I H PVtfT^ftW^'rt'rt ;*T«»-^VS*-Tt»-.VM-t-'' H^*-^ '^ . C^ ONDER BO AGNETIS ■>^->i Wi4W H WWWf< *W «W* ) (> le {> f r.*! ipi '' MH.twM*»iFtMr**'i I SiBlliiiiiiJiii: ..iiiiiifiiiipiiii. ^^ Q C 1 s- /k t ^OFTTT.L University, i^/iaca, N. Y. Cornell IHniversiti^ OF THE IRew l^ork State College of agriculture .;i.2..^.; sJS)\s4 V:-:;'' Kz:^ Fig. 27. — Teue and False Dibection op Cibculae Fltix a greater number of them. When the current strength increases, the lines of circular flux spread outwards in all directions around the conductor, thus reaching a greater distance from it, while, when the current strength de- creases, they contract or move inwards towards the con- ductor. I wish you to remember this fact because we shall use it in a subsequent chapter when we describe how Michael Faraday made a great discovery, a discovery that 116 THE WONDER BOOK OF MAGNETISM was just the reverse of the Oersted discovery; for, as we have stated, what Oersted discovered was how electricity can be made to produce magnetism, while Faraday found out how magnetism could be made to produce electricity. Of course, you will understand that, while I have repre- sented the conductor through which the current is flowing as occupying a vertical position, and the card as occupying a horizontal position, an electric conductor will have cir- cular lines of flu^: produced around it, no matter what may be its position. It may be vertical or horizontal, or inclined in any direction between the vertical and the horizontal, but when the circular flux is produced it is always produced in a plane at right angles to the axis of the wire. And now I trust these words " in a plane at right angles to the axis of the wire " no longer hide from you the idea it is intended they shall convey. If you have thoroughly grasped the above idea, you will understand that the phenomena of an electric current and magnetism are so closely bound together that it is im- possible to separate them. It is absolutely impossