yM/////////m///w/////m^^^ '/^//m>m^??//^/?/y/^//??/7/)/ m7z^ Fig. 40.— Diagram of House's " Electro-phonetic Telegraph." D is the Pine Wood Diaphragm ; E, The Electro-magnet ; A, The Armature. pine wood, to the center of which was attached a bar, with the object of imparting the vibrations of the diaphragm to the upright piece attached to the horizontal armature of an electro- magnet. Two instruments of this pattern at either extremity of a circuit, which includes a battery cell, can act alternately as transmitter and receiver, and actually convey spoken and musi- cal sounds. Mr. House, however, seems to have had no conception of the value of his invention ; so he rested content with local celebrity, leaving the fame and rewards coming from the successful telephone to be gathered by another man eight years later. HISTORY OF THE TELEPHONE. 63 ». Gray's Telephone. — Prof. Elisha Gra^ was also conduct- ing experiments in telephony, and, strangely enough, entered an application for a transmitter very like Bell's in general theory on the very day a patent was issued to the latter. Drawbaugh's Claims. — In. 1884, in a suit for infringe- ment of patent by the Bell Telephone Co. against a concern known as the People's Telephone Co., brought to light the claims of one Daniel Drawbaugh, a machinist, of Eberly's Mills, Penn., to having invented a complete and siiccessful telephone apparatus at least ten years before the data of Bell's patent. Drawbaugh sought to establish his claims by the testimony of 145 witnesses, who swore that they had seen the machines in operation in his shop or heard them spoken of by others between the years 1867 and 1876. He also exhibited a series of models, the earliest of which he claimed was made in 1866, and the latest about ten years after. His transmitters were constructed on the principle of the carbon granule instru- ments subsequently adopted by Runnings and others, and his receivers were of the magnet type, now in use on all lines. His claims, if true, certainly present a strange case of almost perfect coincidence. But it was this fact, as much as any other, that moved the decision of the United States courts against him. In one of his experimental model receivers, dated about 1866, the diaphragm is connected to the armature of an electro- magnet by a taut string, a device adopted by Preece as late as 1879. A still later one (1867-68) has the armature of the magnet on the diaphragm, as is the case with Bell's instrument, made early in 1876. About 1870 he claimed to have devised a double-pole permanent magnet telephone, identical with that later patented by Bell, and to have further improved it by the use of spiral magnets, with a single-pole contact, as in some of the more modern forms of receiver. His case certainly deserves a passing mention. Bell's Early Instruments. — Prof. Bell seems to have made a large number of experiments with almost as many different 64 A B C OF THE TELEPHONE. combinations before he finally hit upon the wonderfully simple instrument patented in 1876. One of the earliest of his experimental instruments is shown in Fig. 41. It consists of an elongated type oi permanent horseshoe magnet, on each pole of which is attached a harp of rods, like those used in the familiar mechanical music boxes, each rod in the comb-like contrivance being cut to a length suitable to giving forth its own note. Now, when one sang near this instrument, he effected a vibra- tion in the key corresponding to the note sounded by his voice, thus producing a "wave" of a size, or " amplitude, " propor- tioned to that uttered. This would produce an agitation in the Fig. 41.— Bell's First Experimental Instrument : An Elongated Permanent V-shaped Magnet, Bearing a Harp of Reeds, which Vibrate in the Field of an Electro-magnet. field of the electro-magnet, in which a magnetically induced current was at once set up. The current, or series of currents, thus induced, traveled on the line wire to the coils of the other electro-magnet, which, in turn, attracted the corresponding key of the harp, with the same strength and shape of wave as that which left the "transmitter," thus reproducing the sound by reversing the process. The theory of operation has been briefly expressed as follows: " The strength of the induced currents is determined by the amplitude of the disturbing vibration, and the amplitude of the vibration at the receiving end depends upon the strength of the induced currents." It would seem, from the description of this instrument, that it presented the same difficulty as Reis's — imperfect transmission of the vowel HISTORY OF THE TELEPHONE. 65 sounds. It was also impractical from the vast expense of manu- facture. It is of interest, however, as representing the earliest form of the theory adopted by Bell. In his next experimental instrument Prof. Bell discarded the permanent magnet and induced currents, and adopted as his transmitter the type of instrument shown in Fig. 42 — an electro-magnet mounted on a post by an adjusting screw, so that it may be approached to the vibrating diaphragm. This diaphragm is made of goldbeater's skin, and bears at its center nearest the magnet a small piece of soft iron to act as an arma- ture. This diaphragm, when vibrated by a sound, produced a variation of the current in the coils of the magnet by disturbing Fig. 42.— Bell's Centennial (1876) Transmitter. the field of force. This process enabled the current to transmit variations in the strength and amplitude of its vibratious to the receiver which could retransform them into waves of sound. The receiver was another electro-magnet, in this instance con- sisting of a wire coiled about an iron tube. About this was an iron sheath, which was closed at its upper end by a thin plate of sheet iron lightly laid upon it, so that when no current was passing it would be just above the pole of the magnet. The vibrating current was able so to act on this diaphragm that the sounds varying the current in the receiver would be produced, by similar currents, alternately attracting and releasing it. Such were the instruments exhibited at the Centennial Exposi- tion in 1876. Bell's First Permanent Magnet Receiver. — Later in the same vear Bell discaiied the electro-mp.gnet and battery, and 66 A B C OF THE TELEPHONE. returned to his original idea of a permanent magnet and induced currents resulting from disturbances in its field by the vibration of a metal diaphragm. What such disturbances of the magnetic field of force are capable of accomplishing we have already seen. Bell's magnet telephone is, in fact, a min- iature dynamo, and the sound-vibrations of its diaphragm transmitted along a line wire by the current they generate affect the receiver, which is precisely similar in construction, as a commercial current affects an electric motor. The receiver is thus, for a time, a motor, and when itself employed as a trans- mitter it becomes a dynamo. Fig. 43. — Bell single pole magnet telephone enclosed in wooden case. Bell's final experimental instrument consisted of a powerful permanent horseshoe magnet, at each pole of which was attached a bobbin carrying a coil of fine insulated wire. Directly in front of these coils is the diaphragm — a thin iron disc supported in a wooden frame in such a manner that it can vibrate freely in obedience to the sounds uttered against its face through the mouthpiece. The relative distance of the diaphragm and magnet may be varied by clamps. He also used a bar magnet, and opposed but one pole to the diaphragm. In this instrument the final form of the Bell telephone was reached. It only remained to make a suitable case for it, as shown in Fig. 43, and this need was supplied by the ingenuity of Prof. Pierce, of Brown University, Providence. R. I. In this figure, M is the bar magnet, B the wire-wound HISTORY OF THE TELEPHONE. 67 bobbin, L and L the line wires carrying the speech-bearing current through the binding-posts, C and C; D is the iron diaphragm, and E the mouthpiece of the case. The present type of magnet telephone differs in no essential particular from this instrument, except in the fact that the mouthpiece is now screwed to the head of the case by engaging a thread on the periphery, as will be noted in some of the figures of modern instruments given in the next chapter. Superiority of the Permanent Magnet. — The permanent magnet is superior to the electro-magnet in a telephone instru ment; first, because it dispenses with the battery cell in the circuit, and second, because in the use of an electro-magnet the magnetic force and the stress due to the periodic vibrations • — contacts and withdrawals — of the diaphragm vary in propor- tion, so as to compel the diaphragm to execute double vibrations in the production of any one sound. - The double vibrations, Fig. 44. — Usual form of hard rubber receiver case. while not observable, would have the distinct effect of rendering the reproduction of speech in the receiver less distinct. This was the case in the Bell instruments based on electro-magnets. Simplicity of the Magnet Telephone. ^In considering the extreme simplicity of the Bell magnet telephone one cannot but be surprised that itw^as not thought of sooner, particularly when we learn that the principle on which it works was recognized years before Bell ever dreamed of the electrical transmission of speech. Thus, as early as 1837 a Dr. Page, of Salem, Mass.., first demonstrated the principle, that if a bar of iron be wound with wire, through which a current is passed, a distinct series 68 ABC OF THE TELEPHONE. of sounds will be emitted by it every time it is magnetized or demagnetized. This is the basis of all receivers, from Reis to Bell. Page and his contemporaries attributed the sounds thus made to * ' molecular disturbances, whereby the bar was lengthened. " Somewhat later a Mr. Farrar, residing in New Hampshire, devised a receiving instrument, consisting of an electro-magnet whose armature vibrated in the center of an elastic disc or diaphragm, and thus received musical sounds — ^he never thought of speech — from a distant transmitter, constructed on the plan of Bell's original experimental instrument, the several reeds or rods being set in vibration by a keyboard, at the same time opening or closing a circuit. Prof. Dolbear, of Tufts College, Mass., has described this instrument in a lecture in 1882, and compared the process of opening and closing the circuits to Helmholtz's tuning-forks, later devised to accomplish the same result by their vibrations, or to some of the harmonic devices now employed in quadruples telegraphy — the method of send- ing four messages over the same wire at the same time. The principle of harmonic vibrations, whereby, as we have previously seen, one string or bell of a certain note may call forth vibra- tions from another of the same note by merely sounding in its vicinity, is the one on which the modern system of ' ' wireless telegraphy " is based. Prof. Bourseul's Prophecy. — In 1854 Charles Bourseul, a French scientist, published a paper on the possibility of trans- mitting speech by electricity, in which he says : "Suppose a man speaks near a movable disc, sufficiently pliable to lose none of the vibrations of the voice, and that this disc alternately makes and breaks a current from a battery; you may have at a distance another disc, which will at the same time execute the same variations. * * * * It is certain that in a more or less distant future speech will be transmitted by electricity. I have made experiments in this direction ; they are delicate, and demand time and patience, but the approxima- HISTORY OF THE TELEPHONE. 69 tions obtained promise a favorable result." This favorable result v/as not achieved by Bourseul, however, but was delayed for twenty-two years, until Alexander Graham Bell perfected the telephone with the simplest possible electrical contrivance which could embody the principles laid down by the Frenchman. Sensitiveness of the Magnet Telephone. — Not only is the Bell instrument a model of simplicity, but its degree of delicacy is simply amazing! This fact has been shown by a number of experiments described by Preece, the noted English telephonist. Siemens' Experiment. — The first mentioned was by Dr. Werner Siemens, of Berlin. He took a Bell telephone, the pole of whose magnet was wound by 800 turns, or convolutions, of insulated copper wire, one millimeter (.03937 inch) in diameter, and of no ohms resistance, and placed it in the circuit of one Daniell cell (1.08 volt). By means of a mechanical commutator, placed in the circuit, the current from the cell was reversed 200 times per second, thus giving a series of impulses which pro- duced a loud crackling noise in the telephone instrument. By use of Ohm's Law, previously explained, we find that a current of 1. 08 volt passing through a resistance of no ohms is equal to .00981 amperes, or, approximately, .01 (tw) ampere. In other words, it is such a current as would deposit about .0000033 grams (about thirty-three-ten-millionths of a cubic centimeter, or about two-ten-millionths of a cubic inch) of pure copper in one second, by the process of electro-plating. Such a current has an intensity of almost unimaginable minuteness, but Dr. Siemens further modified it by inserting a resistance of 50,000,000 ohms, and placing the primary winding of an induction coil in the battery circuit, the secondary wind- ing being attached to the telephone. This added resistance must certainly have reduced the current strength to a point about equal to .000,000,2 (two-ten-millionths) ampere in intensity. But the induced current was capable of maintaining a loud noise in the instrument, which still continued audible even 70 A B C OF. THE TELEPHONE. when the secondary coil was pushed out clear to the end of the primary, thus reducing the inductive action of the primary coil to the minimum, and placing it beyond the reach of measure- ment. Preece's Experiment. — Mr. Preece himself determined by a somewhat similar line of experiments that the Bell instru- ment will reproduce a sound actuated by a current equal to .000,000,000,000,6 (or six-ten-trillionths) of an ampere in inten- sity, or such a current as could deposit about .000,000,000,000,03 (or three-one-hundred-trillionths) of a cubic inch of pure copper in each second of time. Similarly, by using an electrical con- denser of one microfarad capacity, which was discharged Fip. 4S.-:Induction coil, or inductorium, arranged with device for withdrawine the iecondary coil from the primary. This instrument is provided with a mechanical rheo- tome, or circuit breaker, which reverses the current from the battery, as mentioned in Siemens's experiment, ' through the circuit of a Bell telephone 160 times per second, a French electrician, Pellatt, calculated that, with a voltage of .0005, representing the difference in potential, or electrostatic capacity, between the two terminals, an audible sound could still be maintained in the receiver. Such a voltage of electro- motive force would be of sufficient strength, as may be found by calculation, to produce one gramme-degree of heat in ten thou- sand years. That is to say, it would take ten thousand years for such a force to raise one gram, or cubic centimeter, of HISTORY OF THE TELEPHONE. 7 1 distilled water from zero, Centigrade, to one degree above, provided that the water retained every atom of heat imparted to it in the meantime. Still another interesting experiment is recorded in Houston and Kennelly's work, "The Electric Telephone." These authors lay down the rule that the sensitiveness of the Bell instrument to feeble alternating currents has been estimated as best deter- mined with a current of 640 alternations per second. This number corresponds, approximately, to the frequency of vibra- tions of the E note in the second octave below "middle C" in the piano scale. At this frequency, an alternating current strength of .000,000,044 (or forty-four one-billionths) ampere has been found able to produce distinctly audible sounds. Whether this is considered the limit or not, it is calculated that the force required to maintain this current strength through the resistance of the usual telephone receiver, which is about 75 ohms, is so exceedingly small that the work done in raising a weight of thirteen ounces through a vertical distance of one foot, would suffice to keep an audible sound in the instrument for 240,000 years. Such figures prove, either that the Bell telephone is the "most beautiful illustration of . the equilibrium and unity of natural forces in existence," or else, that the standards of elec- trical measurements are on an entirely wrong basis. They are, however, of interest as illustrating the curious facts regarding the perfection of this simple contrivance, although of little account in the questions connected with practical telephony. 1.2 A B C OF THE TELEPHONE. Pig. 4fi.— Bi-polar receiver of the 'Westem Telephone Construction Co. The adjust iaent between the magnet coils and the diaphragm is made by the screw thread on die brass block below the coil bobbins. The method of attaching the cord has the advantage of positively preventing contacts of the wires, and short circuiting- CHAPTER SIX. LATER MODIFICATIONS OF THE MAGNET, TELEPHONE. In the experiments just recorded, for testing the marvelous sensitiveness of the magnet telephone, we have found that the best results were obtained with alternating currents — those that are made to flow first one way and then the other — and that the telephone instrument was used as an electro-magnetic motor, rather than as a dynamo. The difference, as we have seen, depends on whether the instrument is used as a trans- mitter or as a receiver, since the motor differs from the dynamo only in the fact that it is made to move by the currents which the latter produces. Agitation of the magnetic field of force by the steady shifting of the armature produces a current, which, in turn, may agitate another magnetic field and thus cause the armature to move in the same way. Conditions for Transmitting Speecli. — In order to im- prove the practical telephone to the highest point of efficiency two things are necessary, as we may readily understand : First, that the currents bearing articulate sounds should be, as far as possible, of the alternating type — such has been found most effective for distinct transmission ; and second, that the mag- netic field in the receiving instrument should be as strong as possible, with every point of its available space utilized. The first of these needs has been ably met in the numerous forms of carbon microphone transmitters, mounted with a voltaic circuit and induction coil. The second has been almost as well sup- plied by the numerous improvements in magnetic receivers, although in general practice the original pattern instrument, of either pne or two-pole contact, is still in common use. Magnet telephones are most often used only as receivers, on account of their extreme delicacy; and the carbon microphone has attained 73 74 A B C OF THE TELEPHONE. almost universal use as transmitter, on account of its great power in this capacity, and the various devices used with it to produce a strong alternating current of varying pressure. Methods of Strengthening Magnet Receivers. — We may say that there are three ways in which inventors have sought to raise the efficiency of the magnet receiver: first, by strength- ening the magnet ; second, by increasing the number of the pole contacts ; third, by increasing the mass of the armature. By far the greater number of new variations fall under the first head — strengthening the magnets. This result may be Fig. 47.— Single-pole compound bar telephone magnet. accomplished in two ways: either by using' a horseshoe magnet, both poles of which are in contact with the diaphragm ; or, by "compounding" the magnet, or building it up by using a number of magnetized steel bars, in single or double-pole con- tact. Fig. 47 shows a compound bar magnet of the variety now most often employed in common receivers instead of the single rod at first used, as shown in Fig. 43. Compounding the magnet is a method we may compare to harnessing a number of horses to a heavy wagon, and works with the same effect of combined strength. It is also preferable to employing single bar magnets of the same degree of power; both because it is cheaper, and also, because the several separate bars act on one another to prevent the magnetism from becoming exhausted as soon as it would be in any one of them used alone. Methods of Adjusting the Receiver. — Another problem which a number of inventors and manufacturers have sought to solve is one of construction : how to obviate the effects of the differing ratios of expansion and contraction, as the weather is MODIFICATIONS OF THE RECEIVER. n wami or cold, between the hard rubber of the shell and the steel of the magnet. Because this difficulty had not been met, the earlier forms of Bell instruments were often rendered useless, for the time .being, by sudden changes of temperature. The first inventors to attack this problem sought to regulate the distance Between the diaphragm and the pole pieces by an adjust- ing screw, passing through the bottom of the case so as to push the magnet forward or draw it back. In such instruments the magnet was secured at the end furthest from the diaphragm. In more modern types the adjustment is effected by securing both magnet and pole pieces to a brass plate or cup, which is part of a rigid system including the diaphragm. The Siemens Telephone Receiver. — Among the best-known types of magnet telephone is the Siemens instrument, which is widely used in Germany. In general the case, made of sheet-iron, is so constructed that it can stand upright on a table or a shelf, and this was the invariable rule with a variety in which was included also a small ringing apparatus, consisting of an armature made to rotate between the two legs of the magnet and turned by a small crank in the side of the case precisely like the ordinary magneto-generator, to be described in a later chapter. The rotation of this arma- ture induces a current in the magnet coils causing the attraction of the diaphragm, in both transmitting and receiving instruments. A small ball laid upon the hori- zontal diaphragm supplies the means for making an audible signal under such stress. The magnet, made of a rather broad and thin strip of steel, is secured to the tail of the case by a screw. The pole pieces consist of two very small oblong Fig 48. — Sectional dia- gram of the Neumayel receiver. 76 A B C OF THE TELEPHONE, coils brought very close together, and attached to the ends o\ the magnet by two thin steel plates carrying the soft iron cores. The instrument is a very powerful one, and, in practice, is frequently used as both transmitter and receiver. The Neumayer Receiver. — Another good instrument, also a German invention, is the Neumayer receiver, a section of which is shown in Fig. 48. It combines several features not found in other instruments. The magnet consists of five cylindrical rods, which touch at the "south," or negative poles, and an Fig. 49.— The Ericsson receiver. This instrument is manufactured in Sweden and i» used by many independent companies in the United States. It is notable for the strength of its magnet and the excellent device for securing adjustment. The case is a brasB tube, enlarged at one end to form the cup or box for enclosing the coils and the diaphragm, and upon this the htad cap is screwed in the usual manner. The adjustment is obtained by pushing the magnet to the proper position in the tube, and firmly securing it by the screws, which work in a slot on either side, as shown. The tube of the case is covered with a coating of hard rubber. The binding-posts are held apart by a wedge of rubber secured between them by the nickel-plated ring at the tail of the case. held in position around the core of the armature coil by the brass ring, F. The coil is wound on a wooden spool, and through its center runs a thin brass tube, within which is soldered the core, consisting of a number of fine iron wires, of the kind used by florists to bunch together the stems of flowers, By this device it is believed that a greater inductive action is obtained, on the principle applied in the construction of induc- tion coils, that this "lamination " of the core reduces the effects of the induced E M F due to the rapid variations of the magnetic flux passing through it, hence overcoming the danger of induced secondary currents and lowering the magnetic resistance. With the high frequency telephonic currents this is a consideration of vital importance. The core, moreover, is in constant contact with the magnet bars, and thus in the field of magnetic force. MODIFICATIONS OF THE RECEIVER. 77 The coil is contained in a brass box, X, on the top of which is screwed the cover, clamping the diaphragm in place. The wires are attached to the binding-screws at S and S, and the magnet is contained in a wooden sheath, which is secured to the brass head box. Fi&s. 50-51. — Bi-polar compound bar magnet receiver of the Keystone Electric Tele- phope Co. The brass cup holding the magnet coils fits in.side the receiver shell, being held in place when the head piece is screwed on. The magnet is in nb way attached to the shell, as may be understood from, the figure, thus securing a rigid and permanent adjustment to the diaphragm. Defects of Early Receivers. — Most o^ the earlier types of magnet receivers had either one of two grave defects in con- struction: either the adjustment of the magnet was made by a screw passing up through the tail of the case, as in Fig. 43, or a screw near the poles engaged a thread in the inside of the case. In both cases the adjustment was uncertain, particularly in the hands of inexperienced persons, who would frequently damage instruments affected by sudden changes in temperature, 78 A B C OF THE TELEPHONE. Boxed Coll Receivers. — In the most improved makes of instrument the brass box for containing the magnet coils, on about the same plan as that employed in the Neumayer receiver, is rapidly being adopted. It is an arrangement superior to having the armature coils in any way attached to a hard rubber shell, because, as has been ascertained, the "coefficient," or ratio of expansion or contraction, under heat or cold, is about the same for brass and steel, and an adjustment between the core of the magnet and the diaphragm, once established, is likely to be maintained under all temperatures. Figs. 52-53.— Bi-polar compound bar magnet receiver of the Holtzer-Cabot Electric Co. The magnet coils rest on a metal flange, shaped as shown, which is secured to a cor- responding flange in the inside of the case by four screws working in slots. Accurate adjustment may be obtained by turning the back cap, which is separate from the rest of the case, anrt tightening the screws at the required point in relation to the diaphragm. Telephone Magnets. — The magnets of the better class of telephone receivers are made of the hardest tempered steel and magnetized to a degree that will enable any one of them to lift a mass of iron weighing at least four — some even as high as fif- MODIFICATIONS OF THE RECEIVER. 79 teen — pounds. In practice the general experience seems to endorse the opinion that by improving the quality of the steel the power of the magnet may be best increased. Watchease Receivers. — In addition to the ordinary long- magnet receiver there is another variety of a much more com- pact and convenient shape, which, from its general form, is called the "watchease" receiver. One of the earliest patterns Figs. 54-55. — Pole pieces of the Holtzer-Cabot bi-polar receiver, showing their peculiar shape, and illustration of the field of magnetic force on the diaphragm, showing the even and symmetrical pull and the utilization of all the lines of force, of this instrument was made with a coiled magnet, on one pole of which was placed the usual pole piece, directly under the diaphragm, the whole being enclosed in a case about the size and shape of the old-fashioned "turnip" watch. Later exam- ples of this variety of receiver have either the arrangement of the Gower telephone — a semi-circular magnet with two pole coils-r-or a polarized ring magnet, the two coils being attached on the diameter. Watchease receivers are most often used, with the head-gear attachments, by telephone operators at switchboard stations. The D'Arsonval Receiver. — ^An interesting variation of the magnet receiver, in which the greater part of the strength of the magnet is supposed to be directly utilized in the operation of 8o A B C OF THE TELEPHONE. Figs. 56-57. — The Stromberg-Carllson receiver, exterior view and section, show- ing method of securing magnet and coils in a solid brass headpiece, to preserve proper adjustment with the diaphragm. .««MS¥iMi^li>i!n' Fig. 58.— Section of the " solid receiver," showing device for maintainine adjust' nt by imbeddrng the magnet in the rubber stem of the case. MODIFICATIONS OF THE RECEIVER. 8l the instrument, is the D'Arsonval receiver, widely used in France and other European countries. The inventor's contention is that the only really useful part of the field of a telephone mag- net is the part situated directly between the two poles; the por- tion of each coil outside of this area being almost completely lost, and only furnishes a useless resistance to the current. Fig. -59. — Interior view of a typical " watchcase " receiver. With this theory in mind, he constructed the instrument shown in Fig. 62. The permanent maghet is spiral-shaped, one pole ending directly under the center of the diaphragm and bearing the core of the pole-coil; the other entering at the side of the wooden receiver case, and having at its end an iron cylinder which completely envelopes the coil, thus surrounding it com- pletely in a magnetic field of great intensity. The diaphragm is clamped in place in the usual fashion, and the conducting cords enter at the side of the case opposite the negative pole^ 82 A B C OF THE TELEPHONE. connections being made inside. The instrument has given excellen'i results, although it has failed of a very wide use. The Phelps Crown Receivers. — The multiple contact receivers are less numerous, although worthy a passing notice. The earliest types were the Phelps "crown" receivers, designed FiGS^i 60-61.— Phelps single and double " crown " receivers. Fig. 52.— The D'Arsonval receiver. Fig. 63.— The Goloubitzlcy receiver. in 1878. The "single crown" type is shown in Fig. 60. Here the magnets, bent double, are arranged around a common cen- ter, the north, or positive, pole of each one carrying a separate coil, and the negative poles ending at the rim of the diaphragm. The "double crown" receiver, shown in Fig. 61, is somewhat MODIFICATIONS OF THE RECEIVER. 83 different in construction, being really two single crown instru- ments, with two diaphragms separated by a common vocalizing chamber, in which was also introduced a cone-shaped piece of non-conducting material. Both these instruments gave good results and loud, distinct articulation, but the expense of making them prevented their general adoption. TKe Goloubitzky Receiver. — An instrument of somewhat similar appearance is the Goloubitzky receiver, as shown in Fig. 63. It consists of two circular magnets, to the four poles of which are attached the usual cores and coils; these being wound Fig. 64.— The Ader receiver. R, the super-exciter. in series, or the wire from the positive coil running over to, and around, the next negative coil, so that there is a negative con- tact at one terminal and a positive at the other. Goloubitzky observed that when the same message was delivered to several receivers at a station at once the sound was equally loud and distinct in each; consequently, he reasoned, that to increase the number of contacts in one instrument — really combining two or more instruments in one — would increase the power of that instrument. The results, however, although good, hardly war- ranted the added expense and care required to get perfect receivers of this type. 84 A B C OF THE TELEPHONE. The Ader Receiver. — The last theory of improving the magnet telephone is that of strengthening the armature. The best known type of this construction is the Ader receiver, as shown in Fig. 64. It consists of a single magnet, bent into almost circular form, and carrying a coil on each pole, after the fashion of all double-pole instruments. The magnet is gener- ally nickel-plated, to serve as a handle. The special feature of this receiver is the small ring of iron, E, encircling the inner end of the mouthpiece, as shown. This ring, which is called the "super-exciter," acts to render the field of magnetic force more intense, and is doubtless one cause of the remarkable effi- ciency of the receiver. It has been remarked by several tele- phone authorities that the intensifying effect would be increased, in both this instrument and the D'Arsonval, if the whole mouth- piece were made of soft iron. Fig. 65.— The cord of a telephone receiver. It consists of two strands of braided tinsel, insulated, and enclosed in a woven sheath. A metal pin is secured to the end of each strand, and this is inserted in the grooves of the binding-post to give electrical connection. There is a large number of receivers of patterns different from those described, in one point or another, but all combine the essential features of a permanent magnet, or magnets, bear- ing coils on the poles, and an iron diaphragm to receive and turn back into sound-waves the impulses brought along the line from the transmitting instrument at the far-away station occu- pied by the party with whom the conversation is being held. The instruments described are almost all in practical use, or have been, and an understanding of their theories gives a fair idea of the progress of telephony in several countries. CHAPTER SEVEN. THE CARBON MICROPHONE TRANSMITTER. Conditions for Good Transmission. — As we have seen, the earliest form of the Bell telephone circuit was simply two lines of wire ending at the poles of two magnet instruments, with no battery. This arrangement is very satisfactory for short lines, where there is little or no interference from other electrical circuits, or the danger of meeting with unexpected objects that will impede or deflect the current. To meet such obstacles, and at the same time transmit a clear and distinct message, a stronger current is needed than the magnetic field of a Bell instrument can generate. As we have seen, also, the effect of an alternating current and varying resistance is another necessity for getting distinct utterances at a distance. The Theory of Varying Pressure. — The need of a varying pressure to perfectly modulate the current was met in the Reis transmitter, but the light contact between the electrodes, as we have described it, created the constant danger of breaking the circuit and thus interfering with the perfect transmission of sounds. Conseqently, shortly after the introduction of the magnet telephone, a number of inventors began working on the problem of a practical transmitter, which should embody the principle set forth by the French scientist, Du Moncel, that, " if the pressure between two conducting bodies forming part of an electric circuit be increased the total resistance of the path between them will be diminished, and if the pressure be decreased, there will be an increase in the resistance." Berliner's Transmitter. — One of the earliest attempts to make a variant pressure transmitter is shown in the instrument invented by Emile Berliner in 1877. It consisted of a vibrating- 85 86 A B C OF THE TELEPHONE. metal diaphragm of exactly the kind used in the magnet receiver, and against the center of this was a metal knob, or button, held in place by a thumb-screw. One electrode was attached to the diaphragm and" the bther to the metal ball. By speaking against the diaphragm we cause it to vibrate, and hence alternately press hard and gently on the button, thus varying the pressure of the current supplied by a battery. Its working was not satisfactory. Edison's Carbon Disc Transmitter. — After an extended course of experiments to determine the most suitable substance Fie. 66.— Edison's Carbon Disc Transmitter. Fig. 67.— Huniiings' Carbon Dust Transmitter. for a varying pressure transmitter, Mr. Edison designed the carbon disc instrument, which embodies the theory on which transmitters have been constructed up to the present time. His first pattern instrument consisted simply of a button of plum- bago, held against a small platinum disc and secured to the diaphragm by an adjusting screw. Later he modified his plan, and designed the instrument shown in Fig. 66. Here the carbon button, C, made of compressed lampblack, is placed between two discs of platinum foil, jP, and attached to the THE CARBON TRANSMITTER. 87 enlarged end of the adjusting screw, S. In front of the forward disc of platinum rests a plate of glass, G, to which is attached a rounded button of bone or ivory, A, resting against the center of the diaphragm, D. This was a more practical instrument than was Berliner's, but, as was later shown by Prof. David B. Hughes, of England, the carbon transmitter requires, not only a variant pressure, but, also, a loose contact of the electrodes at the start. In the theory of loose contact we see a memory of Reis's irstrument. Fig. 68.— Diagram of the Hughes Microphone. Hughes's Microphone. — ^Hughes based his theory on a device invented by him, known as the microphone, the simplest form of which is made as shown in Fig. 68. Here we have a circuit of wire, including a voltaic cell and a telephone receiver, and having as terminals two steel wire nails. The circuit is closed by laying a third nail across the other two. If, now, a sound, as of the ticking of a watch, be made near the nails, it may be distinctly heard in the telephone receiver, although a. considerable distance intervene. A more perfect form of micro- phone made on the same principle, consists of a pencil of battery carbon, sharpened at both ends, and loosely attached in sockets hollowed in two carbon blocks, which form the electrodes of the instrument. The whole, mounted on a reso- nant sounding-board, is extremely sensitive to minute sounds — being literally, as its name indicates, to small sounds what the microscope is to small objects. 88 A B C OF THE TELEPHONE. Hunnings' Dust Transmitter. — Following up this theory of loose contact, Henry Hunnings, in 1881, devised the original carbon grain transmitter, thus making a distinct depiartiire in Construction and giving the pattern for what has become the prevailing form of instrument, particularly in America. Fig. 67 shows a sectional view of this in- strument. It is a circular wooden block in which a shallow cavity has been hollowed out. In this cavity is set a metal disc, making one electrode connection, and over it are a number of carbon granules, generally coke ; the whole being capped by the metal diaphragm, which forms the other electrode, and is clamped in place by the mouthpiece. The effect of the numerous granules of carbon is to give a multitude of contacts to trans- mit the varying stress of the dia- phragm by the electric current. The result, in point of clear and loud transmission, is similarly increased, a fact which has caused the granule or "dust" system to be so widely adopted. The principal trouble, how- ever, is that, in the simpler forms of this pattern, the granules are apt to .pack under the oft-repeated stress on the diaphragm, and the efficiency of the instrument is thus impaired. ^^- ^'•^.^^imitfer.''^ ^'^''^ The Blake Transmitter. — Another instrument that has seen considerable use both in this country and England is the Blake transmitter, shown in Fig. 69. Here, M is thfe dia- phragm, behind which is screwed an iron ring, carrying the two projecting pieces, B and D. Upoij the upper projection, B THE CARBON. TRANSMITTER. 89 is fixed the angle-piece, C, by means of the flexible brass strip, H, with its lower angle abutting against the adjusting screw, N, which is fitted into D. It is by this screw that the instru- ment jnay be adjusted whenever its woj-king becomes defectivle. The diaphragm, an iron disc, ii,s surroinnded by a rubber ring, R, which is secured behind the mouthpiece by two "damping" springs, one of which presses on the rubber ring and the othlsr on the diaphragm. On the upper arm of the angle-piece, C, are fixed two springs, F and G, the former of which is insulated from the meta!l of C by a clamp of hard rubber, or other. suitable non-conductingmaterial, Z. The latter, 6^; is in direct electrical connection with the metal of C. The j spring, F, carries on its extremity a short section of plafcinuiji wirp, L, which rests directly against the center of the tiiapliragm,\pn one side, aiid against the carbon button, K, fastened in the brass cup, P, at the end of the spring, G. The motion of the diaphragm varies the pressure between the platinum point, L, and the carbon button, K, and thus varies the current which passes from the battery through the iron frame ring, thence through B, H, C, G, P, K, L, F, and out again from the contact, Z, of the spring, F. The rather complicated series of springs and levers is intendied- to hold the contact ^pieces in position, so that no variation of the sound may be lost, either through a too loose contact or the wearing of the carbon disc. Varieties of Carbon Transmitters. — Both the Runnings and th,e "Blake types of transmitter are constructed on the prin- ciple of the microphone — both have one or more carbon elec- trodes in loose contact, susceptible of varying pressure, accord- ing to the movements of a thin metal disc, moved by the stress of the voice. There are three general types of carbon trans- mitter, all embodying these same principles: i. Carbon pencil transmitters. 2. Carbon granule transmitters. 3. Ball or but- ton transmitters. Of these three kinds, the first is most like in pattern to the original Hughes microphone, consisting of two or more carbon pencils arranged in carbon sockets in a variety of 90 A B C OF THE TELEPHONE. Fig. 70. Fig. 71. Fic 73. Fig. 73 Pour types of European carbon pencil transmitters. Fig. 70.— The D'Arsonval transmitter with permanent maenet regulator ; Fig. 71. — The Ader transmitter, show- ing arrangement of pencils behind the diaphragm ; Fig. 72. — The Crossley transmit- ter, showmg arrangement o( the pencils ; Fig. 7}.— Tt>¥ Gower transmitter, showing arrangement. THE CARBON TRANSMITTER. 91 orders around the center of a wooden diaphragm. _ The idea of thus varying the arrangement is that the sound impulses, striking the diaphragm, spread in various directions and should be met by appropriately arranged carbon pencils. The D'Arsonval transmitter is among the most elaborate, each pencil being covered with a cylinder of sheet iron and a permanent horseshoe magnet being mounted behind the row of pencils, to provide an added means for producing a variable pressure, as it attracts the iron covers and holds them so long Fig. 74.— Carbon Diaphragm, Back Electrode Plate and Carbon Balls for the Ball type of Transmitter, as the stress of the voice on the diaphragm continues the altered pressure. Such transmitters are common in France and other European countries, but have gained no favor in America. Carbon Ball Transmitters. — The ball, or button, trans- mitters are equally interesting and various, although the prin- ciple of their construction is only a type of the same kind of instrument a:s the pencil transmitter. Fig. 74 shows a form of this instrument, which consists of three parts: a carbon dia- phragm, a carbon block, or plate, having a number of round 92 A B C OF THE TELEPHONE. cavities, and the same number of carbon balls, of a size to fit loosely into the cavities and be held in place when the carbon plate carrying them is fastened into a frame with the diaphragm. Another form of this instrument has one large ball, fitting into a cup of carbon or metal and held against a carbon diaphragm in the same manner. Fig, 75. — Section of Berliner's Universal Transmitter. Fio. 76.— Back Electrode Carbon Plate for Berliner's Transmitter. Berliner's Universal Transmitter. — Several types of mod- ern dust transmitters employ, as the back electrode, a block ol carbon with a series of cones or concentric circles, as shown in Fig. 76. The plate, thus worked, is separated from a carbon or metal diaphragm by a quantity of carbon granules, which are evenly distributed in the spaces between the cones or circles, thus preventing packing at any one point and distributing t!ie pressure. Of this type is the well-known Berliner Univej-sal Transmitter, depicted in Fig. 75. Here we see the peculiarities of its construction, which renders it desirable to place the car- bon diaphragm in a horizontal position and allow the bai k elec- trode, the circular-grooved carbon plate, to rest upon it. The grooved plate, C, surrounded by a cover of felt, F, which touches the diaphragm about its circumference, is held just above the diaphragm by a quantity of carbon granules to give THE CARBON TRANSMITTER. 93 the variable resistance. On the center cf the diaphragm rests a short rubber tube, held in place by the micrometer screw, E, thus serving to damp or modify the vibrations of the diaphragm. The same screw holds the carbon block in position and also holds the back electrode. The other electrode is at the brass ring which clamps the carbon dikphragm into place. Fig. 77.— Details of the Solid-back Transmitter. The Solid Back Transmitter, — -A form of carbon trans- mitter most often used in the United States is shown in Fig. 77. It is called the " solid back," or White transmitter, and is one of the most efficient in use. The back electrode is a metal case, W, secured by an adjusting screw to the supporting bracket, P. It is lined on the inside with white paper, to serve for insulation. Inside, against the rear wall, fits the small brass disc, B, which carries a pin on its center to attach it to the sup- pore, and on its face carries a button or plate of carbon. In front of and around this parbon disc is a quantity of granules, which rest on the other side against another carbon disc, carriea on the brass button, E, from whose center projects the screw- threaded bars, R and Q. Over these fit the mica washer, M, the nut, U, and the cap, C, which is screwed in position. The bar, Q, projects through the center of the vibrating diaphragm, D, and is held in position by the nuts, T and T, which act as dampers. 94 A B C OF THE TELEPHONE. Improvsd Transmitters. — The number of new transmit- ters constantly being put upon the market by various manufact- urers is simply immense. Many of the features embodied are undoubtedly excellent, and many marked improvements have been made, both in efficiency and durability. Where carbon grain is used the principal problem has been how to prevent packing, or solidifying, which inevitably results in disabling the transmitter by destroying the loose contact, Fio. yS. — Improved Transmitter of the Century Telephone Construction Co. . The Century Transmitter. — The improved transmitter of the Century Telephone Construction Co., shown in Fig. 78, presents many points of excellence that are worthy consid- eration. It is one of the best of the several improved forms of the "solid-back" transmitter, just described. Here, D is the hard rubber mouthpiece; A, the cup containing the electrodes; B, the cover, secured to the bridge-piece, C, by the screws, as shown. The extra strength of the bridge-piece is one of the features of this instrument, securing, as it does, perfect rigidity, and obviating all danger of warping or "buckling," as must frequently occur in ill-constructed instruments. In the center THE CARBON TRANSMITTER. 95 of the bridge is screwed the brass cup, F\ which contains a button of compressed carbon, K^, and forms the back electrode. A similar cup, i^", is screwed to the diaphragm, as shown, and forms the front electrode. The two are separated by the mica washers, H and H, and the felt washer, G, which form a cham- ber, Z, to be filled with carbon granules. As will be seen, the chamber thus formed is enlarged into pockets at top and bot- tom, and into the lower one the carbon granules, having a tend- ency to pack, will settle out of the path of the current. Rapid and accurate adjustment may be obtained by turning the screw at/". Fig. 79. — Ericsson Transmitter. The Ericsson Transmitter. — Another form of transmitter which is winning wide favor is shown in Fig. 79. It is manu- factured in Sweden, and imported by the Ericsson Telephone Co., of New York City. The principal features claimed for it are high efSciency and distinct transmission, with small battery power. The carbon granules used are prepared by a secret process — a fact that has enabled the instrument to maintain its high reputation for many years. Reference to the figure will show that the general construction is much like that of the Ber- liner Universal Transmitter, already described. Here, D is the diaphragm of ferrotype iron, to the rear side of which is secured a gold-plated disc, forming the front electrode. To the outer face of the diaphragm is cemented a cover of oiled silk, which proves an effectual protection from moisture. The g6 A B C OF THE TELEPHONE. grooved -carbon block, C, forming the second electrode, is secured to the rear of the case, from which it is properly fftsulated, and connection is made by the screw protruding at the end. ■ Two double damping-springs, S and S, hold the dia. phragm in place. A ring of felt, FF, surrounds the rear carbon block, and, bearing against the diaphragm, serves to retain the cafboh granules. Another ring of felt' at the center of the block surrounds a small coiled spring, which still further damps the;diaphragm, and prevents harshness of tone in speaking. Fig. 8o.— Section of Ihe Wilhelm double-diaphragm transmitter, showinRmethod of conducting the sound from the mouthpiece, A, through an annular chamber to both diaphragms. Between the diaphragms, each of which is an electrode, is a layer of carbon granules. CHAPTER EIGHT. THE CIRCUITS OF A TELEPHONE APPARATUS. Electrical Source of the Transmitter.— ^As we have seen, the Bell magnet telephone, and the numerous variations of it, operates by a current produced by the agitation of a magnetic field, somewhat after the manner of a dynamo. We; have also learned that, while it is immensely sensitive to small currents, it cannot produce currents of sufficient pressure to successfully operate on long lines of wire, an alternating current of high voltage and variant pressure being best adapted to transipit the sounds of the voice and produce effects on the diaphragm of the receiving instrument. In the various types of trans- mitters we have described, we have seen that the apparatus for prodiicing the variation of pressure is composed of two car- bon contacts, or one carbon and one metal contact, but in this arrangenient there is no provision for producing a current, as in the magnet telephone. The transmitter can only vary a current, consequently there must be some other source of electrical activity. This is supplied by a commercial voltaic cell, or bat- tery of several cells, and in general usage we have the type of cell known as the "open circuit." Open Circuit Batteries. — The open circuit cell is so called because i,t has the power of recuperating its strength, or "depo- larizing," whenever the circuit is left open or is not in use. The closed circuit cell works best when in constant use, and has the power of constantly depolarizing itself. It is used where a strong current is constantly needed, as in telegraphy. The condition known as "polarization" is, briefly, due to the collection of minute bubbles of hydrogen gas on the face of the negative plate, which interferes with the production of elec- tromotive force by reducing the active surface of the plate, and 97 98 A B C OF THE TELEPHONE. hence increasing the internal resistance of the cell. This effect is remedied in closed circuit batteries by a substance, known as the "depolarizer," which in process of electrolysis constantly releases oxygen gas to combine with the free hydrogen. In the ordinary blue-stone Daniell cell the depolarizing agent is the sulphate of copper ("blue vitriol"), which constantly releases oxygen and causes a deposit of metallic copper on the face of the "copper electrode. The process of depolarization is, thus, dependent on the continuity of the current, and ceases with it. For this, among other reasons, this type of cell rapidly deteriorates when not in pse. In the open-circuit cell, on the contrary, the most common and efficient type of which is the Leclanch6, so named from its inventor, a celebrated French electrician, the depolarizer is diox- ide of manganese, which surrounds the carbon plate When the circuit is open and current has ceased to flow, the oxygen is constantly given off from this substance to combine with the free hydrogen, and the manganese then absorbs a fresh supply frona the air. Thus the battery is in best condition after a season of rest, and is not capable of continuous work, like the cells of , the other type. This is the reason that open-circuit cells are used in telephone practice on short lines, although in long-distanc-e telephony, where strong currents are required for long periods, the closed circuit cells are coming into more extended use. Fig. 8i shows a section of a common type of the Leclanch^ cell. In this figure the carbon plate is inserted in a porous cup and surrounded with a quantity of granulated carbon and manganese oxide. The zinc electrode is a pencil at the side of the porous cup, both being immersed in a solution of sal ammoniac. s m ^ .J .- m^«.Hj H = r^ : piffl ::::;". ^' -^^ffl — ~jz ^- 5™BH _--=) Fio. 8i.— Section of ordin- ary l^clanch^ cell. Dry Cell Batteries. — Instead of the type of cell just de- scribed " dry cells " are frequently used in connection with the THE TELEPHONE APPARATUS. 99 <-s c c 5 I- O 41 » H H = 5 s > fO 1 n. surr ell h ythe ft of w 3 u ji 4; eleme This minal be, to o S- 5»2 2 ■S " £ s f, ^ « *> s — C •n -^ £ 2 S.S in U at 'i3 ^ U cell ha " acid d toth the ver - h/1 •g fa = N « - oa 52 i-s N v « 01 V u cd a S^ "n SS-'^S 1 Rl ^ ^ P Fig Fig )f th caci E S lOO A B C OF THE TELEPHONE. g ■a £■ S- S. III ■a g. « 8 :§ bd o c ^ •a a -S % * -o 3 s i w ■3 . t a=3 s i ■s S " < V S u •l| I u S a a a » ■s -^ >> S c -o sS-S ^§ !•£ " jiS 1?^ (2" c ^■^s •p -3 =li 11 lis S| en |: fi^ Si ^, and thus to release the point, F, ifom its contact with the spring, G. Because the spring, G^ is SWITCH HOOKS AND SHUNTS. I I g attached to the terminal post by an insulating block, K, the circuit is broken through it, and the coil of the generator is thrown in. This cut-out is a true shunt, affording a path of the lowest resistance in place of the long fine wire of the armature^'coil. The Post Cut-out. — Fig. 104 shows the Post cut-out, so- called from its designer. Its operation depends upon centrifugal force instead of a pull spring. Here A is the shaft of the armature shuttle, which is turned by the pinion, £, worked by the gear wheel of the crank shaft. One end of the armature Fig. 105. — The Holtzer-Cabot centrifugal shunt. When at rest the copper granules .<;ettle around the end of the spindle. When it is revolved they are thrown out by centri- fugal force, thus breaking the shunt circuit. winding coil is attached to the shaft by the pin, C, and the other to the pin, D, which is insulated from the metal of the shaft, as shown by the shaded parts around it, and leads the curreftt to the terminal connection through an insulated path. . This pin has a platinum head, which, when the apparatus is at rest, is in contact with the bob, 3, carried on the end of the light leaf spring, F, which, in turn, is secured to the shaft of the shuttle by the screw, G. So soon as the shuttle is revolved by turn-ing the crank, the end of the spring, J^, naturally flies outward, impelled by centrifugal force, until the bob, 3, comes into contact with the stop, H. Thus the shunt circuit is broken, and the coil of the armature is thrown into action. It is necessary that the shunt be broken ; otherwise the generator would be short-circuited through the spring, F, and no current could emerge to ring the bells. I20 A B C OF THE TELEPHONE. The Holtxer-Cabot Shunt.— Like all appliances for tele phones, the automatic shunting devices are manifold in numbei One of the simplest and most effective of the more recent con- trivances is shown in Fig. 105. It is manufactured by the Holtzer-Cabot Electric Co., of Boston, Mass. On the end of the armature shaft is mounted a small cylindrical brass box, in metallic contact with the spindle, and partly filled with copper granules, or short sections of copper wire, which are silver- FiG. 106. — Magileto-generator supplied with a granule centrifugal shunt. plated to prevent corrosion, and insure more perfect elefctrical contact. So long as the armature is at rest, a circuit of the smallest possible resistance is established between the end of the spindle shank and the sides of the box. The slightest move- ment ot the armature disturbs this; and, as soon as it begins to revolve, the metallic granules are thrown outward, by centrifu- gal force, thus effectually breaking the shunt. Fig. 106 shows a generator furnished with a centrifugal shunt of this descrip- tion. CHAPTER TEN. THE SWITCHBOARD AND THE APPLIANCES OF THE CENTRAL STATION. Telephone Systems, Large and Small. — As we have seen, each telephone apparatus includes a transmitting and a receiving instrument — the one to talk into, the other to receive the messages from some other telephone apparatus. It follows, therefore, that between any two apparatus there must be a line of wire suitable to convey the electric current bearing spoken messages. In a small system, with but few instruments, as in a country town or in a large manufactory, there may be a number of wires leading from each instrument to every other, so that one telephonist may call up any other in the system by simply manipulating a switch attached to his own apparatus. Such small systems are known as "party lines " and inter-communi- cating systems, and their operation and appliances will be fully explained in another section. At present we are concerned only with the most.familiar method of connecting the talking circuits of telephones. It is known as the "central station," ot exchange, system. Grounded and Metnllic Circuits. — It is hardly necessary to explain to any one that if an inter-communicating system were adopted in connection with any number of telephone stations above ten or twenty, or in any town of size and business activity, the amount of wiring necessary to complete the circuits would be beyond the possibilities of commercial expenditure. It is positively essential that, in all but the smallest systems, each apparatus have but one circuit — an incoming and an out- going wire, or an incoming wire and a ground return. The "ground return " is the method made familiar by its adoption in telegraphy. Here, as we know, the wire carrying the mes- 122 A B C OF THE TELEPHONE sage current is strung on poles, or buried in cables, and the return current, that completes the circuit, flows through short lengths of wire to the ground, and thence back to the sending station. The attachment in the ground is made either by water mains or sewers, or by sheets of metal buried at the required depth. This system is impracticable in cities, where the ground is filled with pipe lines and other obstacles that would immensely weaken any current, or subject it to outside interferences — sneak currents and contact with other circuits — and most often destroy its power to transmit articulate sounds. To reduce these inter- ferences to a minimum, metallic circuits, consisting of two dis- tinct lines of wire, of the same size and material, are most com- monly adopted. All lines are then carried to a central station, where are installed devices suitable for connecting, as desired, any two subscribers. The Switchboard \ Its Construction and Operation. — For the purpose of making these connections a device known as a switchboard is employed. Fig. 107 gives an idea of the general appearance and construction of a common form of this apparatus. As may be seen, it consists of two distinct parts : a series of upright panels carrying drop shutters and round apertures under the annunciator numbers; and a horizontal board or table, upon which appear a number of upright instru- ments, and in front of them a row of short levers. The panel apparatus thus consists of a number of small instruments such as are used on a hotel annunciator, which are known as "drops," and of another series of instruments, fixed behind the round holes on the front of the panel, which are called "jacks," or spring jacks. " The upright instruments, that stand in rows below the panel-boards, are the " plugs"; and each of them is secured at the end of a flexible cord, passing through a hole in the switchboard table, attached at the back of the boards as shown in the next figure, and held in place by pulley weights. The object of these plugs is to close the circuits between any two subscribers' lines, one of a given pair being inserted in the THE TELEPHONE SWITCHBOARD. 12J Fio. 107.— Sterling Bell-type Switchboard of 100 Drops, " }ack " corresponding to the calling station, and another in that corresponding to the called station, connection being made between the two lines by means of the flexible conducting cords. The row of small levers at the front of the table are the operator's "listening and ringing keys." u>4 'J4 '^ C OF THE TELEPHONE. FKt. loS.— Rear view of Sterling loo-dropBell-type.Switchboard. Making a Telephonic Connection. — The method of oper- ating the switchboard is as follows: When any subscriber desires to have communication with any other, he will, as we have seen, operate the magneto of his instrument, thus sending a current along the line wire, which causes the shutter of the THE TELEPHONE SWITCHBOARD. I 25 drop to which his wire is connected to fall, thus giving his number to the operator, who sits in front of the switchboard table. Since each subscriber's circuit is completed at the drop of the board, any one can understand the method of causing the shutter to drop by thinking of the operation of the ordinary burglar alarm, or of a hotel teleseme. As soon as the drop shutter falls, the operator lifts the inner, or "answering," plug of the two immediately under the row of drops in which this particular one happens to be, and inserts it in the hole of the jack bearing the corresponding number. Then, by moving the listening key in the row immediately in front of the drop and plug in question, she throws her own telephone set into circuit, and is thus able to communicate with the feubscriber calling. Having learned the number of the subscriber with whom he wishes to speak, she takes the forward plug of the two imme- diately below the drop of the calling subscriber, and inserts it in the jack of the number corresponding to that of the subscriber called. This done, she moves the ringing key in the first subscriber's row, so as to sound the call bell of the one called ; the current for this purpose being supplied, either by a hand magneto-generator attached to her section of the switchboard, as shown in Fig. 108, or else by thus throwing into circuit the wire leading to and from a power-driven dynamo attached to the exchange. . The latter is the plan adopted in all large exchanges. The Clearing-out Drop. — As soon as the operator has attracted the attention of the called subscriber, she again shifts the key, thus throwing the two into one 'circuit, and enabling them to have their conversation. As soon as this is finished each hangs up his receiver and turns the crank of the magneto of his apparatus, thus causing to fall another drop, usually placed at the base of the panels, which is known as' the "clearing- out " drop. On receiving this signni, the operato'r restores the keys to their first positicn and removes the plugs from the jacks, allowing them to be drawn down by weights attached to their cords, to their plac2 in front oi ilie panel,~as shown in the figure. 126 A B C OF THE TELEPHONE, The Circuits of a Switchboard. — As one might easily guess, the switchboard, like the telephone apparatus already described, is a combination of several different circuits, each of which works only when the others are cut out of line. When Fig. 109.— Ericsson loo-Drop Table Switchboard. the telephone apparatus is at rest the call bells are in circuit, so, in the switchboard, the drop is normally ready to respond to the impulses sent along the line when the magneto generator is set in motion. Just as the removal of the receiver from the switch hook makes the talking circuit of the telephone appa- THE TELEPHONE SWITCHBOARD. 127 ratus, so the insertion of the plug in the switchboard jack cuts out the irop circuit and makes the talking connections. Grcunded Line Switchboards. — So far as concerns the construction of the line, jack and plug, there are two kinds of switchboards: those for grounded circuits and those for metallic circuits. Fig. no shows the details of a grounded circuit switchboard drop and jack. Here the current from the generator of the subscriber's apparatus enters the switchboard apparatus at a ; thence through the leaf spring, b, of the jack belonging to that particular subscriber; through the contact screw, c, which is insulated from the metal of the jack, as shown; through the wire leading from it to the coil of the electro- magnet, d. The result is that the magnet attracts the armature, e, raising the attached lever, /, and thus freeing the hinged drop shutter, g, which falls, attracting the attention of the operator, and disclosing the number of the callihg subscriber. The condition of the drop apparatus, before it is affected by the current, is shown in Figs. 116-117. Here We see that the armature, e, of the magnet, not being attracted, holds the position shown by the weight of the bar, /, which, by the hook at its end, retains the shutter,'^, in its normal position. Circuits of a Grounded Line. — Immediately on noticing the drop number the operator thus closes the shutter and inserts the plug in the jack. ;By this act, as may be understood, the metallic point of the plug forces the spring out of contact with the point, thus cutting out the drop from line, and making a new circuit through the conducting cord attached to the plug, as shown in the figure. The current, then, no longer goes to earth by the ground wire, but through the grounded connections of the two communicating telephones, when their lines have been joined by the two plugs and cords standing in the row just beneath the drop of the calling subscriber. It is essential that the two plugs, used to make connection between any two subscribers, should belong to the same pair; since each pair is connected through the conducting cords with one set of 128 A £ C OF THE TELEPHONE. Fig. iio.— Circuits of a grounded or common-feturn switchboard. listening and ringing keys, which play the parts already described. To attempt a connection with any two pli^gs not in the same pair would mean failure to make a circuit between the two subscribers' apparatus. THE TELEPHONE SWITCHBOARD. 1 29 Switchboard Plugs. — The plugs used in switchboards having a grounded circuit consist simply of pins of metal properly shaped, and joined on to the conducting cord by suit- able screw connections. In a switchboard of a metallic circuit the plugs are made with double connections, in order to maintain the lines of the two wires, the line and the return, throughout. Their construction will be explained later. Grounded Switchboard: Operator's Circuits. — The cir- cuits of a grounded line switchboard are also indicated in diagram in Fig. no. Each of the plugs, A and B, is connected by its flexible conducting cord with the apparatus of the ringing keys, C and D, respectively. As will be seen from this diagram, which uses the simplest forms of listening and ringing devices, eac^i of these keys has two contacts, i and 2, so as to enable it to stand in line with either of two circuits. Upon perceiving that a drop bearing a number, say, 10, has fallen, the operator inserts plug A in the same numbered jack, and at the same time pushes down upon the listening key, Z, so as to throw in her receiver and transmitter, R and T, thus enabling her to converse direct with subscriber number 10, the circuit being established from the ground connection of his apparatus, through the out- going line wire, the spring jack of the switchboard, the plug. A, and its cord,' to contact i of key C, and thence through key Z, to the section talking apparatus, which is provided with receiver, transmitter, battery and induction coil, in precisely the same manner as the subscriber's apparatus, and ending the circuit in the ground connection. Grounded Switchboard : Line Circuits. — On ascertaining that 10 desires to converse with 84, for example, she inserts plug B in jack 84, at the same time pressing key D to its contact, 2, thus throwing into circuit the magneto-generator, G, which is operated either by hand or on a "bus wire" from a power driven dynamo in the exchange. A circuit is thus madp from ground in the exchange, through the generator, G, contact 2 of key I>, cord and plug B, jack 84; line wire 84, apparatus I30 A B C OF THE TELEPHONE. 84, to ground ; thence back again, causing tlie bell of 84 to ring. This done, the operator restores key D to its normal position in connection with its contact, i, thus throwing^ 10 and 84 into a circuit which is bridged across by the clearing-out drop. This "»*^ Fig. III.— Metallic Circuit Switchboard Jack, showing double electrical contacts. Tip contact at spring, F; sleeve contact in thirable,// connection to hue drop through anvil, G, and wire, B, is constructed precisely like an ordinary line drop, although generally of a higher resistance — between 500 and 1,000 ohms. This high resistance and self-induction is used in order that these drops, permanently bridged across the circuit, may not shunt the telephonic current. They are also enclosed in soft Fig. 112. — Metallic Circuit Switchboard Plug, showing double contacts. A^ tip contact; C, sleeve contact : E^ insulating bushing. iron tubes, for the purpose of increasing the electro-magnetic effect, and also to prevent induction from other drops and lines. After the two subscribers have been connected in the manner indicated, the operator may keep her listening key depressed in order to find out whether connection has been made, if 84 has answered the call. She then breaks circuit with her own telephone set, leaving all the keys in the raised position. So soon as the conversation is ended the subscribers hang their THE TELEPHONE SWITCHBOARD. n^ receivers on their switch hooks, and turn the cranlcs of their magneto-generators, thus sending along the line a current of sufficient strength to operate' the clearing-out drop, the talking current being' too weak for "that purpose: ' As ' soon as the ftiQS. 113 and 114. — Spring Jack and Short Answering Plug of a Steriing Metallic Circuit Switchboard. The Jack is a Brass Tube, with Springs of German Silver, operator sees the shutter fall, she knows that the conversation is ended, and removes the plugs from the jacks. Metallic Circuit Switchboards '. Plug and Jack. — The apparatus of a metallic circuit switchboard is arranged to accomplish the same results, although differing in numerous Fig. 115. — ^Metallic Circuit Jack and Plug of the Keystone Electric Telephone Co. details of construction. As, however, metallic circuits are almost universal in present-day telephone practice, it will be necessary to examine such apparatus in detail. Fig. Ill represents the construction of a metallic switch- board jack. As will be seen at once, its principal point of dif- ference is that it has three points of circuit connection — A, £, and C. This is a feature common to all its various forms. As regards the connections of A and £, it will be readily seen that they are the same as those in a grounded circuit jack. The third wire is connected to the binding screw at C, and separated from the other two by the insulating blocks of hard rubber, 2? 132 A B C OF THE TELEPHONE. and E. When the telephone circuits are not in use, the leaf spring, F, connected to the terminal. A, is in contact with the point, G, insulated from the tube of the drop by the hard rub- ber piece, H, and in electrical connection with the terminal Pis. ii6. — A typical Switchboard I,ine drop ; the Tubular Drop of the Keystone Co. The iron tube contains an electro-magnet, to whose coil the current is admitted by the wires, as shown. The armature is hinged at the rear of the tube, and when it is attracted raises the longitudinal lever, thus releasing the shutter. Fics. T17.— Tubular Drop of the Couch & Seeley Co. The tube is drilled from a solid rod of iron, and by the attachment of the magnet core gives a double-pole effect at the armature. Connections of the night bell circuit are shown beneath the hinge of the shutteh wire, B, so that a current coming from the generator of a sub- scriber's apparatus through the wire. A, passes entirely around the jack into the coil of the drop, as in the form of switchboard just described. The plug used in a metallic circuit board is similarly compound, haying two metallic contacts, insulated from one another, instead of the one used in the grounded cir- cuit board. Fig. 112 sliows the construction of a plug of this description. Here we , have the ball-pointed tip, A, as in the other type of plug, for the purpose of engaging the leaf spring THE TELEPHONE SWITCHBOARD. 133 of the jack, and cutting out the circuit of the drop coil. A is enclosed in a hard rubber tube, E, which acts as a bushing to insulate it from the other contact metal part, the sleeve, C, which, in turn, is enclosed in the rubber handle, F. The contact, A, is attached to one wire, G, of the conducting cord by a screw at B, and the other contact, C, is similarly attached to the second cord wire, H, at D. Circuits of a IVIetaiiic Switchboard. — When the double contact plug is inserted in the jack of a metallic circuit switch- board, it closes a circuit having one terminal at F, in connee- FiG. 118. — Diagram of Circuits of a Metallic Switchboard (Fig. lotj). 5*. D.^ Sub- scribers' Drops; C. O. D.-, Clearin^-out Drop; Key at right of C O, D., Ringing Key; A, K.^ l^istening Key; /*. /, Calling Plug; P. 2, Answering Plug; R^ Receiver; M, Transmitter ; /», Primary of operator's induction coil ; S., Secondary ; ff, Generator. tion with the wire, A, in Fig. iii, and the other in the tube of the jack, at J, through the wire, C, held in metallic contact with the jack by the screw, as is shown. Thus the speech- bearing current enters at A, and returns to line at C, the current being continued through the two wires of the plug cord, and completed in the operator's table apparatus, her talking set, or the line and apparatus of another subscriber, in a manner similar to the system previously described. Metallic Switchboard '. Operator's Circuits. — The cir- cuits of a metallic circuit switchboard are shown in Figs. 118-119. Here, as in the former figure, the inner is the "answering plug," to be inserted in the jack of the calling subscriber, and the outer the "calling plug," which is intended to be inserted in the jack of the subscriber who is called for. As ^34 A B C OF THE TELEPHONE. Fio. 119.— Circuits of a Metallic Switchboard, showing the mechanisms for opeiatine the talking and calling connections. This figure illustrates the wiring of a Sterling Metallic Switchboard, which will be explained later. may be seen, there are two wires from each plug, corresponding to the double contacts, tip and sleeve, of each plug. They may be traced through the flexible cords, which are held down by THE TELEPHONE SWITCHBOARD. 135 the pulley weights, when not in use, and terminate in the springs projecting from the bottom of the combined "listening and ringing key," as shown in Fig. 119. The construction of this and other switchboard keys will be explained later, but it is intended to accomplish the self-same results as are accomplished by the three keys, C, D and Z, in Fig. 110 of the grounded circuit board. Operation of the Switch Key. — By the use of the lever of this key, the circuits of the two plugs, or of either of them, may be changed, as in the other type of board, and, by the use of the two press keys, on either side of the lever, the ringing generator may be thrown into the circuits in precisely similar fashion. These results are accomplished, as may be readily understood, by the fact that the circuits of the ringing genera- tor, of the operator's telephone set and of the clearing-out drop, also terminate in the key, as indicated. This mechanism enables the same program to 'be followed as in the grounded circuit board, after the answering plug has been inserted in the jack of the calling subscriber, to wit : i. The cutting-out of the drop; 2. The throwing-in of the operator's talking set; 3; The inser- tion of the calling plug in the jack of the called subscriber; 4. The making of the ringing circuit with the apparatus of the called subscriber; 5. The connection of the two lines with the clearing-out drop bridged on their circuit. The difference to be constantly borne in mind, in comparing the metallic with the grounded circuit switchboard, is that the latter must be so wired as to permit of all circuits made ending in a grounded connection, either at the exchange or at the sub- scriber's apparatus; the former is so constructed that all circuits, short or long, are composed of two metallic lines throughout, permitting every current used to emerge from and return to its source direct. CHAPTER ELbVEN. THE OPERATOR'S SWITCH KEYS AND TELEPHONE SET. Cpmbined Listening and Ringing Keys. — Since, in order to meet the needs of every calling subscriber, as we have seen, the operator must perform no less than seven distinct acts in the way of shifting and changing circuits, it has been the con- stant effort of inventors and manufacturers to produce devices to simplify her work, by allowing the greatest number of results to be accomplished with the fewest movements of her hands and arms. The object is, not only to save the operator's strength, but also to economize time in the " rush hours " at an exchange. Thus it is that a great variety of keys has been devised, on almost as many different principles; and attach- ments for making the operator's listening and ringing circuits have also been combined with the plugs and jacks. It has been found, however, that an experienced operator can do the neces- sary work with the ordinary devices, quite as readily and easily as with some others intended to save her work. The Coolc Switcli Key. — The key shown in Fig. 120 is of the form known as the Cook key, invented by Frank B. Cook, and manufactured by the Sterling Electric Co., of Chicago. The interior view of this kind of key is shown in Fig. 121, which also shows its details and operation. Its mechanism consists of five pairs of metallic spring contacts, a hard rubber insu- lating partition running in the length of the case, serving to divide it into duplicate halves — that is to say, one of each pair of springs is set on either side of the partition. The arrangement may be understood from the exterior view of the key. 136 LISTENING AND RINGING KEYS. m Its Construction. — Turning, now, to consider the details shown in Fig. i2i, we find that the lever, A, ends in a cam, B, turning on its pivot, and so shaped as to work on the two springs, C and Z>, as shown. This cam extends through the hard rubber partition, so that the changes effected on one side Fig. 120. — Cook I^istening and Ringing Key. A hard rubber partition runs in the length of the case, so that it divides the instrument into two equal parts ; one £, one C, one (?, etc., being on either side, making double connections, as shoirn. are also effected on the other; thus, in studying its operations, we must bear in mind that there are two springs C, two D, two E, two P, and two G, each pair, one spring being on each side of the partition, forming the terminals of some particular circuit. The pair of springs, E, terminate in the tip and sleeve of the answering plug; the pair F, in the tip and sleeve of the calling plug; the pair C, in the coil of the clearing-out drop; the pair G, in the magneto-generator. The operator's desk telephone set is connected with the springs, D D, opposite to C C. The pair E, and the pair F, being always in contact with the pair of double springs, H, which are connected together, as shown by the upper dotted line in the diagrams, and make a permanent circuit between the tips and sleeves of the two switchboard 138 A B C OF THE TELEPHONE. plugs. The iever being pressed all the way to the left, the coil of the clearing-out drop is bridged into the plug circuit, as is the case when two subscribers are conversing. Thus at the completion of any conversatien the key mechanism is in position for another call without alteration. Its Operation. — When a subscriber calls, the operator inserts the answering plug connected with the pair of springs, Di«.jra,7n Due. Lult1ini3 In. llM.gra.m Iwo. Circuit MftdLe. Fio. 121.— Diagram of interior construction and operation of the Cook Key showing the relative positions of the cam and springs on the (operator's) left side of the insulating partition within the key case. ' The continuity of the pair of springs, /f/f, is indicated by h. Continuity of Gand two outer springs, \>y g. The pair, E, and the-pair, F, on either side of the partition, are always in electrical contact with the double pair, HH. E, into the jack, and by moving the lever into the position shown in the first diagram of Fig. 121, bridges in her speaking set, so as to make a circuit with the apparatus of the calling subscriber, through the jack, from the tip and back again to the sleeve of the plug. Having learned the number of the sub- scriber desired, she inserts the plug connected with springs, F, in his jack, and presses key, K, thus forcing the pair, F, out of their normal contact with the double pair, ZT, and into contact with the pair, G, thus throwing in the ringing generator, which LISTENING AND RINGING KEYS. 139 sends a current along the line to his apparatus, through the con- ducting cord, plug, jack, line wire, his ringing apparatus, and back to the switchboard generator. This done, she releases the key, K, thus restoring the springs, F, to their normal contact with the springs. If, and breaking the generator circuit; at the same time bringing the lever. A, to the opposite position, as Fie. 122.— The O'Connell Switch Key. First diagram, normal and talking ; second diagram, listening in; third diagram, ringing up the called subscriber. By compressing the plunger to make contact of EB and KK, both subscribers are called. shown in the second diagram of Fig. 121, thus making a circuit between the apparatus of the two subscribers, and bridging in the clearing-out drop, connected to the pair, C. As soon as she has ascertained that connection has been es- tablished, she moves the lever to the position of diagram one, its normal position, thus cutting out her set, and leaving the two subscribers to continue their conversation. If, at any time, she wishes to "listen in," in order to find whether they are done talking, she can do so by bringing, the lever to the opposite position, as shown in the first diagram of Fig. 121. Whenever 140 A B C OF THE TELEPHONE. she has occasion to call up the calling subscriber, as, for example, when there is a delay in making connection with the subscriber called, both plugs being in the jacks, she can do so by depressing fto. 123.— Combined Listening and Ringing Key of the Keystone Electric Telephone Co, the key, Z, thus throwing the pair of springs, E, out of contact with H, into contact with G, and sending a ringing current to his apparatus. The conversation finished, and the shutter of the clearing-out drop having fallen, she has only to close the latter, , and remove the plugs, leaving the lever in the position in which it was found. By the use of this key, all the operations, except the insertion and removal of the plugs, can be performed by the operator, with her hand resting on the lever of the key. The O'Connell Switch Key. — Another form of switch- key, which has been in extensive use on metallic circuit switch- LISTENING AND RINGING KEYS. 141 boards, is the O'Connell key, shown in Fig. 122. Its advantage lies in the fact that all necessary shiftings of connection may be performed by depressing or raising a single push key, A, which operates the suitably shaped wedge of hard rubber mounted on the rod, C, which in turn, moves through slots in the top of Fw». i24.-*-Comtmed I^istening and Ringing Key of the Western Telephone Construction Company. the switchboard table, and in the piece, D. On either side of this wedge, and in such positions as to be engaged by it, are three pairs of springs', EE, FF and GG. The pair, FF, are connected with the tip and sleeve of the answering plug; the pair, EE, with the tip and sleeve of the jailing plug ; the pair, GG, attached to D, close the circuit of the operator's talking and Hsteriing set. The two pins, J J, placed outside of E and E, are the bus bars of the switchboard magneto-generator. The springs, EE, bear the rollers, HH, in order to facilitate the movement of the wedge on the springs. Its Operation. — The normal, or resting, position of the instrument is shown in the first figure. Here we find the 142 A B C OF THE TELEPHONE. springs, FF, in contact with the springs, EE, thus leaving all connections ready between the terminals in the two plugs, through the coil of the clearing-out drop. The two springs, GG, rest against the smallest portion of the wedge, as shown, thus leaving open the circuit of the operator's telephone set. \ 3 ^^^ FiS. 125.— Combined I,istening and Ringing Key of the Couch & Seeley Co. As soon as a call has been received, and the answering plug has been inserted in the jack of the calling subscriber, the operator presses the key so that the wedge is forced down sufficiently to permit the next larger contact to engage the two springs, GG^ thus throwing in her telephone set, for the purpose of learning the number desired by the calling subscriber. This done, she Inserts the answering plug in the jack of the called number, LISTENING AND HINGING KEYS. t43 and, in order to call up, again presses the key until the springs, GG, ride upon the third contact surface of the wedge, and the rollers, HIT, on the springs, EE, move upon its widest portion. The latter action forces the springs, EE, into contact with the bus bars, /J, of the generator, breaking the normal contact between EE and EE. Consequently the ringing current is sent through the calling plug to the apparatus of the called sub- scriber, the circuit of the calling subscriber being meantime cut out. If, at any time, there is a delay on the part of the called subscriber, and it is desirable to call the calling subscriber, when connection is made, both at once may be called by press- ing the key downward until the rollers of EE ride on the extreme edges of the wedge, £. This act forces the springs,. EE, still further back, and brings them into contact with the pins, XK, which are in insulated connection with the two other pins, LL, against which the springs, EE, rest, after the pressure of EE has been withdrawn by the downward movement of the wedge, E. Thus the generator current flows from J, through E, to X, thence to L and E, through the cords, plugs, jacks, lines and ringing bells of both subscribers, and back again, on the other side, to the generator. As soon as connection has been established, the operator depresses the key to the position shown in the second diagram, thus establishing a talking circuit between the two subscribers, while still keeping in her own telephone set, as shown, so as to listen in. Having ascertained that all is right, she moves the key again to the position it held before the call came, thus leaving the line free for conversation without the bridging-in of her own set. This is the final position of the key. The Couch and Seeley Key. — Fig. 125 illustrates the self- restoring, combined listening and ringing key recently intro- duced by the Couch & Seeley Co., of Boston. Unlike the keys just described, the operating lever. A, is automatically restored by the force of the spiral spring shown in the cut. Like the Cook key, however, it has five pairs of contact springs, which 144 A B C OF THE TELEPHONE S3 to p Fig. 127.— Operator's Headgear and Receiver. en bd n . ft " >• Fig. 128.— Ericsson's Breastplate Transmitter for a Switchboard Operator. LISTENING AND RINGING KEYS. 1 45 form the terminal connections. Thus C and C connect to the operator's telephone set ; B and B, directly back of them, to the clearing out drop ; E and E, to the generator ; D and D to the answering plug ; Z)^ and D^^ to the calling plug. The operation is as follows: On receiving a drop-call, the operator inserts the answering plug in the corresponding jack, at the same time throwing the lever to position i, thereby switching in her telephene set. Having learned the number of the subscriber called for, she inserts the calling plug in his jack, at the same time depressing the key to position 2, thereby throwing the generator into line and cutting out her talking set. The lever restores itself automatically from position 2 to position i, and only a slight additional touch is required to restore it to the normal position, A. Other Switch Keys. — The ringing and listening key of the Keystone Electric Telephone Co. operates on the same prin- ciple of positive contacts, but, as shown by Fig 123, it has two levers instead of one. These are, however, so constructed as to restore automatically or maintain the circuit, as desired. Still another highly efficient type of key, that of the Western Telephone Construction Co., is shown in Fig. 124. With it the listening connections are made by a lever, and the ringing connections by the push button. In this form of key the cam operates on the principle of sliding contacts. Switchboard Operator's Telephone Set. — The talking set of a switchboard operator consists of a receiver of the watch- case pattern, previously described, which is secured to her left ear by the head band attachment, as is shown in Fig. 127, and of a transmitter suspended in a convenient position before hei from the top of the switchboard cabinet. The attachment of this apparatus is shown in Fig. 126. Another form for attaching the transmitter, which is much used in Europe, is shown in Fig. 128. Here the transmitter battery circuit is made, and ready for talking when in the position shown; it is broken by the operator turning the transmitter on its hinge until the 146 A B C OF THE TELEPHONE. mouthpiece is against her breast. This form has the advantage of keeping the transmitter always in a convenient position for immediate use, saving the operator some inconvenient movements of the hand to hold the transmitter steady, as is frequently necessary when it is sus- pended on a cord before her. -r<^ ■fTnrrtnrnnnr ^-yr-^ Figs. 129-130. — Direct current electro-magnetic call bell of tlie type used on the switchboard night alarm. The figures show the difference between this and the polarized ringer of the telephone apparatus, as already described. The circuit is closed by making contact of the two terminals, after the manner of the push button here shown in section. The current then energizes the electro-magnet, causing it to attract its armature, but- before the hammer has sounded the gong the circuit is broken at the spring, C, the momentum of the hammer then carrying it forward the rest of the way. After sounding, it again springs back, thus making the circuit anew and being again attracted by the magnet. The Switchboard Night Bell. — In small exchanges at most times, and in large exchanges at night, during dull hours and on Sun- days, it frequently happens that the operator is absent from her seat before the switchboard, and, hence, unable to note the fall of a sub- scriber's drop. To remedy this difficulty a call-bell circuit is attached to each switchboard section, the gong being usually placed at the top of the panels, as indicated in Fig. 96. The wires are so arranged that the ringing circuit may be made by the shutter of each drop fall- ing open against two pins arranged directly beneath, as shown in Fig. 119, thus closing the circuit through the metal shutter. During the day, or in busy hours, the circuit connections of the night alarm bell LISTENING AND RINGING KEYS. H7 are broken by a switch. This renders the bell inoperative even when the drop shutter rests against the contact pins. Switchboards for Small Exchanges. — In a very small exchange, of loo or 200 subscribers, the services of one operator are usually sufficient. Thus ordinary switchboards, such as are shown in the figures, are made with about 100 drops. ~ In stations of 5,000 sub- scribers, or more, where there is a corresponding volume of business per subscriber, a number of such operators' ' ' positions, ' ' as they are called, of 100 each, are arranged in a long row, each operator attend- ing to the requirements of 100 subscribers. •1919191 ■-I9I9I9E Fig. 131.— Diagram of the " Eureka" transfer system. Each section, as shown, is con- nected to every other by a circuit. Plugging-in at any one position lights the lamps on the others ; thus making a call at the one having the calling signal^ and warning all other oper- ators that the line is busy. Large Exchanges: Transferring Calls. — If, in such an extensive exchange, one subscriber desires to communicate with an- other wired to a different position, under the management of a differ- ent operator, there are two ways in which the result may be accom- plished: either by the use of the device known as the "multiple switchboard," which will be presently described, or by some system of inter-communicating transfer. By one system of transfer each posi- tion is provided, not only with subscribers' drops and jacks, but also 148 A B C OF THE TELEPHONE. with drops and jacks corresponding to the other positions of the ex- change. When a subscriber, say, in position i, desires to communi- cate with one who has his drop in position 5, the operator at position 1 calls the operator at 5, and makes the talking circuit between the two subscribers by means of a combination of devices which will be fully explained in the proper place. Fig. 132.— Diagram of the " Eureka "plug circuit, with key connections lor cutting in or out a repeating coil ; according as the lines to be connected be grounded or Tull metallic. Trunking Connection. — For the purpose of " trunking out," or connecting with another exchange, a special section is provided in all switchboards, which is in communication with every other section in the same exchange. The system of intercommunication just men- tioned is, in general, fairly descriptive of the method. of connecting the subscriber in one exchange with the subscriber in any other. In the • telephone systems of large cities, each exchange has a number of connections with every other, in order to accommodate the vast trunk- ing business. CHAPTER TWELVE. IMPROVED SWITCHBOARD ATTACHMENTS. Labor-Saving Devices. — As the result of constant efforts to simplify the operations necessarily performed by the switch- board operator in making the circuits desired by subscribers, and attending to other business connected with the switching apparatus, a number of improved devices have been introduced by the various manufacturers of telephones and supplies. Among such may be mentioned subscribers' drops so arranged as to act as clearing-out drops as well, self-restoring call-drops, and combined drops and jacks. The advantage in every case is that the operator is saved a considerable expenditure of energy and time in having fewer points to observe in the course of her work. Thus, in the combined jack and drop, she has to insert the plug in an orifice indicated by the falling of the drop shutter, instead of searching for the jack bearing the same number. This is a desirable saving of time and nerve force. The same is true in the use of combined calling and clearing-out drops. Here she is saved the necessity of paying attention to more than one series of drops at a time, knowing in an instant whether the falling of a shutter means a call or a clearing out in each par- ticular case, and avoiding much of the delay due to pressure of work in the rush hours. Combined Calling and Clearing-out Drops. — In general, when the calling and clearing-out drops are combined in one instrument, the only chang'js of construction are in winding the drop coils to a higher resistance, to prevent the short circuiting of the speaking current; enclosing them in iron tubes, or cap- sules, to neutralize the influence of outside induction, and in arranging the lines between the drop and its corresponding jack on a different plan. As we have previously learned, the resist- 149 150 A £ C OF THE TELEPHONE. ance of the coil of the ordinary clearing-out drop is 500 ohms, or over, while that of the ordinary 'ailing drop is about 80 ohms. In order, therefore, that the requisite resistance may be inserted in the line, an arrangement has been adopted whereby the drop of the called subscriber is cut out of line, while that of the calling subscriber is left bridged in to act as a clearing-out indicator. The means adopted in the boards of the Sterling Electric Co. for accomplishing this result is to make the calling plug longer than the answering plug, in order that it may actuate the mechanism of the jack to a greater extent, and thus cut out its drop, after making the tip and sleeve connec- tions also formed by the answering plug. The Sterling Switchboard System. -^The system of wiring adopted in the circuits of this company's switchboard is shown in Fig. 119. As may be seen, the jack is constructed with three electrical connections: the line wire, ending in the upper leaf spring, the return wire, ending in the base of the jack; and the return wire of the drop coil, ending in the second leaf spring. Insulating blocks separate the two springs from one another, and from the base of the jack. To insert the short answering plug in the jack would mean merely to make tip con- tact with the upper spring, leaving the return connection for the drop circuit, and thus bridging the drop across fehe talking circuit formed by the tip and sleeve of the plug. To insert the long calling plug in the jack would mean to raise the lower spring into contact with the upper, thus short circuiting the drop coil belonging to that subscriber, and leaving line connections only through the jack. The Sterling Drop. — The switchboard appliances of the Sterling Electric Co. are interesting as combining a number of ingenious and effective contrivances, which deserve description. The tubular bridging drop is shown in Fig. 133. As maybe understood, its construction is different from that usually employed, in that the armature is swung midway on the enclos- ing sheath, which is constructed in three pieces, as shown in IMPR O VED SWITCHB OARDS. 151 Fig, 134. The forward cap being secured to the magnet core, derives polarity so soon as a current is sent through the coil, and attracts the pivoted armature with the usual result of releasing the shutter. The restoring mechanism consists of a lever fixed beneath each back of ten drops, and just above the jack panel. As may be understood by reference to Figs. 133 and 135, each drop has, secured back of the falling shutter, a " restoring slide," with flange at top an(3 bottom. Reference to Fig. 107, which shows a typical Sterling switchboard, will reveal the fact that the restoring slide of the topmost drop rests on the slide of the one beneath, soon down, and that the slide of Fl, >. 133. — sterling I^ine Drop, showing the centrally swung armature and the re- storing slide in front. Fig. 134. — sheath of a Sterling Drop, showing the three parts of which it is compof-ed. the lowest rests on the restoring lever, any movement of which will actuate the slides of the whole series, thus raising the shutters. By the attraction of the armature, the drop lever raises the slide sufficiently to release the shutter, which is held in its normal position by the upper flange. These drops are mounted in rows of five, as shown in Fig. 135. Sterling Jacks and Plugs. — Fig. 113 shows the Sterling jack, which is made of solid brass casting, with German silver springs and hard rubber insulation. Fig. 114 shows the short answering plug used. Reference to the cut of the board will show the excellent arrangement of the rows of plugs, which are " staggered," or so disposed that each plug in the rear row is opposite the space between some two of those in the front row, thus enabling the operator to select the short answering plug wichout inconvenient contact with the long calling plugs in front. 152 A B C OF THE TELEPHONE. The Self- Restoring Drop. — The self-restoring switch, board drop consists of two electro-magnets, set end to end, so that there is a magnetic action at both front and back of the instrument. The first magnet is actuated by the generator cur- rent from the calling subscriber, and is in all respects like the ordinary drop magnet, previously described, having a pivoted armature, attached to a rod extending to the front of the instrument, where a hook engages a drop shutter. Attraction of the hinged armature by the magnet causes the attached bar to rise, thus releasing the shutter from the hook, and allowing it to fall open. The variation comes from the fact that the Pio. 135. — strip of five Stertine Drops. This is the most usual method of mounting switchboard drops of all descriptions. mechanism which is the drop shutter in the ordinary coil is, in the self-restoring variety, the armature of the forward magnet, and itself controls the operation of another shutter hinged at the top. The current for the second magnet coil is supplied by a local galvanic battery in a manner to be presently explained. Its Construction and Operation. — Fig. 136 illustrates the mechanism and operation of the self-restoring drop. Here O is the operating magnet, such as is arranged in the ordinary type of drop. R is the restoring magnet, and B the common base- plate to which both are secured within an iron sheath. A is the armature of the operating magnet, and is pivoted at C, where it is also attached to the bar, Z), bearing the hook, H^ at its opposite end. This hook rests in a groove at the top of the shutter, Aa, which is pivoted on its lower end-at P, On the IMPROVED SWITCHBOARDS. 153 upper side of this same magnet is pivoted the indicator flap, P, in such fashion that it normally hangs directly in front of Aa. Now, when the generator current enters the coil of the operating magnet, O, it causes the armature. A, to be attracted, with the result that the bar, D, is raised, thus releasing th'j shutter, Aa, from the hold of the hook, H, and allowing it to fail outward on its hinge. By this movement the flap, F, is also forced out, and takes the position shown in Fig. 137, being held there by the weight of Aa. As soon as the plug is inserted in the jack corresponding to this drop coil, a current is sent through the coil of the magnet, R, whose core projects into a fecess in the center of the armature shutter, Aa. By the action Fig. 136. — Diagram showing the mechanism of a self-restoring switchboard drop. The figure shows the normal position of the drop shutter. of this current the shutter, Aa, is attracted to the pole of the magnet, R, taking the position shown in the first of the two figures; the hook, H, again engaging the notch at the top of Aa, and the flap, F, again falling to the first position by its own weight. So long as the current is continued through the coil of R by the presence of the plug in the jack, the shutter, Aa, is held fast, and any current coming to the coil of O, while it may attract the armature. A, and actuate the bar, D, cannot operate the drop. Such an attachment as this must greatly increase the com- plexity and original expense of a switchboard, but so needful is it that the duties of the operator be simplified as much as possible, that these matters are of very small importance in comparison. 154 A B C OF THE TELEPHONE. Circuits of a Self-Restoring Drop. — In the circuit arrangements used in connection with a self-restoring drop, the operating coil, O, is permanently bridged across the talking circuit, its high resistance and retardation serving to shunt the telephonic current onto the line, while, at the same time, it is always open to ringing currents from the hand generators in the subscriber's apparatus. The need for cutting out the coil of the drop is obviated by the fact that it is rendered inopera- tive, as we have seen, so long as the plug is in the jack, sending a current through the coil, H. Consequently the jacks are made with but the usual two contacts for the tip and sleeve of the plug, respectively. The form of plug used is the same as that 'Vhr" Fio. 137. — Shutter of a self-restoring switchboard drop, showing the position of the shut- ter after a call has been received. previously shown, for the ordinary metallic circuit switchboard, with the exception that it carries a metal ring, or collar, outside of and insulated from the sleeve contact previously described. This second sleeve, or insulated ring, is intended to close the battery circuit by connecting the two terminals of the restoring battery circuit, which are two thimbles at the entrance of the jack, so that, by inserting the plug in the jack, we not only make the talking circuit through the jack springs, but also allow the current from the restoring battery to flow through the wires to the inner thimble, across this outer sleeve to the outer thimble, thence to the coil of Ji, energizing the magnet and holding the shutter armature fast, as already described. The coil of R, being of low resistance, would greatly injure the battery, if connected direct to the wire coming from D, and to obviate this difficulty the needed resistance is introduced into the IMPROVED SWITCHBOARDS. I55 circuit by connecting the wire at the hinge of the drop shutter, ■ F. and allowing the current to flow through the shutter, the armature, Ad, -and thence through the hinge into the coil of the magnet. The circuit is completed through the earth and back to the restoritig battery. V'' Combined Drops and Jacks. — Another piece of improved switchboard apparatus, having the same object — saving the operator's time and simplifying her movements — is the combined drop and jack; which is to say, the two instruments made so as to occupy the same, space on the switchboard panel. The H G /^^^^S^^ Pio. 138. — Combined Drop and Jack of the Western Telepboue Construction Co. arrangement has the added advantage of saving room, and enabling a larger number of subscribers' drops to be mounted in a given space. Several types of combined drop and jack have been placed on the market, each having its particular points of excellence and advantage. Western Drop-Jack. — Fig. 138 represents the combined drop and jack manufactured by the Western Telephone Construction Co., of Chicago. Here the magnet, E, and the jack-piece, _/, the latter a solid brass casting, are secured to the base piece. A, of hard rubber. The armature of the magnet is attached to the bar, F, pivoted at P to the head piece, H, being, however, normally held away from the pole of the magnet by a small leaf spring, S, bearing against its length, which effectually prevents '56 A B C OF THE TELEPHONE. ."freezing" or "sticking" to the pole. When the instrument is not energized by a calling current, the bar, F, holds up the shutter, C, which in this cut is shown to have been thrown down by an incoming current. Cis hinged to a bar running crosswise Fig. 139.— a 300-drop switchboard of the Western Telephone Construction Co., equipped with the type of drop-jacks shown in the last figure. to the opening between B and B, and works up or down on the curved dotted line, as shown. When a current energizes the coil, the armature is attracted to the pole of the magnet, thus causing the bar, F, to move sidewise, sliding from under the shutter, C, and allowing it to fall into the position shown IMPROVED SWITCHBOARDS. I 57 in the cut. The fall of C gives signal to the operator, who then starts to insert the answering plug, with its tip pushing C upward on its pivot into the former position supported by F. The entrance to the jack is then made through the orifice, K, the sleeve connection being with _/, and the tip connection with the spring shown below. By the former connection, the spring, J, is lifted away from the anvil, /, thus cutting out the coil of the drop, as in the ordinary type of switchboard having drop and jack separate. Although not properly a self-restoring drop in the sense in which the term is generally understood, this instrument accomplishes the same result by the simple act of inserting the plug into the jack. It is, therefore, called a " mechanical self- restoring drop. " The bus bars of the night bell circuit run through the two side pieces, B and B^ one of them serving to hinge the shutter, C, and the other, running parallel, so as to be engaged by a lug, D, carried on the top of C, thus closing the circuit, when the wire connections are made with the switch previously mentioned. Fig. 139 shows a 300-drop switchboard containing this type of drops and jacks. As will be seen, the drop shutters are represented in the raised position, thus disclosing the opening to the jack apparatus shown at K in the previous figure. Connecticut Drop-Jack. — Another type of combined drop and jack, manufactured by the Connecticut Telephone and Electric Co., of Meriden, Conn., is shown in Figs. 140 and 141, the former showing the complete instrument with plug inserted in the jack springs, and the latter, the several parts. As may be seen, the drop mechanism works in the usual way, attracting an armature, and thus actuating a lever bai- with hooked end and releasing the shutter, F, which is normally held up by the hook and falls outward and downward on its hinge, when the hold is released. The line wires are attached to the two terminal pieces, / and I ; the hither wire being secured to the head piece of the coil by a screw, as shown, the further wire passing in the same manner, but being insulated from the metal parts by a 158 A B C OF THE TELEPHON^U. bushing of non-conducting material. The armature is hinged, as shown, by the screw pin, E ; the bar carrying the hook to engage the shutter bears a rubber insulation, A, which serves to cut off the current of the night bell circuit from the jack and drop parts. The entire instrument is held in position on the hard rubber panel of the switchboard, G, by the single nut, B, which engages the thread on the forward end of the jack tube. The magnet coil of this drop is a separately wound helix, and may be slipped off of the core and removed whenever necessary, Fio. 141.— Details of the drop-jack of the Connecticut Telephone and Electric Co. without disturbing the rest of the mechanism, the fiber disc shown at D sufficing to hold it in position in the tube. In action the drop coil is bipolar, the core acting as one pole and the iron shell, or sheath, as the other. The shutter is mechan- ically self-restoring, being restored to its normal position, in engagement with the hook at the end of the armature lever, by the simple act of raising it in order to insert the plug through the orifice into the jack springs behind. This brings it to the position shown in the first figure, which has the plug inserted and the shutter up and engaged. The tube of the jack has two contact springs, at top and bottom, as shown, to make positive tip and sleeve connections with the plug. IMPROVED SWITCHBOARDS. 159 % I •o 3 2 B n n •o " 3.H B 2. i6o A B C OF THE TELEPHONE. Operation of the Ringing Circuit. — The operation of this drop-jack shows it to be one of the most complete and ingenious switchboard contrivances on the market. The insertion of a plug between the jack springs bridges the drop coil and makes Pio. 142.— A 4DO-drop Switchboard of the Conuecticut Telephone and Electric Co., equipped with the type of drop-jack shown in Figs. 140-141. This board has four " trunk- ing " panels, as shown, giving a capacity for 600 drop-jacks. the talking circuit, as in any other type of jack and drop. In order to ring up a desired subscriber, the operator has only to push the plug further into the jack, thus making contact with the cross piece attached to the springs, H and H, causing it to slide backward in its slot, and bring these springs into contact IMPROVED SWITCHBOARDS. l6l with the tips of the line wires immediately behind. The vertioal strips, y and K, are in contact with the bus bars of the gene- rator, and each of them has a tongue cut out of its length, against which bear the points of the bars carrying the springs, Ha.n6.II, as shown in Fig. 140. By this contrivance the line wires of any particular subscriber may be bridged across the generator circuit, whenever desired. The plugs used in connec- tion with this instrument have a collar of insulating material midway on the shank, and upon this the line springs of the jack are caused to ride whenever the ringing connections are made, as described. After giving the necessary rings the operator releases the pressure on the plug, thus allowing it to ride back into metallic contact with the springs of the jack, as it is forced outward by a spiral spring enclosed in the tube back of the cross piece attached to H and H. Its Night Bell Connections. — The night bell connections are made by the springs, M, which engage an ear attached to the lower part of the shutter, F, thus closing the circuit, In addition to this, the stop hook below the shutter is connected to one side of the circuit, and the eyes that hold the trunnion screw are attached to the other, thus making double contacts for the night bell. This is an arrangement worthy of consider- ation, as giving additional assurance that the circuit will be made. Connecticut Switchboards. — Fig. 142 shows a four hun- dred-drop switchboard containing drop-jacks of the kind just described. AH shutters are in the raised position, showing orifice in each, for the insertion of the plug. The plugs are arranged in double bank, as with most up-to-date switch- boards, thus insuring greater ease in reaching and manipulating them. The switch keys are arranged at the front of the table. Fig. 143 shows the rear of a board of this company, ex- hibiting the method of arranging the drop-jacks. The induction coil of the operator's transmitter is shown secured to the top of the case, and below it the gong of the night bell. l62 A B C OP THE TELEPHONE:- Fig " 143 —Rear view of a 50-drop switchtmard of the Connecticut Telephone and Electricleo;, sjjowing forty drop-jacks installed. ': This cut shows the night bell gong and operator's induction coil at fhe top of the cabinet and the hand magneto generator below the shelf at the left. IMPROVED SWITCHBOARDS. 163 Couch and Seeley Drop-Jack. — The recently introduced drop-jack of the Couch & Seeley Co., of Boston, Mass., is shown in Fig. 144. It combines the advantages of simple construction and considerable strength. As will be readily understood, A is an iron tube, to the forward end of which is screwed the core of the magnet coil. As in the instrument just described, the open end of the core forms one pole and the tube the other, thus Fig. 144.— Combined Drop and Jack of the Couih & Seeley Co. making the magnet in reality bi-polar. The ends of the wind- ings of the magnet are brought through rubber bushings in the tube and soldered to the tags of drop springs Z) and D, which are in connection with line springs J, J, when the drop is in normal condition. The line terminals are shown at C, C. When a plug is inserted at B, into the jack frame, K, the line springs, /, J, are separated from the drop springs, D, D, thus cutting the drop entirely out of circuit. The shutter, E, has two spurs, one on each side, formed at right angles with its face, which, when it falls, come in contact with night alarm springs fastened to the side pieces, F, F. These side pieces are of brass, and carry the night alarm circuit to upright brass bars so notched out as to receive the drops when mounted up in regular form in a complete switchboard. The wiring of the complete board using this drop is, therefore, practically done when the drops are set in place. The armature, /, is provided with the counter- weight, G, by which a very close and sensitive adjustment can 164 A B C OF THE TELEPHONE. be made. It will throw the shutter on a bridging line with a 2 5 -ohm shunt, the current being furnished by an ordinary 3-bar generator. The Kellogg Drop- Jack. — One of the most compact and readijy operated of the combined drop-jacks recently introduced is the Kellogg, shown in Fig. 145. The mechanism is too simple to need elaborate description, as may be seen. As shown in the figure, the Fig. 145. — The Kellogg drop-jack. drop-coil is directly above the line-jack, the spring of which is ex- tended outward beneath the shutter, where a lug carries an insulated buffer. The shutter falls against this when the magnet operates, and to restore it, the plug is forced into the jack, raising the sprnig by means of its enlarged head, and causing the shutter to be lifted into place by the insulated lug. No other handling is required. CHAPTER THIRTEEN. SWITCHBOARD LAMP SIGNALS AND CIRCUITS. Lamp Signals for Switchboards.— In many exchanges line drops of all descriptions have been supplanted by small incandescent electric lamps as calling signals. These lamps are superior to drops in that they need not be "restored," the light being extinguished so soon as the proper connections are com- pleted, and also from the fact that they occupy far less room on the panel, a consideration of especial importance in the con- struction and operation of multiple switchboards of large capacity. On the other hand, they present the disadvantage of giving the operator considerable physical annoyance, which, in many cases, is hardly compensated by the time and labor otherwise' saved. Figs. 146 and 147 show, respectively, the usual size of such signal lamp, and the method of enclosing it in a suitably shaped cup or cell. Fig. 148 shows a row of lamps mounted on a strip for attachment to a switchboard. IVIethods of iVIounting Lamp Circuits. — There are several structural disadvantages involved in the use of lamp signals, and any exchange in which they are installed must be constantly inspected in order to prevent any line from being rendered inoperative by the burning out or damaging of its lamps. As a usual thing, it has been asserted, switchboard signal lamps may be flashed at least 1,000,000 times without serious damage. But, on the other hand, and particularly when the- lamp is included in the line circuit, any outside electrical disturbance, such as will increase the strength of the current unduly, will cause the lamp to burn out much sooner. Usually, therefore, the circuits of signal lamps are made separate from the main line, and depend for operation on a series of circuit-closing relays, as will be presently explained. 165 1 66 A B C OF THE TELEPHONE. Lamp Signals on the Main Line. — The most typical method of attaching the signals to the main line, between the wires of the subscriber's metallic circuit, is, briefly, to bridge the jack between the line wires and attach the lamps in series to one limb of the circuit. Each terminal of the line then passes through an impedance coil, or long-wound magnetic induction resistance, and is connected to either pole of a battery. This battery, while the subscriber's apparatus is in normal condition, is prevented from illuminating the lamp signal by the high resistance of the call bells, which are usually wound to a resistance of i,ooo ohms. So soon, however, as the subscriber removes his receiver from the hook, thus making the telephonic circuit, the battery current finds a ready path in series through the receiver and the secondary of the induction coil, a line of far smaller resistance, and immediately flashes the signal. Of course in such an arrangement as this the magneto-gene- rator of the subscriber's apparatus is omitted, and the central energy exchange system adopted to the extent of deriving signal power from "central." The subscriber's sole act in signaling is to remove his receiver from its hook. The fact, however, that the central signal battery is always on closed circuit through the impedance coils at both terminals of every line, a further advantage niay be derived from including both the station bells and a single-cell storage battery, of the general type already explained, in a permanent bridge between the two limbs of the circuit. Thus the local storage battery is con- stantly being charged to its full capacity while the telephone circuit is opeh; but, if at any time it become exhausted, and the line be brought into use before charging is complete, the transmitter may derive sufficient current from the central battery, through the bridge to one terminal of the induction coil primary on the one side, and through the battery and switch hook contact on the other, to operate with good effect. The connection of two subscribers' lines through a plug pair operates to extinguish the signal lamp by introducing a sufficient resistance to render the battery incapable of maintaining the LAMP- SIGNALS. 167 1 68 A B C OF THE TELEPHONE. light. At the same time the impedance coils act to balance the lines connected at the bridged jacks. Lamp Signals Operated by Relays. — The second method of arranging circuits operating switchboard lamp signals is by a sys- tem of relays and subsidiary battery circuits. The most typical sys- tem, the invention of C. E. Scribner, of .Chicago, is briefly as follows : The two limbs of the line, attached respectively to the two springs of the switchboard jack, terminate, the one in a ground connection, the Fig. 148.— Diagram of a typical switchboard circuit using incandescent electric lamp Signals. ^ IS the subscriber s apparatus ; B, the circuit breaking hinged armature of relay C; D, the relay closing circuit of the signal lamp through its armature? .ff. Other in a wire common to all lines and containing a grounded bat- tery. As in the. type of circuit already described, each subscriber's apparatus has the bell magnets on a permanent bridge, which also con- tains a condenser, generally of about .75 microfarad. The introduc- tion of the condenser practically breaks the line circuit, so far as the LAMP SIGNALS. 1 69 direct current central battery is concerned, although presenting no ob- stacle to the alternating current from the calling magnets. It would thus permit the passage of the telephonic current, also alternating, were it not that the coils of the bell magnets were wound to a high resistance and impedance for the express purpose of preventing this short-circuiting of the talking current. As soon, however, as the receiver is removed from its hook and the telephone circuit is made, the current from this central battery finds a path of low resistance through the receiver coils and the induction secondary, and is able to energize a relay connected in series with the line limb which is joined to its positive pole. This relay attracts its armature, a pivoted bar having a grounded connection, and brings it into contact with an anvil attached to one terminal of the lamp circuit. The other termi- nal of this lamp circuit is attached to a wire, common to all the lamps on the board, and carrying a battery with a grounded connection. Therefore, so soon as the relay attached to the line circuit attracts its armature, it makes the circuit through the lamp from the ba'ttery just mentioned and to ground through the pivoted armature, thus causing the proper lamp to be illuminated as a calling signal to the switch- board operator. The lamp then continues lighted until the answering plug is inserted in the jack of the calling subscriber. Jack and Plug Circuits. — The act of inserting the plug in the jack causes the lamp to cease burning, as follows : Each jack has three contacts, a spring for tip, a spring for sleeve, and a thimble con- nected to a third relay, which is grounded. Each plug has the usual tip and sleeve strands, and, in addition, a thimble contact, insulated from both tip and sleeve, and connected through a battery to ground. The insertion of a plug in a jack, then, makes the usual line contacts, and also makes the circuit of the third relay, just mentioned. The armature of this relay consists of two hinged metallic contacts, which normally bear on two anvils, one at the terminal of each limb of the subscriber's line. Consequently, as soon as the relay is energized the line connections to ground and through the first-mentioned battery are cut off, and the jack springs are left as the sole terminals of the line. Moreover, as the relay continues to hold its armature so long as the plug is in the jack, it is impossible that the lamp be relighted before 170 A S C OF THE TELEPHONE. the conversation is completed. Associated with the Une signal lamp is a system of pilot lamps, actuated by yet other relays, to assure the operator's attention ; but this feature need not be noticed here. Figs. i4Sa-i48b.— Rear and front view of a switchboard using incandescent electric lamp signals. Both these arrangements of lamp signals belong properly under the head of the exchange battery, or central energy systems, with which they are best adapted to operate. CHAPTER FOURTEEN. THE MULTIPLE SWITCHBOARD. The Requirements of Large Exchanges. — The various forms of one or two section switchboards described and illus- trated in the previous chapters are intended to operate on sys- tems having only a few subscribers — say 200 or 300 — and capable of being managed by one or two operators. When there are only a few hundred subscribers wired to an exchange it is a comparatively simple matter to make the required connections, by reaching over with the cord of the calling plug and inserting the plug in the required jack, even when it is in the section of another operator. As may be readily surmised, however, such a procedure, while perfectly satisfactory in exchanges where there is very little business, are utterly impracticable when the number of subscribers has reached into the thousands, and each operator works to her full strength during certain hours. Here it is necessary to have some means whereby an operator can, with the ease and speed of handling her own 100 or 200 drops and jacks, make any desired connection, even into the thou sands. Otherwise, in order to meet the requirements of rush hours, the number of operators would have to be far greater than one to each hundred subscribers. The Arrangement of Multiple Jacks. — The device adopted to meet these requirements and, in a great measure, overcome the necessary difficulties of handling a large number of subscribers, is called the multiple switchboard. This name is strictly descriptive, and indicates the peculiarity of the system adopted, whereby each subscriber's line terminates, not in one jack merely, but in a number of jacks equal to the number of switchboard sections in the exchange. Thus each operator has 171 172 A B C OF THE TELEPHONE: Fig. 149. — A Typical Multiple Switchboard. One section of a 3600-line Multiple Board of the Stromberg-Carlsson Co., equipped with "visual signals," line drops of the type shown in Fig. 162, and operated by full central energy at the exchange. The left-hand position is left uncompletec"., but, as may be seen, every line is "raultipled" in the three positions shown, so that the middle operator may reach any subscriber on the line, either on her own panel or on the one to her left or her right. Below each multiple panel may be seen the usual loo " answering jacks " having drops in that position. THE MUL TIPLE SWITCHBOARD. 1 73 on the panel before her, not only the jacks corresponding to her own subscribers' drops, but jacks connecting with the lines of every other subscriber in the exchange. Where there are as many as 5,000 or 6,000 subscribers, as in the largest exchanges it is obviously impossible that so many jacks should be arranged in any one panel; consequently the number is arranged to occupy, say three panels, so that every number of the 5,000 or 6,000 is repeated at every third panel. Thus each operator, either by inserting the plug in the jack of required number on her own panel, or by reaching with the cord of her calling plug to the panel on her left or to the one on her right, can make any desired connection out of the many thousands registered in that exchange. This arrangement is shown in Fig. 149. Varieties of Multiple Switchboard. — The method of arranging the wiring of a multiple switchboard so as to accom- plish this multiplication of the jack connections of each sub- • scriber is, briefly, to run each line the whole length of the switchboard, instead of having it end at the panel where its drop is fixed, and to "tap" it for jack connections at every panel, or every three panels, as just explained. There are two ways of attaching the jacks on a line — either in " series " or in "parallel." The latter method is known, in telephone par- lance, as the "branch terminal" wiring, also the "three-wire system," and is, briefly, the method of bridging the circuit between the line and return wires of each subscriber over as many jacks as the system requires. It is the system now adopted in most up-to-date exchanges. The Series IVIultiple Switchboard. — In Fig. 150 is shown the plan of wiring the jack and drop connections of a series- wired, metallic circuit multiple switchboard of 300 drops. Three out of the three hundred subscribers' lines are shown, number 45 having the drop in the first section, no having the drop in the second section, and 216 having the drop in the third section. Thus, Section I has subscribers' drops i to 100; Section II, drops 101 to 200; Section III, drops 201 to 300. Each drop 174 A B C OF THE TELEPHONE. THE MULTIPLE SWITCHBOARD. I 75 has the answering jack to correspond, and the operation of receiving a call and connecting a subscriber is the same as that in the standard type of switchboard already described. In addition to the 100 drops and answering jacks belonging to each section, however, there are also 300 multiple calling jacks, representing, as shown by the above figure, every subscriber's line which is wired to the board. So in case number 45, in the first section, wishes to converse, with number 216, in the third section, there is no need to communicate with the operator at three, but siniply to insert the calling plug in the multiple jack belonging to line number 216, which is wired to Section I, as shown. Testing Arrangements of a Multiple Switchboard. — In order to attain the end for which a multiple switchboard i? designed — the placing of calling jacks for every subscriber, even to 5,000 or 6,000, within reach of every operator in the long row of switch panels — it is necessary that there should be some ready method for determining, at any section of the 50 or 60 in the row, whether any one line is engaged or not. To accomplish this end a still further complication of machinery is necessary. An extra battery and a grounded circuit are bridged between the sleeve strand of the calling plug and the operator's telephone set, for the purpose of testing the lines, in the manner to be presently explained. Test Circuits of a Series Switchboard. — Fig. 154 shows the sleeve, test and listening connections of a series metallic multiple switchboard. In order to avoid the bewildering com- plications, unavoidable in the use of the ordinary diagrams, the circuit terminals are represented as connected with a type of listening and ringing key, which has already been fully explained, both as to its construction "and operation. In this figure the important points are the sleeve strand and the listening circuit. Consequently the other connections of the switch key are merely indicated. Here A is the wire connected to the answering plug, and C, that connected to calling plug, of a given pair, the tif ij6 A B COF THE TELEPHONE. THE MULTIPLE SWITCHBOARD. 177 and sleeve strands being connected to the key, as already explained. To the sleeve terminal of C is connected a battery of cells, B, connected to ground at X. As they are permanently bridged across the sleeve strand of the plug circuit, this battery and ground connection would undoubtedly unbalance the line, with the probable result of short-circuiting the talking current with some other connected line, were it not that the impedance "1. V///?///A, \ 1 « t s i I s a a H 1 -; a « u 4 u P P £ S g. - V/////////A * etHtCHtCK BMtVmri - T« OKRATaR) > TFLFPHONr. A — ft ji mm \ \ \j p _ IfACTANCCMlk 1 , Y//m JR ^B Fig. 154. — Diagram of the test circuit of a series multiple switchboatd, simplified by using a familiar form of switch key. or reactance coil, R, is placed as shown. This coil offers a much higher resistance than the telephone line, and, as a result, effect- ually prevents all shunting or interference. Consequently when the cord circuit is connected, and the battery, B, left in 1 78 A B COF THE TELEPHONE. connection with the line, as it is easy to see must be the case, the working of the telephonic current is in no way interfered with. The battery, B, however, is intended to act only when some other operator's telephone set is bridged into line with the tip of her calling plug, as will be afterward explained. Inter- posed on the line leading to the operator's receiver will be seen the condenser, D, which is intended to prevent disturbances in the line from producing conditions liable to give a false " busy" test. Furthermore, the secondary winding of the induction coil is split into two parts, one end of each being connected to line with the plug strands, tip and sleeve, the others with the pole coils of the receiver magnet. The ground connection shown at the receiver leads from the center point of the magnet coils, so that such electrical impulses can be run from the battery, B, through the condenser, Z>, pass through one side of the secondary winding of the induction coil, and through one side of the magnet coils to ground. Method of Testing. — By reference to Fig. 150 it will be seen that the series wiring to the. jacks is made only from the line wire, the return wire being strung parallel to it along the entire length of the board, and connected "in multiple" with the thimbles provided to give the sleeve contacts of the plugs. The effect, therefore, of inserting a calling plug in a multiple jack is to connect the thimble, or sleeve contact, of every other multiple jack on that subscriber's line with the battery, B, through this return, or test, wire. As soon as a circuit is formed to ground from the battery, B, by making contact with the thimble of any multiple jack in a plugged line, an electrical effect must follow. The method of making the testis as follows : Suppose thai subscriber 10, in Section I, wishes to communicate with subscriber 326, in Section IV, he turns the crank of his apparatus generator, thus calling the attention of the operator at Section I, who inserts an answer- ing plug in his answering jack. Having ascertained his wishes she' must make a test to discover whether line 326 is plugged at THE MULTIPLE SWITCHBOARD. I 79 any other section of the switchboard, and to do so holds her telephone set on circuit, and touches with the tip of the calling plug, belonging to the same pair, the thimble of the multiple jack of subscriber 326 on the panel in front of her. If sub- scriber's line 326 is engaged, she will, receive the "busy test" as follows: The tip of the plug, touching the thimble of multiple jack 326, will close a circuit with the grounded battery at 326, Section IV, through the test wire, as shown to the thimble of the multiple jack, numbered 326, in Section I; thence through the tip strand of the - plug, through the condenser, D, through one-half of the secondary winding of the induction coil of operator. Number I, through one pole coil of her magnet receiver, through the ground wire connected to the middlepoint of coil wire, to earth and to the ground connection of the test battery in action at Section IV. The condenser, D, being placed in the line, while preventing the transmission . of a continuous current, will discharge' into circuit with the result of giving a sharp clicking sound in the operator's receiver, thus informing her that the. line is busy at some other section. If the line is not busy, no result whatever will follow the, act of touching the thimble of the multiple jack with theiplug. tip, and corfnections ■ miay be made at once.' • The Use of the Condenser in a Test Circuit.^While the electrical condenser, like the'- induction coil, forms a complete break in the circuit, so far as a continuous current is concerned, it is useful in a variety- of ways in both telephony and teleg- raphy. Even though the plates composing ' a condenser of ordinary pattern be insulated from one another, the rapid variations and reversals of the telephonic current produce an inductive action between them, and allow the transmission of speech with ease and perfectness. It is also the device best suited to the test needs of a multiple switchboard of the series type. Defects of a Series Multiple Switchboard. — The series multiple switchboard is open to criticism in a number of particu- i8o A B C OF THE TELEPHONE. pio i« Couch & Seeiey's 200-drop multiple switchboard. This cut well illusttates the multiple system, showing 400 jacks to 200 drops, or each line carrying two jacks, thus avoiding cross-connections. THE MULTIPLE SWITCHBOARD. l8l lars, most prominent among them being the constant liability to disarrangement of a line by a particle of dust or some foreign substance lodging between the springs of a multiple jack and insulating them, and the difficulty in using self- restoring drops on the lines, as would be a most desirable addition in the complicated machinery of a multiple switchboard. p£„^ Jvt»*v&c^ -* AlJUi^ Ac/r. Fig. 156. — Diagram of one circuit through three positions in a branch terminal multiple switchboard. To remedy these and other grave difficulties, the other type of multiple board, the parallel, or multiple-wired, branch terminal, or three-wire board, was devised. While much more compli- cated in some respects, and requiring more appliances for its successful operation, this type of switchboard is coming into more extended use. The Branch Terminal Multiple Switchboard.— Fig. 156 shows the wiring plan of one type of branch terminal multiple switchboard. For convenience, only one line is shown through its multiple connections in three sections. Here are the two line wires, as in the series board,, marked Z and E; also a third, the test wire, T, running parallel to the other two through 1 82 A B COF THE TELEPHONE. the whole length of the board, and the common ground wire, G, which, unlike the test wire with which it is connected when any test circuit is closed, is common to all the jacks in the board, belonging to all subscribers whatsoever. The jacks of this type of board have five contact points — three springs and two thimbles, as shown. The thimble, A, is connected to the wire, L; the short spring, B, to the wire, R ; the spring, C, and the test thimble, D, to the test wire, J", and the spring, E, to the common ground return wire, G. Self -restoring drops, of the type already described, are used with this board. Thus, as may be seen, the insertion of a plug in any jack closes two circuits — the telephonic circuit, on metallic line, and the drop- restoring circuit, on grounded line ; besides lea,ving the grounded battery connected to the test thimbles, Z>, ready to give the necessary "busy" test at any section of the board, in the manner already described in connection with the series multiple board. In order to make those connections, the plug of a three-wire switchboard must be of slightly different construc- tion from the ordinary type. This is shown in Fig. 156, where is represented a plug inserted in a jack of a three-spring variety. As may be seen, the tip of the plug registers with spring B ; the sleeve with thimble A j the springs, C and E, are forced apart by the entrance of the plug, and are held in elec- trical connection by the metal collar, X, which is fitted over the shank of the plug, as shown, but separated from it by an insulating ring. The test thimble, Z>, is made sufficiently wide to permit the insertion of the plug without making contact. The operation of the speaking circuits of this type of board is the same as in all the types of boards previously described. The circuit of the self-restoring drop is made, as shown by the connection of the two springs, C and E, instead of by the connection of two thimbles, as in the standard type of board, when strung for these drops, as already described. The drop- restoring and test battery, V, is, like the ground connection, G, of the test wire, common to all subscribers' lines. THE MULTIPLE SWITCHBOARD. 1 <5 ■• The Branch Terminal Testing Circuit. — The test circuit of a branch terminal board is constructed somewhat differently from that of a series board, but the manner of making the test is the same, the "busy" signal is the same, and, as in the other type of board, it is switched into operative connection when the table telephone set is bridged into line. Fig. 158 shows the general features. Here K is the switch key, already described ; A and C, the connections to the answering and calling plugs. Fie. 157.— Diagram of the multiple switchboard shown in the frontispiece. L is the line ; /i^ Intermediate Field ; O, Table-Key ; J//, Multiple Jack ; 5 /", Speaking Plug ; .AT.ff, Night Bell ; Z,y, Home Jack ; .R />, Rir.gmg Plug ; K, Busy-test Coil ; 5 /T, Subscriber's Drop ; I. (venerator ; L B, Test Battery ; A K, Clearing Out Drop ; M, Microphone ; MB, Microphone Battery ; T, Telephone. It will be seen that the jacks used on this board are of the type shown \t\ Fig. i6o, and that the insertion of a plug throws the battery current into all the multiple thimbles, the sleeve spring, when actuated closing connection with the test battery at L B. respectively; Z>, the terminal of the clearing-out drop, here of the self-restoring variety; G, the terminals of the ringer gene- rator; and T, the terminals of the operator's talking set. As will be seen, the wires leading from D are. connected direct to the operating coil of the clearing-out drop, which is permanently bridged between the plug cords when talking connections are made. As in the .test circuit of a series board, the secondary winding of the operator's induction coil, J, is divided into two halves, two ends of which connect to the terminals at T, and the other two to the poles of the receiver. Here, also, the 184 A B C OF THE TELEPHONE. ground connection is made at the middle point of the receiver coil wires; with the marked difference, however, that the battery, B, is interposed, thus making the test battery circuit in this board the reverse of that in the other type. Of the two terminals of T, one is connected by a branch wire, as is shown, direct to ground through the common ground wire, Y, thus making the two terminals of the test circuit, from ground at X^ through battery £, to the middle point of the receiver coil, through one-half of the coil, through one-half of the secondary winding, at _/, to ground at Y. Making a Test. — In order to make the "busy" test the operator works the key, as formerly explained, so as to bridge her talking set across the strands of the calling plug. She then touches with the tip of this plug the thimble of a multiple jack belonging to a subscriber who is called at her section. If his line is disengaged no electrical effect will follow, and connections may be made at once. This is true because, when the line is idle, the test ring and the tip of the plug, both connected to ground through the battery, B, are at the same potential, and no current will flow. By the connection of the springs, C and E of the jack, however, by the insulated collar of the plug, a difference of potential between the terminals, plug tip and test thimble and ground is created, and on the contact of tip and thimble an active circuit is made, with a resulting click, which may be heard in the operator's receiver. When the operator's talking set is bridged into the cord strands of the plug another result is produced. Connected to terminal 2 of 7' on the switch key, will be seen a line leading to one side of the restoring coil, R, of the clearing-out drop. This enables the closing of the drop, should it be open, by the simple act of switching in the talking set, an arrangement having its advantages when we consider that it is usual to place self-restoring drops behind glass, as protection from dust and other interferents. With the bridging-in of the talking set, there- fore, a circuit is formed from ground at X, through battery fl, THE MULTIPLE SWITCHBOARD. 185 Uirough the restoring coil, R, through one-half of the secondary- winding of the induction coil, J, through both pole coils of the receiver, through the other half of the induction secondary, to terminal i, and thence through its connected line to ground at Y. AC 5 J 1 (NSWERINS PLUG. MLLINCPLUC. ' \ ^\ BUS WIBES ft ■■ r. 5. T. - ' TO opf i?AT08is 9 a f^M|f|||f TELt PHONE. 03 ■■» Batteby Fig, 158. — Diagram of the test circuit of a branch terminal multiple switchboard, simplified by the use of a familiar form of switch key. Visual Busy" Signals. — A number of multiple switch- boards, more particularly such as operate by a common exchange battery, are equipped with a " busy " signal system of a different description from any of those mentioned above. :'t consists, in brief, in arranging a small electro-magnet and drop shutter in connection with each multiple jack, so that when a given line is plugged, a local grounded battery is thrown into circuit, and all its multiple jacks are closed by drop shutters, which enables the operators to tell at a glance whether or not the line is busy. Such an arrangement, of course, is advantageous in obviating the necessity of making the "busy" test, as 1 86 ■ A B C OF THE TELEPHONE. described above, and also in simplifying the wiring of the muU tiple jacks. Further it does away with the complicated test circuit arrangements, which must involve added perplexity in the operator's cord attachments. It has not, however, come into general use. Divided Multiple Switchboards. — The vast complica- tion of the ordinary type of the multiple switchboard has been the occasion of numerous inventions proposing to simplify the necessary plant of a large exchange. The most representative of these devices are, respectively, the divided multiple switch- board, and the various locally inter-connected, transfer or express systems, whereby trunJdng plugs are used to connect the several positions of a switchboard, enabling the making of con- nections without the use of multiple jacks. The divided switchboard is the invention of Milo G. Kellogg, of Chicago, and, as manufactured by the company of which he is the head, may be briefly described as follows : In an exchange of, say, 6,000 subscribers, which means 60 positions of 100 drops each, and at least 20 multiple jacks to every line, we have 120000 jacks at least, exclusive of answering and trunking jacks. Mr. Kellogg's system accomplishes the very desirable result of quar- tering this figure, giving a total of say, 30,000 multiple jacks to a system of 6,000 subscribers. This he does by dividing the 60 positions mentioned into four divisions, and constructing the calling apparatus with polarized drops and ordinary pole changing switches. Polarized Drops. — Let us suppose that each of the four divisions of the Kellogg switchboard represents an even 1,500 subscribers. Then subscriber No. 120, whose drop is at position 2 of division i, has a drop not only in that position, but also at a corresponding position in divisions 2, 3 and 4. By this device he may call any subscriber he desires in either of these three divisions. The device is simple. His magneto-generator or other source of calling current, is furnished with a pole-changing switch, which may be so manipulated as to enable him to send THE MUL TIPLE SWITCHBOARD. J 8 7 a positive or a negative current over the line wire with a ground or third-wire return, or a positive or a negative current over the test wire with a ground or third-wire return. The first of the four currents mentioned will energize, say, his drop magnet in division i ; the second, his drop in division 2 ; the third, in division 3; the fourth, in division 4, each of these drops being so polarized as to respond to that particular current and no other. Thus he may transmit a call to any one of the four, as he desires, according to the position of the subscriber with whom he seeks communication. Answering and Multiple Jacks. — Subscriber 120 has an answering jack in position 2 of division i, as we have seen, and also in the corresponding position in divisions 2, 3 and 4. In division i his line is also tapped with multiple jacks at every section in the ordinary fashion. But he has no multiple jacks in the other divisions, the sole object of connecting his line to them being for purposes of calling subscribers belonging there. Any other subscriber, therefore, who wishes to communicate with 120 will so arrange his calling circuit, by his pole-changing switch, as to release his drop in division i, where a multiple jack of 120 may be plugged as desired. Advantages of the System. — In addition to those arrange- ments there are also relays which operate to break the circuits of all the drops of a subscriber the moment a plug is inserted in any jack on his line, thus obviating the confusion arising from false calls. The testing circuit is arranged in the same fashion as in the ordinary multiple switchboard. By the use of the divided board system, not only may the number of multiple jacks be reduced, but four times as many subscribers may be wired to an exchange. The limit on an ordinary board is 6,000 subscribers, with 120,000 jacks. With a Kellogg divided board exchange there may be 24,000 subscribers, and yet but 120,000 jacks, on the principle just explained. Such an arrangement, if successfully carried out, must mean an increased activity in the telephone business and a corresponding reduction in tariff rates. 1 88 A B C OF THE TELEPHONE. CHAPTER FIFTEEN. LOCALLY INTERCONNECTED OR MULTIPLE TRANSFER SWITCHBOARDS. Objections to Multiple Switchboards. — While the mul- tiple switchboard is a very complete and in many ways a satis- factory device for accomplishing the ends for which it is constructed, its great complexity, and the correspondingly increasing cost of installing and enlarging exchanges equipped on the multiple system have set numerous inventors to work on the line of devising satisfactory substitutes. Their efforts have resulted in the several systems of transferring, by which the line of a calling subscriber may be directly switched to the board on which is the jack of the caflled subscriber. While in all these systems there is required very complex appliances, the absence of the multiple jack feature more than compensates for it. The Sabin-Hampton Express System. — Perhaps the most typical, and certainly the most complicated, of the transfer systems is the Sabin and Hampton "express" system. This operates, briefly, as follows: In the first place, the apparatus at the subscriber's station is constructed without the ordinary magneto generator, the operation of the drop, or visual signal, on the switchboard panel being accomplished by the current from a storage battery at the exchange, by closing a circuit from the subscriber's station to the exchange, on the removal of the receiver frpm the switch hook, as in the operation of the common battery systems now increasing in popularity. Cut-Out Jacks. — Each subscriber's jack contains two spring contacts of the ordinary type, which form the terminals for both Bides of the line — the line wire and the " test" wire — and each of them rests, when not actuated by a plug, on an anvil, from rSUy IQO A B C OF THE TELEPHONE. CALL TRANSFER SYSTEMS. 1 9 1 which the plug draws it away in precisely the same fashion as in the types of jacks already described. The jack of this system is, in fact, a double jack, having two line springs and two drop contacts. A jack of this description is shown in Fig. 160. Rising Visual Signals. — When a subscriber takes his receiver from the hook the circuit is closed at his apparatus, through the two drop anvils of his jack, through the local battery and the coil of the drop, causing the armature to be attracted and the target to be raised so long as the plug is not inserted in the jack. Immediately this is done, the circuit is broken at the jack anvil contacts, and the target falls into its normal position. Such a line signal is shown in Fig. 162. Call Boards and Order Boards. — There are two kinds of switchboards required by this system, which for convenience, we may designate as the "call" boards and the "order" boards, respectively, although these terms are here adopted merely for simplicity's sake. The "call" boards are of the ordinary type, containing the signals and jacks of 100 sub- scribers to a panel, as in all others, but without the operator's listening and ringing keys already described. The "order" boards are equipped with signals and jacks corresponding to every section of the call board, so that connections may be made between the two. Operating the Call Boards. — The plugs of the call boards differ in two particulars from the ordinary type. First, they are arranged in single rows instead of double, as in other boards; second, they form the terminals of a " trunking " circuit between the call boards and the order boards. That is to say their tip and sleeve strands connect direct to the signals — each plug has one — on the order board. Thus when a subscriber calls, and his signal rises at the call board, the operator there merely inserts a ■"trunking" plug in his jack, causing his signal to be restored, and pays no further attention to it. 192 A B C OF THE TELEPHONE. Operating the Order Board. — The insertion of the plug in the jack of the calling subscriber continues his line from the call board to the order board, where the proper signal is displayed. The operator at this board then moves the proper switch key, so as to bridge in her telephone set and inquire the wants of the subscriber thus transferred from the call board. Having ascertained the number of the subscriber with whom he wishes to converse, she presses the "order key" of a trunk line leading to the call board section containing the called subscriber's jack, and by this act having placed herself in direct connection with the telephone of the operator there, directs her to make the proper connections. The operator at the second call board, thus signaled, answers, informing her which one of the several trunk plugs to use ; and this being complied with makes the desired connection. Lamp Signals and Relay Circuits. — The signal and clear- ing devices used on this type of exchange boards is at once ingenious and complicated. Instead of drops of any description incandescent electric lamps, white and red, are employed for this purpose on the order boards. The trunking plug being inserted in the jack of the calling subscriber at the first, or incoming call board, completes the circuit closed at the sub- scriber's station by the removal of his receiver from its hook, from its tip strand, through a balance coil arranged to prevent interference between the signaling and talking currents, through a local battery, which energizes the coil of a relay, constructed on the same plan as the ordinary switchboard drop, thus causing it to attract its armature, and from this coil to the sleeve Urand of the trunking line. When the relay attracts its armature it closes another circuit between a battery and the signal lamps on the table of the order board. As soon as the circuit is thus made the white lamp is lighted, and continues burning until the operator of the order board switches in her telephone set; or until she lifts the plug from its socket, to insert it in the jack of the section containing the called subscriber; or until the calling CALL TRANSFER SYSTEMS. 1 93 subscriber replaces his receiver on its hook, thus breaking the circuit. Operation of the Signals. — The withdrawal of the plug from its socket at the order board, in order to insert it in the required jack, and thus make the trunking connections with the outgoing call board, releases a leaf spring, which breaks the circuit of the white light and makes the proper contacts for the red light, which is lighted as soon as the calling subscriber restores his receiver to its hook. Thus the white light acts as a calling signal and the red light as a clearing-out signal. Remembering that the red lamp depends for its proper connec- tions on the removal of the plug from its socket, we may under- stand how that the removal of the plug from the jack at the order board, before conversation is ended, will again make the circuit of the white light, thus causing it to burn again, as a signal to the operator that she has made a mistake. Similarly, should the operator at the first, or incoming, call board remove the plug from a jack before the conversation is ended, the calling signal, already described, would again be displayed, thus indicating that the circuit is still made from the subscriber's apparatus. The order board relay, already described, being wired in multiple to a relay at the incoming call board, operates a similar sort of clearing-out signal there. The Clear-out Signal System. — The clearing-out mech- anism of the outgoing line, as operated on all call boards what- soever, when plugged for an outgoing line, is somewhat compli- cated, although highly efficient. Briefly it consists of three different relays, the combined calling and clearing drop and two others, which, by their individual energizing at the succes- sive acts of the operator, can make or break the circuit of a signal lamp placed at a convenient point for indicating the state of a particular line. The connections of the calling and clearing relay signal are made as already described. The two other relays form a pair, one placed above the other, the poles in opposite directions, in such fashion that what corre- 1 94 A B C OF THE TELEPHONE. spends to the shutter of' ordinary drop coilfe with the upper one is the armature of the lower one. This shutter-armature normally leans away from both coils, but having been attracted by the lower of the two arid then released, it is caught and held by a hook at the end of the armature lever of the upper coil, just as the shutter of the ordinary drop is held in place. This contact, however, makes a circuit which lights the signal lamp, which circuit is broken so soon as the coil of the upper relay is energized, thus raising the armature lever and allowing the shutter to fall down to its normal position. The Signal System on an Outgoing Line. — The operation of the signal system of a call board with an outgoing line is as follows : The , operator at the call board having made the connection as directed by the operator at the order board, having inserted the trunking-in plug into the designated jackj pushes the button of, her ringing key, thus sending a calling current to the subscriber desired. The ringing key is so arranged that the act of pushing the button does two things — sends a current and makes the circuit between a local battery and the lower of the two relays just mentioned, causing it to attract the armature or shutter. As soon as she releases the button of the key the circuit is broken, and the armature shutter falls away from the pole of the lower relay, being caught by the lever hook of the upper relay, and thus closing the circuit from a local battery, through the shutter armature to the lever hook, through the lever to the armature pivot of the upper relay, through the incandescent signal lamp just mentioned, and back again to the other side of the battery. The lamp thus lighted by the depression of the ringing key remains burning until the called-for subscriber takes his receiver from his switch hook. This act, as with the calling subscriber, in the first instance, causes the calling coil to operate and raise its shutter, as previously noted. This raising of the shutter also makes a circuit through the coil of the upper relay, causing it to attract its armature and raise its armature lever, releasing the shutter from the hook, and by allowing the shutter to fall away again CALL TRANSFER SYSTEMS. 195 breaking the circuit of the signal lamp. The shutter of the calling and clearing signal, thus raised, remains so until the conversation is ended, the falling of the shutter being the clearing signal. Since the initial act of operating this system of signals is pressing the ringing key, very little reflection will show that it could not be used except on the outgoing line. Advantages of the Express System. — Although the mech- anism of the express system may seem uselessly complicated, and there are several disadvantages in the delay entailed in having three operators to handle every one call, yet the great expense and constant attention to involved circuits and delicate contrivances entailed in the installation and maintenance of a multiple switchboard are obviated. The express transfer system is much more readily operated, and the constant dang'er that a careless or inefficient operator will plug the multiple jack of a busy line is completely done away. This system is also con- trived to save manual labor and the attention of the operator to the last degree. Instead of compelling her to answer each individual subscriber to inform him when connections cannot be made, power-driven phonographs are attached to the boards which constantly repeat into a transmitter, mounted with induc- tion coil and battery, the required sentences. These may be connected to the line by inserting a plug in a jack connecting to the one or the other phonograph, and thus allowing the sentences: " Subscriber called for does not reply," or, "Busy; please call again," to be transmitted to the calling subscriber. The latter phonograph is attached to the order board, and is switched into line by the operator there as soon as she has learned from the operator at the second call board that the required line is in use; the former is attached to the call board, and is switched into the line of a would-be outgoing subscriber so soon as, by the lamp signals already described, the operator has learned that the called subscriber does not respond. These phonographs are useful, as enabling an operator to give the required information by a simple manual act, without inter- rupting her other duties to speak along the line. 196 A B C OF THE TELEPHONE. The Cook- Beach Transfer System. — Another transfer system based on some of the same general principles, although employing only one type of board for both calling and order> Figs. 163-164.— Diagrams illustrating the circuits of the Cook-Beach Transfer Sysl through two operators' sections. The transfer drops indicate on one section of switchboard that the corresponding transfer jack has been plugged on another to wh: it is connected. The operator, noting the fall of a transfer drop inserts the plug conned through its strands with the signaling section into the jack of the called subscriber, whose number she learns by depressing her "'order key," thus throwing her telephone set into em ;he LCh ;ed ing, is the Cook-Beach system, which is used in connection with, the apparatus of the Sterling Electric Co. The plan is to have all subscribers' drops arranged on section panels of one hundred CALL TRANSFE-R SYSTEMS. 197 sach. In addition to the regular switchboard apparatus — drops, jacks and plugs — there are also transfer drops, jacks and plugs, directly connected to each other several sectiot) of the board. circuit with that of the operator at the signaling board. The "hang-up drops" fall to designate a disconnection made too soon, and the plug is then to be inserted in the corre- sponding " hang-up jack," so as to restore it. The circuit connections may be understood from the figures by remembering that all opposite lines are supposed to be continuous between the two. As may be seen, the transfer plugs at each section form the terminals there of a transfer drop and jack at the other. This is the " truuking plug." Thus there can be both visual signaling between the several operators, and also trunking connections between the several sections of the switchboard. 1 98 A B C OF THE- TELEPHONE. Making a Connection. — When any subscriber, say Num- ber 50, "rings up," his drop falls on his section of the switch- board. Section I, thus signalling the operator in the usual manner. She immediately inserts in the jack bearing his num- ber the answering plug of a given pair, which, as we have previously seen, is shorter than the calling plug, the latter being made longer, so as to act on the jack springs and cut out the drop coil. Having ascertained that Number 50 wishes to converse with Number 600, the operator inserts the long calling plug in the transfer jack corresponding to Section V of Fig. 165.— operator's order key used in the Cook-Beac^ transfer system to connect the talking .set of one section into circuit with that of another. Fio. :66.— Cut-out plug used in Cook-Beach transfer system to break a false connection with a busy line. the board on which are hung the drop and jack of 600. This causes the "transfer drop" of i, on V, to fall. At the same time she depresses her order key, which throws her talking set into circuit with that of the operator at the fifth section, and informs her that 50 wishes to be connected to 600. The opera- tor at V then takes the trunking plug on her board correspond- ing to the line that has just signalled, and inserts it in the jack of the called subscriber, Number 600, thus completing connection by the use of three plugs. Operation of the Signal System. — The clearing-out signal system on this type of transfer board is simple. As the CALL TRANSFER SYSTEMS. 1 99 drop of the calling subscriber, left bridged across the talking circuit, .is usually employed for this purpose on the boards of the Sterling Company, its fall indicates the close of the conver- sation to the operator at Section I. .As soon as she. notes this signal, she removes the plug from the jack, which act causes the transfer drop on Section V to^fall, signaling the operator. If at any time before the close of the conversation, either operator should remove the plug from the jack, the act causes the shutter of the " hang-up drop " on her section to fall, thus apprising her that an error has been made. She can then insert her plug in the "hang-up jack," and learn from the subscriber what his number is, and then remake the connections. Oh the other hand, if a calling operator should make the mistake of making trunking connections with a line that is already busy, the operator at the called board may, by depressing a cut-out button on her transfer key, break the connection. The Western Express Transfer System. — The transfer system of the Western Telephone Construction Co., of Chicago, is simpler in many respects than either of those previously described. On the table of each loo-drop section are arranged twelve pairs of plugs, one of each for calling and the other for answering. Ten of these pairs have bridged between the two plugs a trunking line, which leads out from that section, say, from the first, to the fourth, seventh, tenth sections — to corresponding plugs at every third section. Each transfer plug represents a distinct wire, so that there are ten distinct trunking lines lead- ing to and from every section of the switchboard. Moreover, the transfer line of each pair of trunking plugs is attached midway between the calling and the clearing-out drop in every case, so that the clearing signal may be given at the section con- taining the drop of the calling subscriber, while the drop at the transfer section is cut out. A board equipped with the transfer apparatus of this system is pictured in Fig. 139. Operation of the System. — On receiving a call at any section, the operator merely inserts the answering plug of a 200 A B C OF THE TELEFHONE. given pair in the corresponding jack, after the usual manner, and, having switched in her telephone set, inquires his wishes. If he asks to communicate with another subscriber at the fourth, seventh or tenth board, she depresses a push button on the table in front of her, which makes connection with the talking set of the operator at that particular board, and informs her that a subscriber on the first, second, third, fourth or other trunk wishes to communicate with one numbered on her board. The operator, thus addressed, takes the calling plug of the des- ignated pair, and, inserting it in the jack of the called sub- scriber, makes the connections in the usual manner. It will readily be seen that but two plugs are used in any connection, even to the end of the switchboard. If the called-for subscriber stands in any other section than one that is third, sixth, ninth or twelfth, etc., from the section of the calling subscriber, the operator at his section calls up the operator at the section near- est to that containing the desired number, which will be either to her right or to her left, and she inserts a plug in his jack. The Clear-out Signals. — The clearing signals of this system consist in a series of incandescent electric lights con- trolled by relays whose circuits are made by spring contacts arranged at the base of the answering and of the calling plug of each pair. When, as we have seen the shutter of the clearing-out drop falls at the section of the calling subscriber, she removes the answering plug from the jack, and replaces it in its socket on the table in front of her. As soon as it is thus replaced it depresses by its weight a contact spring, thus making a circuit through a relay and a local battery to a miniature lamp at both that board and the one of the called subscriber. The lamp thus lighted at the latter section informs the operator there that the conversation is completed, and she removes the calling plug from the jack of the called subscriber and replaces it in its socket. By this latter act she allows it to depress a contact spring, which breaks the circuit of the lamps and informs the operator at the first section that the disconnection is made. CHAPTER SIXTEEN. EXCHANGE BATTERY SYSTEMS. Advantages of a Common Battery.^In conducting and maintaining an extensive exchange system it is, of course, ex- tremely desirable that the details should be, as far as possible, simplified, and that all points of avoidable expense and care should be eliminated. Such ends are very largely attained by the use of the various common battery exchange systems now being so widely adopted. The simplest application of this principle consists in using a common calling battery for all subscribers' stations, the signal being given at the switchboard by closing a circuit on the removal of the subscriber's receiver, thus dispensing with the ordinary magneto-generator. Such an arrangement has been described in connection with simple lamp signal circuits, and also the Sabin-Hampton "express" system, which, as far as the subscriber's calling circuit is concerned, are fairly typical common battery systems. As in both of these systems the insertion of a plug in the subscriber's jack cuts out the signal and makes the circuit for the talking battery. A further elaboration consists in attaching a constant _ current battery to the cord circuit of each pair of plugs, ■ so that, when the signal battery is cut out by plugging the jack, energy is supplied for operating the transmitters. These general prin- ciples are largely typical of all the various systems of common battery exchanges, although the devices employed for their successful application in practice vary widely. Signaling Circuits and Batteries. — Fig. 167 represents a co.mmon type of central energy subscriber's station apparatus, the practical operation of which must be obvious to any one. The transmitter and induction coil are attached to the head of the backboard, and the ringer and switching apparatus within 301 202 A B C OF THE TELEPHONE Fig. 167- — Central Energy Telephone Apparatus. The back plate of this apparatus is a hollow iron casting, suflElciently large to contain a condenser, which, on the common battery system of the manufacturers, is connected in series between the bridge containing the polarized ringer on the one hand and one-half of the transmitter and primary circuit, ou the other. Energy for ringing the signal bells comes in over the telephone circuit wires. CENTRAL ENERGY. 203 the box below. No battery whatever is included. To signal ccvArsX from such an apparatus the subscriber merely unhooks the receiver, with the result of closing a circuit with the switch- board signal, as has been already described. This result is accomplished generally by making a path of small resistance in place of one of large resistance, which is maintained so long as the apparatus is not in use. Such a resistance is obtained by a high winding of the ringer magnets — they are sometimes wound as high as 5,000 ohms — which prevents an efficient current from reaching the switchboard signal. As soon as the receiver is unhooked there is provided a path of comparatively no resistance along which the current can readily flow from the exchange and back again, so as to energize the signal coil or flash the call lamp. The result is also accomplished by 'clos- ing a normally open circuit between the two limbs of the line. Calling a Subscriber. — There are two general methods of connecting a subscriber's ringer to the exchange switchboard ; either by connecting it in series on a bridge between the two wires, the high resistance of its magnets barring the current as soon as the telephonic receiver and transmitter are switched into circuit; or else by placing it with one side normally con- nected to the test wire of the circuit and the other grounded, in order to enable it to take current from a grounded battery at central, the talking apparatus being normally cut out of circuit by the depression of the switch hook. In both cases the ringing circuit is made by the insertion of the calling plug in the line jack, the apparatus talking set being meantime out of circuit, as above explained. According to these arrangements of circuits the apparatus is normally in condition to receive a call, and can transmit a call by the simple act of raising the switch hook. These variations from the ordinary scheme of apparatus arrangement are necessitated by the new conditions introduced by the use of a central battery, in accordance with which problems arise on the point of how to use a central energy current to advantage without having it too weak to energize the 204 A B C OF THE TELEPHONE. switchboard signal or too powerful for use on the calking circuit. Sources of Energy. — In all systems of common battery exchange the talking and switchboard signal energy is derived from storage batteries which are charged from power-driven dynamos in the usual fashion. This practice is superior from Figs. 16S-169. — Motor-generator central battery ringing machines, the first designed to work from street power circuit ; the second, from storage battery at the exchange, both generating an alternating current. The machines are of the type supplied with the exchange battery plants of the Western Telephone Construction Co. The armatures are dotible wound, the commutator side being the motor, the collector ring side, the dynamo, the same field serving for both. the fact that the fluctuations of current characteristic of dynamos, which are very annoying on telephonic circuits, are completely avoided without resort to transformers of any descrip- tion. Again, since the source of power is seldom required for work at its full capacity at any one time, the operation of a dynamo would involve needless waste for most exchange systems. However, for signaling subscribers it is customary to use a power-driven alternating dynamo, such as is shown in Figs. 168-169, an alternating current being required to operate the subscriber's polarized ringer, while the direct current from the storage battery is required for the talking circuit. CENTRAL ENERGY. 205 Methods of Applying Central Energy. — In ringing from the central exchange to any subscriber's apparatus, the method of applying and transmitting the power need differ in no particular from the devices followed in the ordinary local battery systems previously described. How to centralize the talking energ)' is, however, a problem filled with many difficulties, since we must have a center source, a current in two directions, and an uninterrupted flow of energy from one subscriber's apparatus to any other with which his line may be joined at the switchboard. The most logical means of applying the current is to bridge the battery between the two strands of the plug circuit, so that one -pole is joined to the sleeve strand and the other to the tip strand. By such an arrangement, as will be readily understood, a circuit willi be made so soon as either plug of a given pair has been inserted in a spring jack; the current traveling from the battery on to the tip strand, thence " through the jack contact spring, to the subscriber's apparatus, back on the other limb of the line to the contact spring, through the sleeve, its strand, and to the battery again. To make the central energy operate like a local energy at each end of the line when both plugs are in jacks, or, in other words, to prevent the short-circuiting of the talking current at the central bat- tery, is the difficulty first to be met and overcome. The various solutions of this situation constitute a big part of the several systems now in practical operation. Connecting the Battery in Series. — One of the earliest applications of the central energy theory consisted in connecting the battery in series ; that is to say, attaching both its poles in one strand of the plug circuit, as, for example, the sleeve strand. The circuit was then made from the positive and back again to the negative pole, either when both plugs were inserted in the jacks of two subscribers, or else when the operator's talking set was bridged across the cords in communication with a calling sub- scriber. The receiver and transmitter at the subscribers" stations were also arranged in series, as in the ordinary local battery apparatus, the ^nly difference being ^ha* the energy fot 206 A B C OF THE TELEPHONE. operating the transmitter circuit was derived from the switch hook contact of the induction coil primary, as shown in Fig. 102, instead of from the galvanic cell. While this system was perfectly practical, it would seem that a great disadvantage lay in the fact that the circuit, so energized, was not balanced, and that the speech-bearing current would be unevenly transmitted. Fig. 170,— " Dynamotor,'" charging machine for charging the storage battery. The motor, worked from street power circuit, is attached on the same shaft with the dynamo. Methods of Balancing the Circuit. — As a consequence of its shortcomings this system of battery connection was abandoned for that at present adopted — the method of bridging the battery -between the plug strands. By this construction, as we have seen, it is perfectly practicable to transmit a current in two directions — from central to both subscribers' stations and back again — but the theoretical difficulty is to ensure a through current from one station to the other, and prevent short circuiting at the bridged battery. To accomplish this result three typical devices have been adopted: i. To interpose an impedance coil between each pole of the battery and the strand to which it is connected, as in the system devised by John S. Stone. 2. To interpose a divided repeating coil between each pole of the battery and the strand to which it is connected, each strand being thus divided, and the double windings of the coils being joined at the pole of the battery, as in the system devised by Hammond V. Hayes. 3. To connect the center ooinrs of CENTRAL ENERGY. 16f both sWands of the plug circuit with the winding of an imped- ance coil, whose center point in turn is connected to one pole of a grounded battery, as in the system based on the conceptions of J. J. Carty and W. W. Dean. Most of the more recent central energy devices are based on these broad principles or consist in some variation of the systems about to be described. The Stone Central Battery System. — By interposing impedance coils between the battery poles and the connections on the plug circuit strands two ends are achieved ; the direct current flows from the battery and back again with ease, the low ohmic resistance of the coils offering no bar whatever; the rapidly alternating telephonic current is effectually confined to its line on the plug strands by the magnetic action of the coils, and short-circuiting through the battery is completely prevented. Thus the battery current^ dividing on the plug strand and flowing in two directions to both stations and back again, is able, as it is modified by the variant pressure in the transmitters, to convey the telephonic impulses from one end of the circuit to the other. In connecting a number of plug circuits to one battery the method is to connect both strands of all plugs with' the impedance coils and join them in multiple to the battery poles. That is all sleeve strands are joined through the sleeve coils to the positive pole of the battery, and all tip strands, through the tip coils, to the negative pole. The subscribers' apparatus in this system are constructed with the receiver and transmitter connected in series to the line wire, as in the ordinary exchange instruments, the current for the transmitter being usually derived as in the series battery system described above. The Hayes Central Battery System. — In the Hayes system the battery is bridged or wired in multiple between the plug strands, as in the one just explained, the prindipal difference being that each pole is connected to the cord circuit through a divided repeating coil instead of through an ordinary magnetic impedance. In' other words, the current is passed in two directions through a transformer or induction coil re-istance, 208 A B C OF THE TELEPHONE. FiO. 171.— Power switchboard for a central energy exchange. This is a slab oil slats npon which are mounted the various devices shown for controlling and measuring the strength and amount of the current, also rheostats for coutroUing the charging current from the dynamo and the cuneut from the storage batteiy, and cutout switches. CENTRAL ENERGY. 209 the end attained being to obtain the same kind of an inductive effect between the two extremes of the line as is obtained between the primary and secondary circuits of the ordinary telephonic transmitter. In this way, while the direct current from the battery emerges from the posi- tive pole, travels along both windings of the repeating coil to both stations and back again, the continuity of the alternating telephonic current from one end of the line to the other is maintained by induc- "^ WRMm^f. a l^'i:=-<'--"i:-■■J■*--«iii- ■ .. ■tMlft,-*-»\ K \ MmuIAx ■' s>- Fig. 175A.— Diagrams illustrating tlie several systems, of transmitting energy from the exchange. A, the Stone system, plug Circuit ; /"i, P^, plugs, C, C, retardation coils, R, operator's 'teceiver, /, transmitter. B, Hayes system, plug circuit ; C, C, repeating coils. C, the Dean system, plug circuit, C, the retardation coil. /), feed circuit with condenser in series; P, primary, 5, secondary, /, transmitter, R, receiver. , E, apparatus wiring of the Hayes system. F, apparatus wiring of t)ie Dean sytem. 2l8 ABC OF ^ THE TELEPHONE. storage cells at the exchange. The transmitter circuit of each sub- scriber's station is then connected in multiple to the power circuit, with the induction coil primary on a bridge. The danger of cross- talk in this arrangement is obviated by making the batteries of low resistance and the power conductors as short and heavy as possible, in comparito.i to the transmitter and primary circuits, so that the fluctu- ations of current produced in one apparatus will return direct to the source rather than leak across any of the other bridges. All drops in potential are inappreciable at the battery terminals. Series Feed Circuits. — ^There are two noteworthy systems of imparting energy for the transmitter from a series connection. In both of these the circuit of the transmitter and primary winding forms a shunt circuit, its two terminals being connected to the wire, the subscriber's transmitter circuit being in series. Across the shunt circuits, however, and between the two terminals just mentioned is bridged, on the first system a storage cell, on the second a condenser, each also in series with the power wire. Consequently when the transmitters are operated by the voice, sufficient additional energy may be derived from the storage cells, in the first instance, for distinct transmission. In the second instance, the current fluctuations pro- duced in the transmitter vary the potential of the charge in the con- denser, around which it is shunted, and gives the full effect of an alternating current on the primary winding of the induction coil. Such alternation is then said to be "superimposed " upon the direct dynamo current always flowing through the feed circuit and trans- mitters. In the latter system an impedance coil is interposed be- tween each pole of the dynamo and the terminal of the line power circuit. CHAPTER SEVENTEEN. PARTY LINES AND SELECTIVE SIGNALS. Party Lines or Common Circuits. — In all of the sys- tems we have considered heretofore, the apparatus of each sub- scriber is connected to the central exchange by its own circuit. There is, however, another method frequently adopted in systems where there is but little business in proportion to the length of the line, and it is, briefly, the connection of a number of subscribers' station apparatus on one circuit, so that all have a common drop and jack on the exchange switchboard. This is called the " party line" or common circuit system, and it is also available for small private telephone systems, connecting a number of neighboring houses with one another or with stores or places of business. The Series and Bridging Systems. — In the party line system of telephones, as in the case of electric light, power and other circuits, there are two general methods of connecting the different stations. They are connected either in series or in multiple. The latter method is better known in telephony, in its simplest application, as the " bridging bell " system. The Series Method of Wiring. — The series method of connecting a number of telephones upon a party line is shown in Fig. 176. Here, as may be seen, one side of the line extends from one apparatus to the next, and passing through the coil of the signal bell in each — the bell circuits being closed so long as the receiver is hung on the switch hook, as we have seen — emerges at the other binding post at the top of the generator box, whence it goes on to the next station. The other side of the line extends from the right-hand binding post of the right- hand instrument back to the left-hand binding post of the left- 220 A B C OF THE TELEPHONE. PARTY LINES. 221 hand instrument. In grounded lines the ground connection is made at these two extremes only. The Wiring of a Series Apparatus. — The wiring of a series circuit party line station telephone apparatus is substanti- ally the same as that of the ordinary exchange circuit apparatus, as may be seen by reference to Fig. 177. As in the latter instru- ment, the hook makes two contacts when the receiver is removed, thus making the talking circuit, and but one when the hook is down, thus making the calling cir- cuit. In calling in a series system a special code is used, by which each telephoner rings out an agreed signal to call each other one. Such signals are generally mere indica- tions of the numbers of the other operators' apparatus, and may be given by turning the crank of the generator with alternate pauses in- stead of continuously, as in the case of the subscriber who desires to call up the central station, as we have seen. ^^°- '77- Objections to tlie Series System. — As may be readily understood, the principal objection to the series system of party line telephones is that each conversation must be carried on by a current that passes through the call bell coils of every other apparatus on the line. Such an arrangement is bound to weaken the current in proportion to the number of apparatus connected, and for this reason the bell magnets have usually been wound to a much lower resistance than in the orditiary type of telephone; sometimes as low as 80 ohms, just about thq average resistance for receiver coils, for each double-pole wind- ing. The telephonic current, while it may pass through a 22 2 A B C OF TKS TELEPHONE. number ol such coils and still remain at hearing intensity, is not sufficiently strong to energize the bell magnets. Thus, akhough the high potency currents from the magneto-generators can readily ring through the combined resistance of fifty instru- ments (4,000 ohms), the talking current at that figure becomes so faint, through the combined effects of line resistance, impe- dence, induction and general leakage, that it is extremely unsatisfactory, not to say impracticable. Such troubles may be partially overcome by frequently transposing the line; for example, connecting Station I to Station IV, and Station II to Station V, etc., instead of connecting in succession, as in the cut, or else by connecting stations alternately to either side of the line. Even with this arrangement balance cannot be maintained, and the talking Dualities' are only partially improved. Magneto-generators for Series Circuits. — As it is often necessary to ring through as high a resistance ais 4,000 ohms, combined with line resistance, on such a line as is mentioned $bove, it follows that' the generators of a series telephone circuit must be of high electromotive force, sometimes reaching so great a strength as to ring through 15,000 ohms, or about four times the power of the ordinary exchange instrument. Tlie Bridging BelP System. — With a view to overcoming the serious difficulties thus mentioned, another system of string- ing a telephone party line has been introduced within a few years. On this plan the several telephone apparatus are strung in parallel or multiple, as shown in Fig. 178, which shows the arrangement for a metallic circuit line having four stations. In a grounded circuit the ground connections are made direct from the right-hand top binding post of each instrument, which in this diagram is shown to be connected to the return wire. The ground connection' shown at the center binding post of each apparatus is for protection against lightning and electrical disturbances, or else, as in some systems, to afford the necessary ground connection for some scheme of selective signaling. PARTY LINES. 223 224 A B C OF THE TELEPHONE. The Carty Bridging System. — The bridging system which was originated and patented by Mr. J. J. Carty, of the Metro- politan Telephone Co., New York, is, in many respects, the opposite of the series, and certainly overcomes most of its gravest faults. The first advantage is found in the fact that the telephone current does not pass through the coils of the several bell magnets, as in the former case, and these are wound to a very high resistance, a large number of turns and long bobbins, so as to effectually prevent the shunting or short circuiting of the current before it reaches the apparatus tp which it is destined. In short it may be said that the whole theory of the bridging telephone system may be summed in the principle that a current will always follow the line of least resistance or of greatest ease in making a circuit. This principle is applied in the interior arrangement of the apparatus as well as in the stringing of the line. Circuits of a Bridged Telephone. — Fig. 179 shows the circuits of a bridged telephone apparatus. As may be seen, the rising of the hook on the removal of the receiver, while it makes the circuit of the transmitter and receiver, does not break that of the bell and generator, which are left permanently bridged across the talking circuit. For this reason the switch lever makes no lower contacts. Two possible paths are thus presented to the telephone current: (a) that leading through the bell magnets to line, or (J)) that leading direct to line. It chooses the latter path, as being the line of least resistance. Similarly a current on the line will enter an apparatus in which the telephone circuit is made, and will avoid the path leading through the coils of the bell magnets, which are bridged across. Wiring of a Bridged Line. — As may be seen from a brief examination of Fig. 178, a complete circuit may be formed between any two apparatus in the system without reference to any of the others. Thus, if the telephoner at the second station is conversing with the one at the third, the current passes out from the right-hand binding post of his apparatus, PARTY LINES. 221: through the line wire, into the apparatus of Number 3, through the right-hand binding post, through his tallcing circuit, out oi his apparatus by the left-hand binding post, to the return sid« of the line, and thence back to apparatus Number 2, through the left-hand binding post, thus completing the circuit. Th« high resistance and retardation of the bell magnets of the first and fourth apparatus, which remain bridged across the line, forming so many shunt circuits for the current from the magneto generator of any one of them, which, when operated, rings all the bells in the system, effectu- ally bar the talking current on the same principle as that applied when the coil of a switchboard clearing-out drop is left bridged across the line of two subscribers. The resistance is so high that the talking current has no tendency to enter the coil, and remains on the line between the two apparatus whose talking circuit is made. High Impedfince of the Bell Magnet. — For the purpose of bar- ring the talking current, the coils of the bell magnets are wound with a greater number of turns of wire than ordinarily, generally to a resistance of about 1,000 ohms, although a heavier quality of wire is usually employed than in ordinary coils. This latter practice has been adopted for the purpose of wind- ing a longer core, thus obtaining more iron in the magnet and increasing the magnetic retardation. Coils so constructed, while readily operated by the low frequency currents from magneto generators, are effectual blockades against the high frequency currents of the talking circuit. As has been calcu- lated, the impedance of the bell magnet coils is about four times its ohmic, or true, resistance during the passage of the Fig. 179. 2 26 A B C OF THE TELEPHONE. telephonic current. The question of high retardation is of much greater importance than mere resistance in the coil wire. Thus the use of finer wire, or some highly resistant metal, such as German silver, while giving the same or a higher degree of resist- ance, is not so good a practice as using a coarser wire. The latter practice secures the required' length for the magnet cores without undue increase in the resistance, while the former would only impede the path of any but the most powerful type of magneto-generator current, if there should be an attempt to give the usual length to the magnet cores. Pl6. i8o. — Interior of ttie generator box of a bridging telephone apparatus, showing the long-wound ringer magnets. Compare with Figs. 3, 97, 98. The Generators of Bridged Telephones. — The genera- tors of bridging telephones are constructed with the express view of securing a large quantity of current, a high amperage, so as to ring the bells of the mogt remote apparatus on the line. They must also be constructed to give a high voltage to work through the combined resistance of all the coils. The armatures are usually wound to a resistance of about 350 ohms, with No. 33 B. & S. wire, which gives ..about the right current strength for ordinary bridged lines. §@me manufacturers have sought PAH TY LINES. 22 "J to strengthen the current producing capacity ot the generators by increasing the number of the magnets to four or five, thus giving a longer field and allowing the winding of more wire on the armature — giving a greater number of "ampere turns"— with a consequent augmented E M F. When, however, the bridging circuit is connected to a central exchange, and some form of selective signaling is there installed for calling up the desired station, the generators of the separate apparatus are wound to furnish a low E M F, which will be effectually barred by the bell magnets, while amply sufficient to operate the switchboard drop, or signal, as in ordinary single-station exchange circuits. Another method of accomplishing the same result is to wind the station magnetos so as to generate direct instead of alternating currents, which, while readily operating' the bridged drop of the exchange switchboard, , cannot energize the polarized bell magnets wound for alternating currents. Induction Coils Low Wound. — On the ()ther hand, it is very essential in any apparatus intenderd for a bridging circuit that the induction coil secondary should be wound to as low a resistance as is consistent with good transmission. : Otherwise,, and particularly when the resistance oi the secondary winding! is as high as 500 to 800 ohms, it happens that the talking current] will seek a path through the bell magnet coils as quickly as through the induction coil secondary and the receiver. Independent Bridging Systems. — The Carty patent, cov- ering a system of apparatus wiring best adapted for use in bridging circuits, is the one under which most manufacturers of such telephones operate. As,, however, it does not cover the method of stringing an inter-communicating line in multiple, others have devised systems of their own which accomplish the same ends without infringing. Fig. 181 shows a circuit diagram of the Ericsson bridging or multiple- wiring system. As may be seen at a glance, it differs considerably from the Carty wiring^ already shown. When the hook is in the lowered position under the weight of the. receiver, the circuit of the bell is made at the 228 A B COF THE TELEPHONE. end contact of the switch hook lever. When it is allowed to rise, on the removal of the receiver, it makes two contacts, as shown, thus closing the talking circuit. The principal differ- ences between the Carty and Ericsson wiring are, t?ierefore: {a) the bell magnets of the communicating apparatus are not bridged across the line when the talking circuit is made, and the neglect to return the receiver to the hook at the close of the conversa- tion would prevent the ringing of the signal ; (^) the switch lever has both upper and lower contacts, as in all other types of apparatus; {c) the bell coils of the two communicat- ing telephones being cut out of line, all "leakage" of current is limited to the apparatus not so connected, and all unnecessary resistance is dis- pensed with. The Ericsson Company claims this latter feature as the great- est advantage of the system. They, however, follow the general practice of high winding and long cores for the bell magnets, frequently using a resistance as high as i,6oo ohms, which still further decreases the tendency of the telephone current to "leak," and, as they claim, secures the best possible talking service. Selective Signals. — The greatest possible disadvantage of all party line telephone systems is that the act of " ringing up" one station, even by an agreed code signal, rings the bells at every other, thus constantly annoying every party on the line, making possible frequent mistakes and consequent confusion, and, worst of all, allowing every conversation to be "listened in " by any meddlesome fellow with an apparatus in his house or place of business. Such facts have led numerous inventors to devise schemes for "selective signaling," as it is called, by Fig. i8i. PARTY LINES. 229 which a signal intended for any particular apparatus will be rung by its bell only; and also methods of "locking out,'' whereby the act of making the telephone circuit between any two apparatus energizes conveniently placed relays in every other, thus cutting the other apparatus out of line. While it is not within the province of this book to enter into the details of the numerous ingenious and complicated devices for accom- plishing the end of selective signals, it will be possible to indicate briefly the general varieties of such machines. These fall under three general heads — those working on the "step- by-step " principle; those working by the use of harmonic reeds; those working on the principle of varying "strength and polarity " of bell magnets and calling generators. The Step-by-step Principle. — The first of these, the step- by-step signal system, is at once the most practical and most familiar in its operation. It is constructed on the same general principle as is applied in the dial, or needle telegraph, and in the ordinary stock quotation and news " tickers. " This principle consists, briefly, in transmitting a current, controlled by a regularly graduated make-and-break device, which operates relays at the various stations on the line. The transmitting instrument is generally a dial divided into a number of points, each corresponding, as in the "stock ticker," to some particular letter or figure. Each such point, moreover, is a " make," while the spaces between them are "breaks." By moving the key, pivoted at the center of the dial, over a given space, say from A to M, one makes and breaks the circuit as many times as there are points and spaces between, thus sending precisely the same number of distinct impulses along the line as there are make points passed over before reaching the desired point, say M. These several impulses operate the relays at the several stations exactly the same number of times, and by virtue of a pivoted lever attached to the armature of each, works a ratchet wheel or escapement, such as is used with clock pendulums, so as to operate a needle on a lettered dial or a wheel bearing printing types on its periphery. On bringing the above-mentioned 230 A B C OF THE TELEPHONE. transmitting key to the point desired, say M, the needle of the dial rests, or the mechanism of the "ticker" causes that partic- ular letter to be impressed on the paper strip, which then moves one point ahead for another impression. Step-by-step Signals. — The application of this principle to telephone party line circuits, while, seemingly, requiring much more complicated devices than those described, may be briefly stated as follows: The " make " points on the transmit- ting instrument correspond, not to letters or figures, but each to some particular apparatus in the circuit. Thus, if there are eight apparatus in the line, the first corresponds to the first point, the second to the second, and so on to the eighth. Moreover, the toothed wheels at each station are so arranged by •insulation of every point except the one corresponding to its proper number, say, one, two, three and so on, that its call bells maybe operated only when that point has been brought into position. We may understand this by an illustration. Suppose that a number of printing telegraph instruments be arranged in a circuit. If from each of the type wheels, save one, we cut the letter M, it is obvious that by that one only will the letter M be printed. On the same principle the bell of apparatus No. i can be operated only when the relay and ratchet has brought the signal controller to point i ; similarly with all other appa- ratus. If, therefore, a telephoner wishes to call up Station 3 he moves his circuit-controlling key to the 3-point, and then sends out the ringing current, with the effect of calling Station 3 and no other on the line. The other call bells are effectually locked out by virtue of the insulated contacts at all points save that corresponding to their own numbers. The Transmission of Harmonic Waves. — The harmonic system of selective signaling is based upon the same principles which we have already seen applied in Prof. Bell's first experi- mental telephone and other contrivances for the transmission of musical sounds. The calling current consists of waves of dif- ferent length and frequency, according as it is started by the PARTY LINES. 231 FiO. 182.— Western Bridging Bell Party I,ine Telephone Apparatus, equippe4 with a five-magnet generator. 232 A B C OF THE TELEPHONE. (vibrations of a circuit-cutting reed or rod at the calling station. Furthermore, the making of the calling circuit at each appa> ratus depends upon the reception of a current of the right degree of frequency and wave length to affect a circuit-cutting reed or rod, similar to the one vibrated at the calling station. Such receiving reed or rod at each instrument differs in these respects from the one at every other station. The idea may be briefly expressed by saying that each apparatus has its own note — the rate of vibration of its reed — which must be sounded at the calling station before the current can ring the bell of that apparatus. Thus if a party line circuit be operated by har- monic signals, and be not connected to a central station, each apparatus must be equipped with its own receiving reed and also with some device for shortening and lengthening the vibra- tions of the calling reed mechanism. Most selective signal party line circuits, however, are constructed with reference to the connection of the line to a central office, where suitable sig- nals may be operated by the calling current from any one of the apparatus on the circuit, and the complicated calling mechanism may be operated to the best advantage. The Pendulum Signal. — One of the simplest and most efficient signaling devices of the harmonic, or wave length, description is the Stephen vibrating pendulum system, which has been used to some extent in England. It may be briefly described as follows: In each station is a pendulum of fixed length, different at each station, and having a proportionate rate of vibration. Attached above its pivot is a fork or hook standing at right angles to the length of the pendulum rod. The pendulum rod itself carries the armature of an electro- magnet, and is set vibrating by any calling current that may - enter the apparatus. The fork or hook above mentioned is of such a length that, when the pendulum is actuated by a current of the right wave length and frequency, and vibrates accord- ingly, it will release a hooked lever arranged at the right interval and thus make the circuit of the call bells by breaking its normal or resting connections. This normal position is PAR TY LINES. 233 rtstored by a double-pole relay, which acts on another lever, so arranged as to renew the first connections under impulse of a current sent from "central," when the conversation is con- cluded. The principal point of the selective signaling in this apparatus is the length of the fork or hook at the upper end of the pendulum rod, which, with the relative position of the lever to be actuated by it, differs in every station apparatus on the circuit. The Metronome Transmitter. — The apparatus used to send a call from "central," or from any calling station, con- sists of an instrument resembling the metronome used by musicians for beating out the time required for any musical composition — a rod which, moved by clockwork, vibrates slowly or rapidly as a weight is slid up or down to one or another of the graduated positions in its length, thus shortening or lengthening the rate of vibrations. With such an instrument to control the undulations of the current — being used as a rheotome — the selective signal may be readily transmitted according to the position of the weight on the indicated marks corresponding to the rate of vibration for one or another station apparatus. Such a method of signaling, while simple in theory and per- fectly practical, is open to grave objections on the ground of the inevitable delay m making connections on the line. Signaling by Current Strength or Polarity. — The third head under which' party line selective signal apparatus are grouped is that of varying the strength and polarity of the generators and bell magnets at the several stations. A number of highly elaborate systems have been devised to apply this principle to party lines of from three to eight stations on a single circuit, the calling impulses in every case being sent from a central office switchboard, where suitable ringing switches are arranged. In all these systems the feature of bell magnets, so polarized as to operate in obedience to a current coming from one direction and remain irresponsive to one coming from another, is prominent, the principal points of divergence being 234 A B C OF THE TELEPHONE. Srcni-iHe El-ectric Company SekBCTtUB F^nTYLlNESvSTVM bMs4 CCNTMM. OmOB CONMeCTIOH* A-20S Selcctiue Rinojns Key Fig. 183.— Diagram of the four-station polarity signal system of the Sterling Electric Co. Stations are to be connected to line as shown on diagram, that is, the right hand post on magneto bell on stations one and three are to be connected to the tip side of line, and the left hand post to the body side of line. At stations two and four, the right-hand post is connected to the body side of line, and the left-hand post to the tip side of line. The middle post at all stations is to be connected io the ground. The generators are supplied with commutators, so arranged that they will furnish both the alternating and straight currents with a grounding point, which must be connected with a good ground. A selective key is furnished, connected in such a manner that anv desired current can be thrown from the generator to any regular ringing and listening key on the board. If hand generator only is used for operating this system, the positive and negative brushes of the commutator, which are indicated on the diagram by the. signs (+) pins and (— ) minus, respectively, are wired directly to the two short bus bars on the selective key, which are also marked with a (-|^) plus and (— ) minus sign to correspond. The alternat- ing spring or brush which is designated by the (+) plus and (— ) minus si^s in combina- tion, is carried io the isolated point on the selective key correspondingly indicated, where both power and hand generator are to be equipped with the party line commutator, the connections must be complete, as shown on diagram, going through a three lever switch. The power "jenerators must invariably be run in the direction indicated by arrow stamped on the face of the pulley. A reversal of this direction will result in transposition of the numbers of the stations responding to calls owing to the fact that the current will be reversed. In the operation of the system in order to call station No. t on any party line, the button marked No. i on the selective key must first be depressed, the plug inserted in the jack in the i;sual manner, and the ringing done on the regular ringing and listening key as usual. Pressing the button No. i merely throws the current required to operate stati on No. I to that pair of plugs. If No. 2 is wanted, No. 2 button is depressed, and so on. Wh en generator is to be used for ordinary exchange work, calling only for alternating current, the button on selective key marked " A " must be first depressed. Where the subscriber on one of these party lines wishes to call the central office, he must first take the receiver from the hook before ringing, as otherwise it would be impossible to ring down the drop at the central office. This will also give him an op'\)rtunity to ascertain whether the hue is lu use before calling for connection. PARTY LINES. 235 in the means employed to apply this scheme more readily, or to vary the number of possible combinations of circuits and polar- ized magnets. A Two-limb, Three-circuit Signal. — Most of the later applications of the principle contain some variation of the idea first introduced with the Sabin and Hampton three-station system of making three distinct circuits with the two limbs of a single metallic line, consisting of a line wire and a return wire, as must be now understood by the reader. This idea is, briefly, to placethe call bell magnet of the first station in a "bridge" between the line and return wires; to attach one side of the bell circuit of the second station to the line wire and the other to ground ; and to attach one side, of the third station to the return wire and the other to ground. To operate the signals on this system it is necessary that there be three separate generators at the central station, each one being connected to the wires in precisely the same fashion as are the call bell magnets. Thus the current from one goes out on the lirje wire and returns on the return wire; that from the second goes out on the line wire and returns by earth connection ; that from the third goes out on the return wire and returns by earth connection. As will be obvious on reflection, a current sent from any one of the three generators completes its circuit through the instrument it is intended to affect, and through no other. Six Signal Circuits to Two Wires. — A further elaboration of this system may be obtained by stringing six bells instead of three, two on every circuit just described, by alternating the polarity of the bells and generators. Thus there may be two call bell magnets at separate stations, each bridged between the line wire and the return wire, but one of them must be so polarized as to respond only when a positive current comes- on the line wire and a negative on the return wire; the other, so as to respond only when a negative current comes on the line wire and a positive on the return wire. Similarly, also, the grounded circuits on each limb may be duplicated by making 236 A B C OF THE TELEPHONE. one magnet of each pair to respond only when a positive current comes from earth and a negative from the wire. To accom- plish the result of selective signaling in both cases it is necessa y to have either six generators, properly polarized, or some method of changing the circuit connections, from one side of the line to the other, so as to send a positive or a negative current by the desired route. Thus a simple three-circuit arrangement may become a true polarity method of signa'ing. Such an arrangement might operate to advantage on a short line. Fig. 184.— A common form of telegraphic relay. A telegiapWc "sounder". The Principle of Quadruplex Telegraphy, — In addition to this feature some systems also embody the idea which is basal in quadruplex telegraphy — the introduction of relays which respond only to current of given strength and are indifferent to its polarity. In telegraphy this principle is applied by having a " polarized relay " and a " neutral " or current strength relay at every station, each relay being operated by a separate key in the transmitting office. Thus, as may be understood on reflection, this arrangement admits not only of selective signaling, but also of four messages being transmitted at one time over one grounded circuit, two in one direction and two in the other, the strength of the current and its polarity being the important considerations in each case. PARTY LINES. 237 The Merits of Selective Signal Devices. — As regards the matter of selective signals in general, we can do no better than to quote from Herbert Laws Webb's excellent "Telephone Handbook." He says: "Many inventors have turned their attention to this problem, and T. D. Lockwood announces in a paper before the American Institute of Electrical Engineers, read in 1892, that between January, 1879, and December, 1891, no fewer than 161 American patents were taken out for systems of selective signals for tele- phone lines. Most of these devices are extremely complicated, almost all of them are inoperative, and none has come' into general use. " In view of the more recent devel- opments in telephony, this statement must seem increasingly true. As most party line circuits cover no great length of wire, except in the case of some toll systems entering city ex- changes from the outlying villages, where jt is more economical to string all apparatus on a single circuit, the adoption of elaborate systems of selective signaling seems almost a waste of money and pains, a doubt- ful way of saving the expense of separate line circuits for each station. Toll Lines. — The term, " toll line," is used to designate a trunking line between exchanges, which may have a number oi private telephones bridged in its length, or where a party line telephone circuit has its terminals in a central exchange switch- board. This designation is said to be derived from the fact that each separate telephoner on the circuit is charged for every Fig. 185. Toll line wall switchboard 238 A B C OF THE TELEPHONE. occasion on which he uses the line, ]ust as formerly persons driving on country roads paid a designated fee, at the various toll gates they might pass, for the maintenance and repair of the highway. Also, where the term is applied to a trunking line, as stated above, it is proper from the fact that such lines frequently leading to exchanges conducted by other companies involve an additional "toll", or charge, to the subscribers using them. Toll Boards. — On the other hand, on account of the exposure of such lines to interference from lightning and other weather disturbances ; on account of the fact that they are liable to leakage in proportion to their length and the number of apparatus bridged on the circuit; and because of the difficulty of properly adjusting the circuit arrangements, when required, so as to connect a grounded line to a metallic, it is necessary to use specially constructed switchboards and employ skilled operators. Toll boards differ from others in having fewer drops — generally no more than ten to a position — including devices for switching repeating coils and other necessary devices into circuit, when needed, and in having arrangements for making such selective or code signals as may be in use to cal' the various telephoners on the line. Several telephone manu facturers have given particular attention to this branch ol exchange work, with the result that any company desiring to install toll-line appliances has a wide variety from which to select. One of the most complete and practical of these devices is shown in Fig. 185, which represents a combination switch- board for local exchanges and toll lines, manufactured by the Western Telephone Construction Co., of Chicago. CHAPTER EIGHTEEN. PRIVATE TELEPHONE LINES AND INTERCOMMUNICATING SYSTEMS: COMMON RETURN CIRCUITS. Common Return Intercommunicating Systems. — The general plan of wiring a number of telephone apparatus, so that each one has its own wire, forming a circuit between itself and any other by using a common return wire as the other limb, is adapted primarily to the needs of manufactories, business houses having a number of separate offices or desks, and to small domestic systems. Such small systems require no central office nor any scheme of selective signaling. Moreover, the danger of being "listened in " is reduced to a minimum, if not rendered impossible; and the expense and trouble of caring for the apparatus is similarly reduced by Using a common calling battery. Signal Devices for Common Return Circuits. -^In most of the recent schemes for stringing intercommunicatmg tele- phone lines every station is, in fact, both " central " and receiving station. That is to say, means are provided in each apparatus for as readily calling any other station as exist in an exchange, and in the point of receiving there is no further trouble necessary than to answer when the signal rings. This end is attained by some form of switch which at any apparatus can connect the common return wire into circuit with the terminal of the desired station at that same apparatus. Such switching is accomplished either by plugging a jack, as in exchange switchboards, or by making the required connections by some form of sliding contact lever key. Both varieties of switch operate by making contact between a leaf spring, such as is used in switchboard jacks, or else between the lever of a pivoted key connecting to the common return wire and any one of a number of anvils or contact points which is the terminal of some station line or other. 33!) 240 A B C OF THE TELEPHONE. Plgg and Switch Key Signals.— Figs. 186 and 187 repre- sent two instruments of similar appearance and construction, one furnished with plug board switch, the other with an ordinary keyboard. The latter is manufactured by the Connecticut Tele- FiGS. 186-187. — Two types of intercommunicating circuit telephones, showing plug and button contact switches. phone and Electric Co., and the former, by the Western Tele- phone Construction Co. They are intended for circuits using a common calling battery and individual station talking batteries. The proper wiring connections are indicated beneath the binding posts at the base of the backboard in Fig. 187. In most plug board telephone sets the plug cord is a conductor to connect that particular station into circuit with any other line; in others it is merely a conventional device intended to accom- INTERCOMMUNICATING SYSTEMS. 241 plish the same result by pressing a leaf spring against its anvil, thus connecting two circuit terminals. Wiring of a Common Battery Circuit. — The proper wiring connections for a three-station common signal battery system is shown in Fig. 189. At the left of the figure is an ordinary standard desk " phone " with the line wires connected to a suit- able "terminal block," which also contains the induction coil. The switch keyboard is on a separate piece, as must be the case with desk apparatus of this type. The second station is equipped with a similar desk set, although only the wiring Fig. 188. — Diagram of the construction of a fonn of button contact switch. The common wire and home apparatus line are wired to the lever, the others, to the binding nuts on the circumference. connections with the base of the apparatus are shown, and, unlike the one at the first station, the induction coil is enclosed in the base, to which the local talking battery is also connected. The wall set in the third station is one of the kind already depicted. The connections are merely indicated, and the switch may be of either the plug or keyboard type. As may be seen, the connections of each talking circuit have the "common battery wire " as one limb with any one of the three station wires, while the calling circuit includes both the former and the ''common return wire." The talking battery of any station may spend along this "common battery wire," because the calling battery itself is then on "open circuit" and emits no current. Fig. 190 illustrates the wiring connections for an ordinary 242 A B C OF THE TELEPHONE. a 01 IE a « B-C * a Mm O tf] a-^ £■3 o t: 2p INTERCOMMUNICATING SYSTEMS. 243 standard desk set with talking battery, magneto-generator and extension or wall bell. Such a system is adapted, first place, for a full metallic line, and when arranged in an intercommuni- cating system with a common ringing battery the terminals, here shown as connected to the ringer generator, are connected to the station line wire and common return wire, respectively, as indicated in Fig. 189. Wiring of an Individual Battery'Circuit. — As we have seen, all individual line intercommunicating systems have a common return wire which may complete the circuit between any two of them with the wire belonging to the called station as the other limb. On a system including three stations, therefore, there would be four wires, as in Fig. 191. This figure shows a series of apparatus, each equipped with its own magneto-generator and transmitter battery. On systems containing a common calling battery, as in Fig. 189, such a three-station line must be equipped with five wires, or two more than the number of stations. If now the telephoner at station i on the former diagram wishes to call up station 3, he moves the switch lever of his apparatus to the contact point numbered 3 on his dial, thus bringing wire number 3 into circuit with his apparatus and the common return. If his apparatus is equipped with a magneto the calling circuit is thus made from his station, along wire 3 to line 3, into apparatus 3, where it rings the bell signal, out of apparatus 3 to the common return wire, and thence back to apparatus i. The talking current follows the same path as soon as the switch hook is raised, as is the case in central exchange apparatus. If, on the other hand, the line be of the common calling battery type, the operation is as follows: Telephoner number i moves his switch lever to contact point 3, thus closing a circuit with apparatus 3, and in order to send out a calling signal depresses a button at his apparatus, thus causing a current to move from the common battery along the common battery wire until it reaches the wire leading into apparatus i. Entering there it passes through the apparatus to its connection with wire 3 out upon line 3 to apparatus 3, which it enters, causing 244 A B C OF THE TELEPHONE. INTERCOMMUNICATING SYSTEMS. 245 the bell to ring, and, having passed through the magnet coils, emerges on the common return wire, whence it moves to the r^posite pole of the battery. The raising of the receiver hook ai. each station closes the talking circuit, which, as we have seen, uses the common battery wire for its return limb on the system of wiring shown in Fig. 189. Ericsson Intercommunicating System. — Such a iive- wire three-party system is shown in greater detail in Fig. 19a, which illustrates a system using an Ericsson microtelephone set at stations i and 3 and an ordinary wall apparatus, with switch attachment, at station 2. Here the three individual wires are numbered L\ L^, L' and the ringing battery and common rety.rn wires, R B and C R, respectively. The apparatus connections and the wires numbered i, 2, 3, 4, at stations 1 and 3, corre- spond to those shown in connection with the diagram of micro- telephone circuits in Fig. 130. The apparatus at station 2 is furnished with a switch hook, and is in all respects of the ordinary type, except that it has no magneto-generator. The calling signals from all stations are sent out by merely placing the switch lever on the required contact point and then pressing the push button, P. Thus if station i wishes to call up station 3, he places the switch lever on point 3 an4 depresses the push button, P, of this apparatus, with the result of making the circuit of the calling battery along the wire, R B, to the branch wire leading into station i ; through that apparatus to the switch lever, to point 3, to line wire 3, to apparatus 3, where it enters and rings the bell ; passing out of apparatus 3 from the other side of the bell magnet to wire i of the microtelephone, thence as shown in Fig. 130 of the circuits of such an instrument to wire 2, and thence by the common return wire, C R, to the opposite pole of the ringing battery. When the circuit is arranged to include an individual ring- ing battery or magneto-generator at each station, it is placed at the point, R B oi each apparatus, and the line connection is made with the common return wire, C R, instead of the ringer battery wire, R B, which is then omitted, leaving a. four-line three-party circuit, as in Fig. 191. When it is desired to use a 246 A B C OF THE TELEPHONE. ^ X!n o "^G . 1 c® i-^ J a ^^sdi_ ^tttrlD «MD INTERCOMMUNICATING SYSTEMS. 247 common battery for both talking and signaling, the wires, T £, and R £, may be connected, thus joining the local transmitter circuits direct to the battery when the switch key, P, is in its normal position. Device to Insure Ringing of Signals. — According to the wiring of this system, every bell is connected, as shown, with the line wire of that station, so that a calling current coming along its wire, Z\ I? or Z', finds a path directly through the coils of the bell magnets and out to line, the position of the switch lever on any one or other contact point being a matter of indif- ference so long as the talking circuit is broken and the push button lever, P, is in the raised position, as shown. Thus is avoided the complication found in some of the earlier inter- communicating systems, the necessity of returning the switch lever to the home point after a conversation, a duty that must be frequently neglected throygh carelessness or preoccupation. Thus the call bell of each station is bridged across the circuit between its proper wire and the common return, so long as the individual talking circuit is not made. The making of that circuit, either by the rising of the switch hook or the act of pressing the lever shown on the shank of the microtelephone instrument, switches out the bell. On the other hand, the releasing of the fever of the hand microtelephone, or the hang- ing- up of the receiver places the bell once more in circuit, ready to be rung by another call coming from some other station, no matter on what point the switch lever may be left. The advan- tages of such an arrangement may be readily understood, when it is remembered that one of the most serious problems of early intercommunicating systems was, how to insure the restoring of the lever to the home point after each conversation. The Ness Automatic Switch Hool<. — The difficulties involved in a neglect to return the switch contact lever to the home point after a conversation has been overcome in a different manner by other inventors and manufacturers, whose plan is, in brief, to provide a device for automatically accomplishing this result. One of the most typical and excellent of these is the 248 A B C OF THE TELEPHONE. INTERCOMMUNICATING SYSTEMS. 249 Ness automatic hook switch manufactured by the Holtzer-Cabot Co., of Boston, Mass. Fig. 195 shows a wall apparatus for a' ten-station line equipped with this device, and a magneto generator. In other points there is no departure from the general type of switching telephones, except for the fact that a stop piece for the lever is provided in Ness instruments at the right of the semi-circular row of contact points. This is an essential part of the contrivance, and is intended to hold the lever at the home point when it has automatically sprung back. 5 /? ■JG. 193. — Mechanism ot the Ness hook switch for intercommunicating telephones. Mechanism of the Ness Hook. — -The mechanism of the Ness switch hook is shown in Fig. 193. Here ^ is the lever just mentioned, which is normally held in contact with the home point, or right hand contact, on the outside of the door of the box. This right-hand contact connects with line wire i in station i, with Ime wire 2 in station 2, etc., the order of the points being varied for every station. (See Fig. 194.) If, now, for example, station i desires to call up station 8, he moves the switch lever to point 8, as in any other type of switch, being then ready to send out the calling signal. The act of rotating the switch lever turns the ratchet wheel, R, which is held in 250 A B C OF THE TELEPHONE. INTERCOMMUNICATING SYSTEMS. 251 position at the desired point by means of the pawl, P, which is reinforced by a spiral spring, as shown. Having moved the lever, S, to point 8, the telephoner at station i has his apparatus in position to send out the calling signal current, which he does by pressing the tip of the lever into contact with the semi- circular metal strip shown in Fig. 195 below the row of contact buttons. This act accomplishes the closing of the common battery circuit, as in other systems of intercommuni- KiG. 195. — Wall apparatus, equipped with Ness switch hook and magneto generator. eating telephones, sending out the calling current, being con- nected to the common battery wire, or else makes the proper circuit for the magneto current when no common battery is used. Operation of the Ness System. — Immediately after send- ing out the calling signal, the telephoner at station i removes his receiver from the hook, which then rises, making the circuit of the talking battery, as in any other type of apparatus. The conversation finished he again hangs his receiver, and the hook is brought back to its lowered position. This act causes the 2!;2 A B C OF THE TELEPHONE. switch handle to fly back to its normal position over the home button by virtue of the mechanism indicated in the figure as follows: On the short arm of the switch hook is pivoted the dog, Z>, which, when the receiver is rehung, engages a notch in the pawl, F, and lifts it away from the ratchet wheel, Ji. A spiral spring held around the axle shaft of the ratchet wheel, H, then acts, causing the ratchet wheel and the attached handle to be restored to the first position. Meantime the dog, D, slips Fig. 196.— HoUzer-Cabot desk telephone equipped with the Ness switch. out of the notch on F, and allows it to fall into contact with the ratchet in readiness for the next call. In order, however, that the pawl, P, may not engage the ratchet before the lever, S, has fully reached the normal position, a second dog, /, is placed, as shown, being pressed by a spring to a position under the pin, /, carried on the pawl, and thus holding the pawl out of engage- ment until -5" has reached the home point. Then a cam on the under side of the ratchet — it is not shown in the cut — pushes the dog, /, away from P, and allows the pawl to fall into engage- ment with the ratchet. INTER CO MM UNICA TING S YS TEMS. 253 254 A B C OF THE TELEPHONE. Wiring of the Ness Circuit. — The Ness switch is a very complete and accurate device for achieving the end sought. All parts exposed to wear are made of the best hardened steel, thus insuring the utmost durability. Best of all, it reduces the number of necessary acts on the telephoner's part to the lowest figure, and leaves nothing dependent upon his memory or carefulness. The full circuits of the Ness automatic system are shown in diagram in Fig. 194. Here we have a four-station common calling battery line, in which, as may be seen, station 4 is in the act of calling station i. The circuits, when made, are the same as in the common battery lines just described, Fig. 198.— Holtzer-Cabot wall microtelephone apparatus for hotels and private systems. being completely indicated at stations i and 4, and merely omitted from the drawing at 2 and 3. The wiring diagram also indicates how the pressure of the switch lever, S, on the semicircular contact piece, D, makes the calling circuit with any station desired from the common battery, C B, along the wire, as shown. Common Tali (D > ®) i6b A B C OF THE TELEPHONE. Another type of hotel or factory office switchboard apparatus is that manufactured by the Western Telephone Construction Co. , of Chicago, and shown in Fig. zoo. This is an ordinary mag- neto calling apparatus, with the exception that, in place of the usual transmitter arm and coil box, it carries a small drop 3nd jack switchboard. The figure shows an installation of Fig. 212, — Annunciator-switchboard for hotel systems. By plugging a jack talking connections may be made between any room apparatus and the office. twelve drops of the type already described, and calling or answering connections may be made between the apparatus and any other on the line by inserting the plug in the required drop jack. Also, any two stations may be connected as in the ordinary switchboard exchange. A conveniently located switch key enables the clerk or operator to send either a magneto calling current to the other station or to make the talking circuit, as is necessary. CHAPTER TWENTY. LARGE PRIVATE SYSTEMS AND AUTOMATIC EXCHANGES. Limited Capacity of Private Teleplione Systems. — Asa general rule not more than twenty or thirty telephone apparatus may be included in an intercommunicating system, either with common return wire or full metallic circuits. The reason lies in the very obvious fact that even if an indefinite number of stations should be wired to the ordinary apparatus switches, such as have just been described, there would be continual con- fusion and interference on lines apt to be busy. The alternative, therefore, is to have a private central exchange, with a switch- board arranged to connect the lines of any two stations in the system. Such objections hardly hold good for hotel and large business office telephone systems, which have a "one-way' connection with the main office, since such systems are, in fact, dependent on a central office. A Private System of Exceptional Size. — A notable exception seems to be presented in such an intercommunicating system as was, some months since, installed by the Stromberg- Carlson Telephone Manufacturing Co., of Chicago, in the offices of the People's Gas Light and Coke Co. of the same city. In order to connect the numerous desks in the book-keeping department and give telephonic communication with the man- agers' offices and general storerooms, all without the use of a . private exchange, it was necessary to provide intercommuni- cating lines for nearly one hundred apparatus. The problem seems to have been satisfactorily solved by connecting each desk to every other one with which it could possibly need to have communication, and arranging the circuits to be controlled by a system of plug sw^itches. On several of the desks are plug 267 268 A B C OF THE TELEPHONE. switch boxes with a capacity as high as sixty points; that is to say, providing for switching connections with sixty other appa- ratus. The wiring is so arranged that the various managers' desk sets may also be put into telephonic circuit with the public exchange by proper plug contacts. Automatic Telephone Exchanges. — There seems to be a well-defined tendency among telephone users, particularly in the matter of installing private plants, such as has just been described, to avoid the use of an ordinary switchboard exchange, and render the system, as far as possible, automatic. Thus it is that a number of inventors have set themselves to solve the problems involved, with the result that several automatic exchange systems have been put upon the market within recent years. As must be understood, with very small reflection, the most likely method of operating such exchanges is by some variation of the step-by-step principle, which, as we have seen, is the most feasible yet found in the domain of party line selective signals. As a matter of fact, an automatic exchange is only an application of the selective signal idea, so extended as to meet the needs of a larger number of stations than are usually included in an intercommunicating telephone system. The Strowger Automatic System. — One of the most successful systems of automatic exchange is the Strowger system, which is installed in several American cities. A Strowger exchange in Atlanta, Ga., has as many as 500 subscribers, and estimates an average of twelve calls daily for each one. Another, recently installed in New Bedford, Mass., began business with about the same number, and has been in successful operation for several months. The mechanism of the system is exceedingly, complicated, and could not possibly be described within limited space. Briefly, however, we may treat it as follows : The line wires at the central station are arranged in rows in definite order. Over the terminals of branches from these, from the exchange apparatus of each subscriber, is placed transversely a shaft, which, by a suitable arrangement of relays, ratchets AUTOMATIC EXCHANGES. ' 269 and levers, has both a rotary and a longitudinal motion under impulses coming from a common battery. Each such shaft carries a series of arms set upon it spirally; that is to say, occupying different relative radial positions on its circumfer- ence. The wires are divided into groups of tens, thus, loi, in, 121, 131, 141, etc., or 102, 112, 122, 132, 142, etc., and each shaft has an arm corresponding to each such group. Thus a contact between the shaft and the desired line wire may be made through one of these radial arms, according as the switch mechanism at the subscriber's apparatus is operated. Subscriber's Circuit- Making Apparatus. — Each station apparatus has four keys — one lettered H, for hundreds; a second, T, for tens; a third, U, for units; and a fourth, R, for release. The wiring in connection with these keys is so arranged that circuits are made from the central exchange grounded battery over either limb of the talking line alternately to ground at the subscriber's station. Supposing a given subscriber wishes to call up another, numbered 253, he will press key H twice, thus causing the -shaft of his switchboard apparatus to rotate . through the space of two make points or teeth, at the same time closing and locking a circuit connection through a second relay over the other limb of the line. He then presses key T five times, thus causing the shaft to be moved longitudinally, through five steps and locking circuit connection, through a third relay. When key U has been pressed three times one arm of the shaft is brought into contact with wire 253, and commu- nication may be had by ringing his magneto bell to call up subscriber 253. If his apparatus bell does not ring he knows that 253 is engaged, and accordingly awaits another opportu- nity. When the conversation is completed he presses the key R, thus operating a restoring mechanism at "central," and leaving his switchboard apparatus in position for another call. The later apparatus of this system have a simple dial arrange- ment, instead of buttons, for transmitting the current impulses, and the method is to send out three distinct sets of impulses for every call: the first for hundreds, by turning the dial to the 2^0 A B C OF THE TELEPHONB, c« (0 o "S'S Is .S-S ig U Ik s I AUTOMATIC EXCHANGES. 2^1 required number from o to 9; the second for tens, by turning the dial to the required number from o to 9 ; the third for units in the same fashion. Each separate turn energizes the proper relay the required number of times, then locks connection for the next set of impulses. The Clark Automatic Exchange. — A much simpler system, adapted for use on lines of not more than 150 stations, is the one manufactured by the Clark Automatic Telephone Switch- board Co., of Providence, R. I. This may be described as a thoroughly representative step-by-step mechanism, and has the advantage of combining simplicity of construction with exacti- tude of action. Each subscriber's station has a call-transmitting device, consisting of a dial around whose circumference is a succession of numbers, from i to 75, corresponding to the numbers of the stations in the system. This is shown in Fig. 213. When one subscriber desires communication with any other he pushes in the button shown at the side of the trans- mitting dial, thus releasing a locking arrangement which normally holds the indicator mechanism at any desired point. He then turns the dial by the knob at the center until the desired number is opposite an indicated point. By this opera- tion he causes a circuit-making arm to pass over as many "make" points as there are between the start and the required number, thus alternately energizing and de-energizing an electro- magnet at the central station a corresponding number of times. This electro-magnet is shown in Fig. 215, which gives a good general idea of its operation. There is one such automatic switch, or "receiver,"' to every subscriber's apparatus. To the armature of the magnet is attached an escapement, as shown, which is normally held away from the pole by a small coiled spring. Each time the armature is attracted the escapement ' moves the ratchet wheel forward one-half a tooth, each release making the other half unfler the force of the spring. The contact buttons arranged around the circumference of the wheel are the terminals of the various apparatus wires in the system, and each movement of tihe ratchet through the snace of one 272 A B C OF THE TELEPHONE. tooth makes contact with one of these circuits. The forward movement continues until the required number is reached, for since the number of make points on the transmitting dial and the contact points on the ratchet wheel correspond exactly, the movements will always be in unison. The magnet takes energy from a battery included in the circuit. When the conversation is completed the subscriber again pushes in the plunger on his transmitting dial, and moves the numbered dial through the remainder of its revolution until it is restored to the neutral point from which it started, and is ready to start again. Fig. 214 shows the form of frame for mounting the subscriber's switching apparatus at the exchange. CHAPTER TWENTY-ONE. DEVICES FOR PROTECTING TELEPHONE APPARATUS FROM ELECTRICAL DISTURBANCES. Protective Devices. — Beginning the study of line wiring at a terminal station, the first things to attract the attention are the devices constructed to afford the apparatus protection from the damaging effects of lightning and sneak currents. The need of the former may be readily understood even by those who have little or no electrical knowledge; since, as all know, steel instruments and electrical contrivances, when in a position exposed to lightning, are apt to attract the bolts and suffer damage accordingly. The latter danger, exposure to sneak currents, due to the lines crossing with power wires or other conductors of electricity, is quite as grave. Such currents, entering a station apparatus or exchange, are liable to "burn out " all the magnet coils and other appliances by so heating the wires that their insulating covers are charred and rendered useless. To guard against such disaster devices are installed which are able to break the circuit when charged by a current of more than a specified amperage. Fuse Wires. — In order to thus break the circuit under the stress of over-heating, wires of fusible metal — usually an alloy of tin and bismuth in such proportion as to melt at a desired temperature — are interposed at proper points in the circuit, as, for example, at the entrance of a station apparatus. The arrangement of such a fuse is shown in Fig. 216, where the line wire and the direction of the current are indicated by the arrows, the fuse wire being between the two connecting points. If a current of more than the safe strength come along the line, the fuse is melted, or " blown out," as its resistance to the current generates heat, and the circuit is thus broken. Owinp- 1^73 2 74 A B C OF THE TELEPHONE. to the constant necessity of replacing blown-out fuses, such wires are usually attached between plates, or clips, of harder metal, such as are shown in Figs. 217 and 218, and these are inserted in circuit, either by screws, as in Fig.' 222, or between leaf springs. Each such fuse plate or clip has a flange, intended to hold a sheet of mica of proper length as mounting to the fuse wire, as shown in the same figure. .,iiJi-r~iL:^j. Fig. 216. — Fuse wire attached to connectors. Figs. 217-218. — Fuse wires attached to metal contact plates or clips. Heat Coils and Circuit Breakers. — While it is generally a sufficient protection against sneak currents to arrange the fuses as shown in the foregoing figures, the requirements of line work, in pole and station terminals, frequently demand some additional device which will insure the breaking of the circuit the moment the heat is of a sufficient degree to melt the fuse wire. Such devices are desirable from the fact that a badly arranged fuse — one not of the proper length or diameter—^ under certain conditions, such as will enable a short fuse to heat its metal clips by radiation, and thus increase its current- carrying capacity, will frequently admit of damage before the circuit is opened. It is also very difficult often to arrange a fuse to blow off with the desired degrees of current. To meet these conditions some inventors have enclosed in a coil of Ger- man silver a short hooked pin, the two being soldered with a few drops of fuse metal. A strong spring is then attached to PROTECTIVE DEVICES. 275 the hooked pin, and the moment the fuse metal is heated to the melting point by a current in the highly resistant coil, it is pulled forcibly from its socket, thus cutting the circuit. A device of this description (Ericsson's heat coil) is shown in Fig. 219. Tubular Fuses. — Under the general head of tubular fuses, we may place such devices as provide for surrounding the fuse wire with a sheath or jacket of non-conducting material. A representative protection of this kind is the Cook tubular iuse, a group of which is shown in Fig. 220, arranged to be fitted to a cable terminal. The case is a cylinder of hard rubber, carrying a threaded bore in its length. Into one end of this is screwed one terminal brass cap. Into the other end is screwed a brass plug with flanging end to hold in position a coil of Gernjan silver wire, which coil connects the two brass terminal caps, passing through a hole running in the length of the tube parallel to the screw-threaded bore. Through the center of the screw-plug is a longitudinal hole, into which is secured, with a low fusion solder, a brass rod attached to the terminal brass cap, which closes that end. A current of unusually high amperage passing through the coil of German silver wire produces sufficient heat to melt the fusile solder, and allows the cap and rod to be drawn from the tube by tension springs attached to either end, thus breaking the circuit through the heat coil. Lightning Arresters. — A simple form of lightning arrester, still used on some telephones, consists of three serrated metal plates, which are arranged in a staggered row on the top of the generator box, each being secured in place by one of the bind- ing posts of the apparatus. The third, or middle, binding post is connected to ground ; the other two are the terminals of the line wires. As these plates are not in contact — contact would interfere with the operation of the apparatus — the method of Fig. 219. 276 A B C OF THE TELEP/fONE. securing efficient protection from lightning is to place a metal- plug between the two line terminals, thus short-circuiting the apparatus ; or, to connect either or both with the grounded plate by similar plugs. The theory of this simple device is that the lightning will not affect the apparatus when it is short- circuited, rather preferring to jump the short distance to the grounded plate and pass off harmless. This result, however, does not always follow, as lightning is one of those things that often Fig. 220.— Strip of Cook heat coils arranged for use on a cable terminal. seems to be an exception to all rules. Another difficulty is found in the fact that should a subscriber neglect to remove the plug at the end of a thunder storm his instrument is rendered inop- erative. The line circuit being completed between the plates at the top of his apparatus, no current will enter. Such neglect has frequently worked great inconvenience, disabling an entire line when a number of instruments are included in a bridging circuit. Carbon Lightning Arresters. — A much more efficient method of grounding a lightning charge short of the apparatus to be protected is by the use of carbon arresters. As produced by nearly all telephone manufacturers, the carbon arrester con- sists of two fiat blocks of carbon, between which is placed a thin sheet of mica. Such pairs of carbon blocks are attached to each terminal of the line, one on each being connected to line, the other to ground. The direct line to the telephone PROTECTIVE DEVICES. 277 apparatus is, however, not broken on either limb, the carbons forming a branch circuit. The theory is that the lightning cur- rent will pass through the small hole in the mica strip between the carbon blocks, and follow the line of least resistance to ground. In practice this theory seems amply warranted. The Sterling Electric Co. have introduced a further protective feature: Fig, 221.— sterling combined heat coils and carbon arresters for protecting a subscriber's, station apparatus ; also long tubular fuse for protection on the line outside the subscriber's stdtion, not only perforating the mica plate at a point midway on its length, but also inserting in a hole in one of the blocks a small drop of fusible metal. Under a high heat pressure this metal will melt, thus assuring perfect electrical connection between the two blocks, and grounding the line. The fuse and carbon protectors are frequently combined in one instrument, as shown in the accompanying figures, thus assuring complete protection to the apparatus from all electrical disturbances. Line Protectives. — In well-constructed telephone lines every exposed point is protected as thoroughly as possible from 278 A B C OF THE TELEPHONE. lightning and sneak currents. Thus in addition to the com- bined fuse and carbon arresters attached to the telephone appa- ratus, outside protectives are provided to doubly assure the re- sult. In Fig. 221 are shown the station protectives used by the Sterling Electric Co. Within is the tubular fuse and carbon, and at the joining of the line outside is a long tubular fuse. A current that will pass through the one must meet the other. Similar tubular fuses are provided at all cable terminals, as shown in Fig. 2^0. Fio. 222.— Fuse and carbon protector for subscriber's station apparatus. CHAPTER TWENTY-TWO. THE GENERAL CONDITIONS OF TELEPHONE LINE CONSTRUCTION. Construction of Lines. — The subject of telephone lines includes a large number of details — such as proper construc- tionsi testing, repairing — which make it a profession by itself, to be mastered only by practical experience and careful training. In addition we may include under this general head the matters connected with terminal devices and the various protectives used on lines and apparatus and at central exchanges. In entering upon a review of the matter we must bear in mind that there are two distinct kinds of lines in telephone practice, as also in telegraphy ; lines with a ground return and full metallic lines, in which both limbs of the circuit are wires of the same mate- rial and dimensions. There are also two ways of constructing lines connecting terminal stations; stringing them on poles in the usual familiar fashion, and enclosing them in cables, which are run through properly constructed underground conduits. Very frequently also wires hung on poles are bunched into cables of the same description and suspended by hangers. The majority of lines also are strung on poles through one portion of their length and buried in conduits through another portion. All these points of construction involve the use of special de- vices, which will be explained in place. The Conditions of Line Construction. — By such devices as have been described in the last chapter the apparatus at sta- tions and exchanges may be protected from danger by foreign currents. There are, however, so many precautions necessary to the end of securing a thoroughly quiet and serviceable telephone circuit that we may venture to assert that the science of tele- phone line construction is very largely summed up in the knowl- 279 28o A B C OF THE TELEPHONE, edge of how best to overcome the electrical difficulties and ob- stacles that must be met. In the first place, the question of the material and dimensions of the line wire must be carefully con- sidered, both from the standpoint of durability and from that of conductance. For, when we have overcome the difficulties inci- Jent to the specific resistance of a metal by enlarging the diam- eter of the wirCf we are met by a new one quite as serious. By enlarging the diameter we have increased the total surface, or circumference, area of the line, hence increasing its electro- static capacity for electrical condensation with the earth as the other charging surface and the intervening air as the insulator. This condition, which is a wonderful deterrent to successful tele- phonic transmission, may be largely neutralized by using taller poles ; but here also is a difficulty, as above a certain height poles are exceedingly liable to be uprooted in a storm, with the obvious result of disabling the entire system. In order to avoid another cause of electrical waste it is essential that all joints be tight and secure, soldered where not otherwise protected, in order that the ends of the two wires so connected may not become a kind of loose contact microphone to the disadvantage of good conduction of speech. Finally the disturbances due to induc- tion, both electro-magnetic and electrostatic, from other wires, telephonic, telegraphic, or power, when these cross or are strung near the line, have to be neutralized. It is thus easy to see that the matter of line wiring for a telephone circuit involves many other considerations than merely attaching a wire of any metal to a series of poles of sufficient strength. Every phase of the question must be carefully considered and planned before work is begun on any given line. General Inductive Disturbances. — One matter which should be understood by every practical telephonist relates to the phenomena characteristic of the constant shifting and re- versals in the magnetic properties of a conductor carrying the telephonic current. As we have seen in a previous chapter, every current-bearing conductor, whether insulated or not, is surrounded by a whirl of magnetic forge, th^ lines of which run LINE CONSTRUCTION. 28 1 at right angles to its length. As we might naturally conclude, the directon In which this whirl of force moves is in accord with the direction of the electrical current on the wire. Hence it is that every time the current is interrupted, diminished or alter- nated, there is a corresponding alteration in the magnetic condi- tions of the wire, which demands readjustment or a new adjust- ment of the magnetic field. This process, of course, demands time and a certain expenditure of electrical energy, and for this reason constitutes a species of false or apparent resistance, comparable to the resisting property of water in a pipe when met by any sudden change in its rate of motion, either as an effort to start or to check its flow. The tendency of matter to maintain any condition is what physicists term "inertia." On the same figure, then, do we speak of magnetic impedance or self-induction of an electric circuit as electrical- inertia. It is most powerfully, illustrated in a coiled conductor, in which, as we can readily understand, the lines of each coil cross those of the ones next following, demanding possibly numerous adjust- ments of their relative positions and directions, which explain the delay in closing a live circuit through such a coiled con- ductor. The "magnetic lag," ' as it is termed, is vastly increased by introducing an iron core into the coil — making an electro-magnet of it — since the magnetization of the core con- stitutes a true impedance, the process of demagnetization, on each cessation of the current, gives rise to the condition known as "retardation." This is the principle applied in the long- wound magnets of bridging bells, which form an efficient bar to the rapidly alternating telephonic currents. Indeed, alter- nating currents in general are liable to suffer from the resisting action of self-induction in direct proportion to their frequency. Inductive Disturbances in Telephone Lines. — In tele- phony the effects of cross-induction between different lines and of self-induction are found in the " absorption " of the higher overtones of the voice, which renders the sound received both small in volume and indistinct. The speech-bearing current is extremelv complex, and, as transmitted by the modern tele- 282 A B.C OF THE TELEPHONE. phonic apparatus', alternates with high frequency and an inde- scribable multitude of wave lengths and shapes due to the blending of the overtones of the sound waves with the funda- mental notes of characteristic volume and timbre. The trans- formation of the sound impulses into electrical impulses gener- rates a correspondingly large variety in phase — ^that is to say^ produces a series of waves of varying length, speed and; fre- quency, which must inevitably involve the loss of much of the original vocal quality — many waves being "choked off" before completing their phase — although perfectly carrying the funda-. mentals and many of the lower overtones. Thus it is that con« ditions which permit of the ready transmission of regularly alternating currents intended for power use are important inter-; ferents in telephony, which deals with alternating currents of indescribable irregularity. ■ The Electrostatic Conditions of a Line. — Another form of interference in telephone lines is due to derived induction from other current-bearing circuits, which causes "crosstalk" and such foreign disturbing noises as arise from proximity to an alternating current power wire or a telegraph line. As has been several times suggested, every line is in fact a condenser, with the wire for one charged surface and the earth, or some other near conductor, as the other. This involves a continued neces- sity of recharging and readjusting the polarities every time the currerit alternates. In the section on electrical condensers we stated that the basic fact of condensation was a produced differ- ence in potential between the electrically charged surfaces. Thus it is that to change the polarity of the source or to alter- nate the current on the line involves a shifting in the potential, which means not only a change in this respect in the conducting surface immediately charged, but also in every other such sur- face in the system affected by induction from the line. Electrostatic Induction. — It has been demonstrated that the disturbing influences of " cross talk " and other noises are due principally not to electro-magnetic induction but to electro- LINE CONSTRUCTION. 283 Static induction, such as is seen in the process of "charging " a condenser. The series of experiments by which this fact was established by Mr. J. J. Carty, inventor of the "bridging bellj'' consisted in running two grounded circuits of equal length at equal distances, apart, arranging an ordinary transmitter appa- ratus at the line extremity of the first, and three receivers, one at either end and one in the middle, in the second. Words spoken into the transmitter on the first circuit could be distinctly beard in the two end receivers of the second circuit, but not at all in the middle one, thus proving that the inducing current moved either to or from the middle and neutral point, which is an ascertained characteristic of electrostatic charges, and not at all from either end through the whole length of the circuit, as is the case in such electro-magnetic induction devices as the ordi- nary induction coil. The problem of overcoming the condition is then a simple one, involving merely an observance of the laws governing induction of this variety. Electrostatic Capacity. — As has been well, said in regard to telegraph lines : "When a key is depressed, closing a long telegraph current and sending a signal into a line, it is at least very probable that a portion of the electricity travels to the end of the wire with the velocity of light. But as the wire must be charged, enough current to move the relay may not reach the end for some seconds." The amount of electrical energy required to thus charge, or shift the potential of a line, varies with several other facts, such as the diameter of the wires, the height of the poles, the proximity of other lines, sometimes as the strength of the current, and as the line is grounded or on full metallic circuit. According to the observance of the condi- tions thus enumerated in the construction of a line, it differs from other lines, exactly the same in other particulars, but under different conditions, in a quality known as " capacity." In one sense capacity is to be determined by the area of the conducting surfaces to be charged; but in the practical aspects of the situa- tion, so far as regards condensers of all descriptions, the rule followed is that it varies according to the thickness of the 284 A B cor THE TELEPHONE. *' dielectric," or insulating layer, between the conducting sur- faces. Thus the same telephone or telegraph line may have dif- ferent capacities on different sections in its length, any two such sections differing in most of the particulars above enumerated. The rule seems, in this way, to Ije established that electrical condensa:tion demands a maintenance of uniformity in the con- ditions that render it operative. The converse follows, there- fore, that to disturb this same uniformity, in such fashion as not to interfere with the conducting properties of the circuit, will accomplish the end of neutralizing the effects of electrostatic disturbance. Such shifting must, however, be regular and pro- portional, in order to balance the line and to avoid substituting a large number of short condensing areas for a few longer ones. The means usually adopted is the arrangement known as "transposition," by which the wires of several circuits are so shifted at proportioned intervals that thejr relative positions are quite disturbed, and all electrostatic influences are reduced to the lowest point. This system will be fully explained in the proper place. Fio. 223.— Pocket battery gauge. Snch an instrument is indispensable to every electilclan, but the readings of its records must first be determined by some known standard, since they are purely arbitrary. CHAPTER TWENTY-THREE. TELEPHONE POLE LINES. Pole Line Construction. — The first considerations of im- portance in the construction of a pole line for telegraphic or telephonic circuits relate to the strength and durability of the poles, involving calculations on the size, the diameter at both top and bottom in pro- portion to the height, and on the kind of wood to be used. Again, and by no "means of subordinate importance, is the proportion of the total length of a pole to be planted in the ground in order to afford a secure hold. Wood for Poles. — It will be readily understood that the dura- bility of a pole depends largely upon the kind of wood of which it is composed. Moreover, no commercial system of treating the wood can insure its preservation for so long a time as to materially alter the pro- portional figures on this point. When, pole lines are strung in cities, where protection is afforded from many inclemencies of the weather, the material used is generally Norway pine. This is the case because trees of this variety frequently attain the height required to raise the wires out of the way of all obstructions or interference. In cross-coun- try construction, where, other things being equal, strength and dura- bility are considerations of equal importance with height, poles are most often of chestnut, cypress or cedar. As estimated by several authorities, the average life of the varieties of wood most commonly used in pole-line construction is as follows: Norway pine, 6 years; cypress, lo years; cedar, 12 years; chestnut, 15 years. Preparing the Poles. — To prepare a tree-trunk for use as a telegraph or telephone pole it is necessary to peel away the bark as soon as it is felled, carefully shaving down the knots, and leave it to dry, in order that the sap may be evaporated. The top of each pole is then roofed, or cut to a wedge-shaped point, with both faces of the wedge 28s 286 A B C OF THE TELEPHONE. carefully equalized. A pole thus treated will last its full period, par- ticularly if it be painted. There are several methods of preparing the wood chemically so as to prevent decay, but the cheapest of them in- volves an initial expenditure and complicated appliances hardly war- ranted for small poles, and almost prohibitory for the longer ones. The most familiar of such methods are those known as " creosoting " and " vulcanizing," both extensively applied in Europe. In America the usual practice is to give the pole a heavy coating of pitch over the first six feet from the butt, which serves to protect it from moisture. Dimensions of Poles. — Since it is necessary that poles be suffi- ciently strong to withstand all strains, due to the weight of the lines and to high winds, it is obvious that certain lengths, or heights, involve definite dimensions. These also vary with the variety of wood used, on account of the diversity in strength and other qualities. The pro- portions are generally stated in terms of the circumference at the top of the pole and at a point six feet from the butt, both in inches. As given by various authorities, they are, in general, as follows : Dimensions in inches for poles of various lengths. Length in Feet. 25 30 35 40 45 50 55 60 65 70 Cedar ( top ( 6 feet top 6 feet ( top 1 6 feet top 6 feet \ top ( 6 feet 20 22 22 22 22 22 3° 36 to 38 38 to 40 43 47 50 Chestnut 24 24 24 24 24 24 24 24 24 24 34 37 40 43 47 50 53 56 59 62 Juniper . . 21 22 22 22 22 22 22 22 31 37 40 44 48 52 56 60 Cypress . . 28 28 28 28 28 28 28 28 28 41 42 46 47 48 49 50 51 52 Average 22 22 22 22 22 22 22 22 22 22 • Rule... 28 31 34 37 41 44 50 53 56 62 POLE LINES. 287 In planting poles of any kind of wood, a certain portion of the length is buried below ground ; such portion varying with the total length. The average practice, as given by several authorities, \k summed in the following table : Length of Pole. Diameter, 6 Ft.from' Butt. Length Planted in Earth. 25 feet. 9 inches. 5 to s>^ feet. 30 feet. 10 inches. 5>^ to 6 feet..- 3S feet. II inches. ' 6 feet. 40 feet. 12 inches. 6 feet. 45 feet. 13 inches. 6>^ feet. 50 feet. 14 inches. 6J^ to 7 feet. 55 feet.. 16 inches. (>j4toTyi feet. :eo feet. 17 inches. 7 to 8 feet. ' 65 feet. ' 18 inches. 7K to 8J^ feet. 70 feet. 20 .inches. gJto 9 feet. Cross- Arms. — The familiar cross-arms for stringing the wires are usually attached to the poles before they arfe erected. They are commonly made from yellow pine wood, generally s}( by 4^ inches square, and are freely coated with good mineral paint as a preserva- tive. Attachment is made to the pole by cutting a gain one inch deep and of sufficient breadth to allow the longest side of the crossr arm to fit accurately. It is then secured in place by a lag screw, with a square head, so that it may be driven into place with a wrench. The, cross-arm is further secured to the pole with braces. These are flat strips, of wrought iron or low carbon steel, 30 inches long, )( inch thick and i}( inches wide, according to standard specifi- cations. Holes are bored at points one inch from either end, one for- attaching to the' pole, the. other for ■ attaching to the crosS-arm ; two- braces forming a triarigle -with the cross-arm for the base and with the* apex at the point of connection to the pole. Like all^ther iron work' used on pole lines, the braces are carefully "galvanized," so as to stand three immeiSioris of one minute each in a saturated solvitiori of copper sulphate without showihg copper deposits, the color being- black at the completion of the test. Before the cross-arm is set in place the gain is carefully painted 288 A B C OF THE TELEPHONE. with white lead. As it is important that the cross-arms on a line of poles, particularly when there are several on each one, should be at equal distances from the ground as well as being uniformly spaced, it is necessary that some measuring instrument should be used to secure this end. Such an instrument is the ordinary template, which is a length of board carrying a pointed block at one end, to correspond exactly with the top of the pole, and also cross-cleats nailed at pre- cisely the same intervals below it as it is proposed attaching the cross- arms. The template, laid upon a pole, shows where to cut the gains. Figs. 224, 225, 226.— Insulator pin for attaching to cross-arm ; bracket for attaching to pole or other support ; petticoat insulator, to be made of glass or porcelain. In planting the poles it is customary to so arrange that the cross- arms on alternate poles shall face in opposite directions, for the purpose of equalizing the strain of the line. On curves, however, all cross- arms are placed on the side of the pole facing the middle of the curve. Attachments for the Wires. — The cross-arms are bored with holes for the insertion of the insulator pins, which are made of locust wood and threaded at the upper end to attach the glass insulator. Acjcording to the number of the pins to be inserted in a cross-arm it is made shorter or longer. An arm for two piiis is made three feet long, according to the standard usually followed, with holes for the pins at center points three inches from either end and a space of 28 inches bfetween them in the center. The pins are inserted in the cross-arm in even numbers, a, 4, 6, 8, 10, owing to the fact that it is customary to string the two limbs of every circuit, its line and return wires, to the same cross-arm on every pole. According to the standard system of measuring, a lo-pin arm is made 10 feet long, giving a space of four inches from the last pin to either end, 16 inches between the two cen- POLE LINES. 289 ter pins, and 12 inches between all others. This standard system is followed in telegraph line work, and frequently also in telephone lines. Of late, however, a telephone arm spacing and dimension system has been widely adopted, which differs in some important particulars. In this system the cross-arms are made 2 ^ by 3 ^ inches square, the end pins are placed three inches from the ends, and on all except the 2-pin umjLmu^^l^a^ ; Fig. 227. — Long-handled digging shovel. fefrtjjjfti'^^.* . .....I.I.I ^ Another type of pole terminal is, shown in Fig. 256, the Cook pole-top terminal. As its name indicates, it is intended to 320 A B C OF THE TELEPHONE. be secured to the top of the pole. It consists of a circular cast iron box, as shown, into the bottom of which the cable is led up, being then fanned out and the wires connected to the connect- ing posts, with the line wires, through suitable air-tight fuse plugs. Each circuit connection is suitably numbered, as shown. After the arrangement is complete a quantity of unslacked lime is placed within the box, and the cover is screwed on with a rubber gasket, as in the other type of terminal. A copper cover is then placed over the terminal box, as protection against the elements. Exchange Terminals and . Distributing Boards. — Fig. 259 shows the type of distributing board furnished with the ex- change equipment of the Sterling Electric Co. It consists of a case containing a number of such cable terminals as are shown in Fig. 358, each equipped with suitable protectors. The switchboard wires are bunched together and led out at the top of each terminal box to the proper section of the switchboard, where they are again fanned out and attached to the jacks. In large exchanges it is necessary to employ rather complicated cross-connecting boards, frames supporting shelves for the cables and carrying fused connectors, to which the switchboard wires are attached at one end and the exchange cables at the other. Thus whenever any shifting of the line connections is necessary, the wiring of the switchboard need not be disturbed. Arrangements for Different Circuits. — In telephony grounded circuits are now seldom used for the speaking cur- rent, except on short lines. Consequently, most of the prin- ciples of construction hitherto laid down apply to full metallic circuits. In ground return circuits only one wire is strung on poles, the earth forming the return, but it would be impracti- cable to enclose numbers of such wires in underground cables, since the effects of cross induction, as well as other electrical disturbances peculiar to such circuits, would render talking service nearly, if not entirely, impossible. CABLE LINES. 321 !Ai!iiij;,,^,,v^,,i., '-w Ibic ;.l Ontti S! ? Fig. 258.— Section of a cable terminal box. showing the method of connecting the separate wires to the binding nuts and. distributing wires. Fl<5 259.— Type of exchange distributing board. It is a combination of such terminal boxes as is shown in Fig. 258, 322 A B C OF THE TELEPHONE. Common Return Circuits. — In some cases where it has been found inadvisable to string full metallic circuits for all lines, a method known as the "common return" has been adopted. It consists, briefly, in providing the line wire for each station with a common wire for return, iqstead of the earth. Fig. 259.— a street telephone station, set for the use of the police, fire department or street railway companies. A 11 the component parts of a station apparatus are here shown, although arranged in more compact form than usual. This kind of circuit works very. well with a limited number of stations being handled at a single-wire switchboard, such as has been described in connection with grounded circuits, pro- vided the return wire be of sufficient diameter to insure individ- ual return currents and prevent, as far as possible, leakage in CABLE LINES. 323 multiple through the wires and apparatus of other subscribers. It is unnecessary to make it of too great diameter, and when, for line wires, Number 12, measuring .0808 inch diameter, is used, a Number 8, of .12849 inch, has been found sufficiently large for common return wires on most circuits. Pig. 2593. — Circular gauge, showing approximate dimensions of the -wiTemeasiirea on the American, or Brown & Sharpe, Wire Gauge System. CHAPTER TWENTY-SIX. CIRCUIT BALANCING DEVICES. Circuit Balancing Devices. — In the practical work of balanc- ing telephonic circuits several varieties of magnetic coil are employed. Most important among these are repeating coils and impedance or re- actance coils. The former, in point of construction and operation, are simply transformers or induction coils used for special purposes j being composed of two windings on an iron core, which are either equal in point of length and size of wire, unequal in length, but of the same size of wire, or composed of wires of different resistances, for the purpose of modifying the voltage and current, according to the laws governing all varieties of transformer. The second variety is the one most commonly employed, its function being principally, to neutralize noise and general inductive disturbances, as when a grounded line is changed to a full metallic through a section of its length that is much exposed to annoyances of this description, or when a common talking battery is bridged between the strands of an exchange plug circuit. In both cases the vibrations of the voice are transmitted inductively, as in the ordinary induction coil. Repeating Coils. — Telephonic repeating coils were formerly constructed on the principle of the Faraday ring, which consists of a coil of iron wire for the core with a bobbin carrying the primary wind- ing on the one side and another carrying the secondary on the oppo- site side. Such an instrument is still known as the standard repeating coil, as generally used by the Bell companies. Later practice, how- ever, has given birth to the armored coil, which is enclosed in a suit- able iron sheath, with the primary object of neutralizing stray currents and inductive influences from the outside by a magnetic envelope, and also of preventing similar influences from the coil itself upon outside apparatus. The armoring of a repeating coil is arranged in either one 324 CIRCUIT BALANCING DEVICES. 325 of two ways : by winding it upon a wire core of such length that the ends of the separate wires in the bundle may be turned back, so as to completely cover the windings, being held in place by brass bindings, or, by enveloping it in an annealed iron tube, closed at both ends and connected to the core. Dimensions of Repeating Coils. — The typical forms of re- peating coil have double windings on both primary and secondary cir- cuits ; each pair being connected in series by a loop, or unwound section, as indicated by accompanying diagrams. This gives the effect of two distinct windings on both circuits, which are usually separated by a fibre washer. The dimefisions of such an ironclad coil for ordi- nary circuits, as given by one well-known authority, are as follows : External diameter of tube 2 in. Internal diameter of tube i ^ in. Length of tube 6 in. Space inside case 5 ^ in. Depth of each outside winding ' H ^^' Depth of each inside winding /^ in. Diameter of fibre core 5^ in. Diameter of fibre heads i|J- in. Thickness of fibre heads /4^ i"- Thickness of fibre middle washer ^ in. Diameter on outside coil i ^ in. Diameter on inside coil ^ in. Length of outside coil (No. 30, B. & S.) 1,036 ft. Length of inside coil (No. 30, B. & S.) 600 ft. Resistance outside coil (75° F.) 109 ohms. Resistance inside coil (75° F.) 63 ohms. Total resistance outside coils in series 218 ohms. Total resistance inside coils in series 126 ohms. The core of this coil is composed of a number of lengths of No. 28 B. & S. iron wire, with a diameter of three-eighths inch. As given by another authority the dimensions for a coil armored by its own core are : 326 A B C OF THE TELEPHONE. A core formed of annealed iron wires, No. 24, B. & S., each made of sufficient length to completely envelope the two windings, being bent around them on every side. Both windings are composed of No. 31, B. & S. silk-covered copper wire, of .008928 inch diameter, having a resistance of about 200 ohms, thus making the length about 1,500 feet apiece. These dimensions are derived from dissections of special coils, and can be considered no more than typical. Like all other special circuit attachments, the question of proportions is entirely governed by considerations of requirement in special cases. So also is the ratio between the resistances of the two windings. Thus, while for some special uses, as in exchange work, repeating coils are often made with Fig. 260. — Type of armored repeating coil. both windings of the same dimensions, as regards length and resistance, the rule for ordinary circuit work is to arrange for a definite transfor- mation of the voltage and current. In connecting a grounded circuit to a metallic, the method is to attach the line wire to one end of one winding of the coil, the other end being grounded, and to attach the terminals of the second winding each to one of the limbs of the metallic line. In the same manner, by the use of two coils, a grounded line may be transformed into a metallic through a noisy section of its length, all the troublesome effects of in- duction from telegraph and power wires being neutralized. The same is true when a long metallic wire is connected to a short one. With- out a repeating coil the combination would be almost intolerably noisy. By interposing the coil, however, each of its windings being attached to the two terminals of one of the lines, all foreign noises are perfectly weeded out. A well-.known telephone manufacturing com- pany furnished a standard size of repeating coils for transforming a grounded line into full metallic through a portion of its length liable to CIRCUIT BALANCING DEVICES. 327 inductive disturbances, as from trolley line-feeders, power wires, etc. Their specifications mention coils with a resistance of i8o ohms in the inner winding and 320 in the outer, an arrangement involving a rais- ing of potential as the telephonic current enters the metallic line and a lowering of potential as it enters the grounded line. Arrangements of Repeating Coils.— In bridging a bat- tery between repeating coils on the cord circuit of an exchange switch- board, the coils may be arranged with the high-resistance winding on the one side and the low resistance on the other, with the battery con- nected at the loop, or by such a scheme of "cross-connecting" as shall bring one high resistance and one low resistance on each side, with the battery connected as indicated in Fig. 175. Of course, in either arrangement, the current flows evenly through both windings. ?^^^ tQ^ Fig. 261. — Diagram of three telephone circuits on two metallic lines. thus balancing the circuit. Various advantages are claimed for all such arrangements. Effects of Repeating Coils. — In order to obtain three com- plete circuits from two full metallic, a \vell-known Swedish manu- facturer employs the type of repeating coil shown in an accompanying figure. It has five terminals, with binding posts for circuit connections. ' By the use of four such translators — for so he designates this type of instrument — two metallic circuits may be arranged so as to permit of the three telephonic messages being transmitted at the same time, 328 A B C OF THE TELEPHONE. without interference. The arrangement is, briefly, as follows : Each of these five-terminal transformers has a primary winding of 1 70 ohms resistance and a secondary of 340 ohms. Consequently the circuits designated respectively as GIf and /X have a total resistance of 680 ohms for the translator secondary windings in addition to the general ohmage of the line wires. The line, JI/, which is shown connected to the fifth terminal of every coil, should have a similar degree of resist- ance if the triplex arrangement is to be maintained without inductive Fig. 262.— Five-terminal translator for arranging circuits as shown in last figure. and other disturbances. This result is accomplished by attaching the fifth terminal of each coil to the center point of the secondary winding, so that, entering at this center point of the two coils at the transmit- ting end of the line, the current traverses one-half of the secondary winding of each, going thence over the test wire of Line i and the line wire of Line 2 to the opposite station, where it again traverses one-half of the secondary windings of both coils to the terminals in the switchboard jack, as shown, or in the subscriber's apparatus. The resistance of each secondary winding being, as we have learned, 340 ohms, one-half of each would give 170 ohms, with a total of 340 ohms at each end of line HJ, or a total of 680 ohms for the entire circuit, in addition to the general line resistance. By this arrangement the lines are kept perfectly balanced, and there is no interference, cross talk or leakage possible even if all three are in constant use. Impedance or Retardation Coils. — An impedance or retar- dation coil consists of an iron core wound with a number of turns of in- sulated wire, and in this particular is comparable to an electro-magnet, and induction coil or one of the type of transformers just described. Its principal function in telephony is to bar or impede the alternating CIRCUIT BALANCING DEVICES. 329 speech-bearing current so as to keep it on its destined line and off of a bridge between the limbs, or from an associated circuit, as in the test arrangement of a multiple switchboard or the battery wires of a central energy system. This result is to be achieved not by a high ohmic or metallic resistance in the windings of the coil, but by the reactance of the magnetized core. Thus, although the term "impedance" prop- erly designates any foirm of check or obstacle met by the electrical cur- rent, including true metallic resistance, it is most generally used as a synonym for the effects of magnetic retardation on alternating currents. In order to attain this effect to the best advantage it is essential that the core be made of sufficient mass to allow of the greatest desired degree of magnetic reactance, and 'of siich length as to permit a large number of turns of the winding wire. With a core of the same mass and a wire of the same size or weight, the efficiency of two given retardation coils is usually to be measured by the length of the core and the number of turns. Thus for the winding a wire of comparatively low resistance may be employed, and no particular check offered to the free trans- mission of a direct current, whose strength and induced magnetic prop- erties are moderately constant. An alternating current is impeded in proportion to its frequency. For this reason the long-wound bridging bells, used in connection with the central energy systems and bridged party-line circuits will bar the telephonic current, which is immensely irregular in phase and frequency, while the magnets may be readily energized by the low-frequency alternating current from the exchange ringer generator. In measuring the impedance of such instruments in ohms it is, of course, intended to express the sum total of all checks to the telephonic current, including the "false" as well as "true" re- sistance. The element upon which the operation of such coils depends is the hysteresis of the core, or the tendency of the iron to persist in a given direction of the magnetic force and to resist the efforts of the current to change it. The necessity of changing the polarity of the core at every reversal of the current involves, of course, a considerable expenditure of energy, which is an item of importance in large trans- formers, and, in fact, in every form of coil carrying alternating cur- rents. As would be supposed, a, long core tends to resist reversal of 330 A B C OF THE TELEPHONE. magnetization more effectually than a short one, and, with increasing frequency of alternation, the impedance is so rapidly increased that beyond a certain point, it is practically impossible for a current to pass through the winding. The action may be illustrated as follows: If one work the pedal of a foot-power printing press so that the fly-wheel is put through half a revolution in one direction to a dead center, and then reversed, he will notice that the amount of energy expended in each start and stop of the machine is grfeatly in excess of that required to maintain an even drive through several revolutions straight ahead. In other words, the force needed to pass over the dead center and begin the movement in the reverse direction represents the waste occurring in a magnetic coil with rapid reversals of current. Were it possible fbr one to urge the fly-wheel of a foot-driven press through a series of su. 16, 48 Electrical Currents 17 ^ Compared to Water Currents 24 Electrical Force, Varieties of 16 Electricity Defined 16 Electrodes. _ 19 Electromotive Force 19 Energy, Transmutation of i Farad, The. . . : 53-54 Force, Octaves of. 3 Fundamental Tone 2,3 Generator (see Magneto-Generator). Gray, Elisha 63, 101 Grotius' Theoryl ' 20 Harmonic Tones 2 Heat, Properties of i Henry, The 55 Hydrogen Gas 21-23 Impedance Coil (see Coil, Retardation). Induction Coil (see Coil, Induction). Induction, Electrostatic'. 282-284 Magnetic 28, 30 -38 Self 56 Voltaic ,. 27. 33-38 Rules Governmg 41-42 Inductive Disturbances on Lines.... 56-58, 281-282 Insulation ......^ 49-50 Intercommimicating Systems 239-266 Common Return 239-255 Switch Keys for 239-240 Common Battery 241-243, 253-254 Individual Battery 243-249 Ericsson Wiring 247 Ness Switch Hook 247-254 Full Metallic Circuits 257-266 Century Switch 257-259 Plummer-Monroe Switch 260-262 Couch-Seeley Switch 262 Hotel Systems 262-266 A loo-Po'nt System 267-268 Inventors of Telephones, 59-67. 75-84. 85-92, 104-105 347 348 INDEX. FAG9 Jackf Spring 123,123,125,127,130,131,151 Combined 155-164 on I^amp Switd)boards 169 Multiple 171,176,187,190 L>amyx 13 Light, Properties of i Lightning 17 • Arresters 275-278 Listening-Ringing Keys, 123, 129, 130, 133, 135, 136-145 Cook 137-140 O'Cohnell 139. 140-143 Keystone 140, 145 Western 141,145 Couch & Seeley 143-145 Magnetj Electro 28 Magnetic Lines , zg Magnetism 25,28,30 Magneto-Generator 106-108 Resistance of no Devices for Shunting ii6-i4o Operating 124 For Series Party Lines 222 For Bridging Party Lines, 226-227, 23I1 5'34. 236 Magnetic Lag 281 Magnet, Permanent 31 Production of 31 Superiority of in Telephone 67 Magnet Telephone 61-84 Simijhcity of 67-68 Sensitiveness of 69-71 Modifications of 73-84 Methods of Strengthening 74 Manometric Flames 8 Apparatus for Making 9 Micropl^bne 87 Microtelephone, The 331-333 Multiple Circuits 52 Multiple Switdiboard, The 149, 171-1S7 Varieties of 173 Series 173-181 Branch-Terminal i8i-zi86 Divided 186^87 Test of Series 175-179 Test of Branching 183-185 Visual Busy Signals 185-186 Central Energy 172 Octave, Musical 2 Ohm's Law 42-44 Ohm, The .40 Overtones 2 Party Lines ' 219-23S Circuits of 219 Series 319-222 Bridging 222-228 Carty System 224-227 Ericsson System 227-228 Selective Signals for 238-237 Toll Lines 237-236 Fhonautograph, Scott's 8 Pitch of a Sound 3 Plug, Switchboard 123, 125,139,130, 151,176 Pole Lines, Telephone.. , , . .379-313 Pole Lines, Disturbances on 279-284 Construction of 285-305 Guying of 393-396 Poles, Material and Dimensions. . .285-286 Poles, Preparation of 286-388 Cross Arms 287-390 Planting Poles for 291-293 Stringing Poles 300-305 Pressure, Varying, Theory of 8s Protective Devices 273-378 Fuses 273-274 Heat Coils 274-27* Carbon Arresters 275-278 Puinn's Cable 58 Receiver, Defects of , 77 Methods of Strengthening 74 Watchcase ,,. 81 Reis, Philip 59 Reiieating Coils 324-328 Resistance 22-24, 48-50 ' Laws Governing 23 True and False 23-34 Measure of 40 Resonance 4 Retardation Coil (see Coil, Retardation). Roentgen Rays 3 Scale, Musical a Series, Circuits. 51 Shunt, Western Electric nS Post iig Centrifugal 119-130 Signal Lamps, Switchboard, 165-170, 192-195, 200 Signals, Selective, for Party Unes 228-237 Explained 328 Step-by-Step 229-330 Harmonic Wave 230-233 Strength and Polarity 233-336 Sterling 234 Opinion on 237 Signals, visual 185, 190, 191, 192-193 Sound, Qualities of a ■ Sound, Visible 8-9, 15 Sound Waves , 4-7 Spectrum, Solar i Speech, Production of 7 Conditions of Transmitting 73 Storage Battery 167, 204, 216-317 Swituiboards > 121-135 Grounded Line 127-131 Metallic Line 131-135 Circuits of . . , 13J Operation of 135 Telephone Set 144, 145-147 Night Bell 148 Multiple 149, 171-187,215 Central Energy 201-315 SterUng 133,124, 1341151-152,196-199 Ericsson (frontispiece). 126, 133, 183 Connecticut 161, 163 Western 157 Private Line 256, 265-266 Automatic 267-272 Switch Hook, Its Function 111-113 Ness 247-254 Olobb ••«««.«•»„., ^,^ ^54-^55 INDEX. 349 PACK Telephone Circuits 121 Telephone, Reis's 59-61 House's 62 Gray's 63 Drawbaugh's 63 Bell's Experimental 63-67 Telephony, Triplex 327-338 Terminals, Cable 318-321 Timbre ■ 3 Toll lines 237-238 Transferring Calls 149, 189-200 Sabin-Hampton System 189-196 Cook-Beach System 197-199 Western Express System 199 Transformers 35-38 Transpositions 306-312 12-Line 307 40-lJne 309 Frequency of 308-310 General Rules 310-311 English Method 3iX'3i2 PAGE Vocal Cords 10 Voice, Human 10 Powers of ii Production of 13-14 "Pictures'* of 15 Volt, The 41 Voltaic Battery 19-21 Watt, The 46-47 Wire, Telephone 296-305 Gauges for 297-300 Attaching 303-304 Splicing, Various Methods of 304-305 Wiring of Aiiparatus 116, 217, 221,225,228 of Switchboards 128,133, 134, I74i 177. 181, 183, 185, 196-197, 217 of Circuits. .166-170, 205-218, 217, 220, 223, 234, 242, 244, 246, 248, 250, 258, 265 X-Rays 3 Hawkins' Self-Help Mechanical Drawing. Price, $2. This volume contains 320 pages, 300 illustrations, diagrams and suggestive sketches ; it is attractively and strongly bound in green cloth, with full-gold edges and titles, making a handsome book, 7x10 inches, for the library, for study and ready reference. It is superfluous for the publishers to say aught concerning the importance of knowing how to draw, and the utility of draughting in industrial pursuits ; but the fact cannot be too frequently reiterated that the education of the mechanic is incomplete without a knowledge of drawing. CONTENTS. The work has been carefully arranged according to the fundamental principles of the art of drawing, each theme being separately treated, and many examples given for practice. A list of the subjects are given below, all of which are plainly described and illustrated. Chalk Work. Preliminary Terms and Definitions, Freehand Drawing. Geometrical Drawing. Drawing Materials and Instrumenti Mechanical Drawing. Penciling, Projection. 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This volume has over 2oo pages of practical "pointers" showing the path of advancement, so much desired by aspiring engineers and firemen. It is printed on excellent paper and handsomely bound in heavy red leather, with gold' title and edges. It is strongly bound for coutinuotls study ; the size is 5X7K- The work is a most important aid to all engineers, and is undoubtedly the most helpful ever issued relating to a safe and sure preparation for examination. It presents in a condensed form the most approved t)rac- tice in the care and management of Steam Boilers, Engines, Pumps, Electrical and Refrigerating Machines. On the following page is a list of its " helpful " contents. CONTENTS. This book embraces information not elsewhere obtainable. It tells exactly what an engineer will have to go through in getting a license, with much kindly and helpful advice to the applicant for a license. 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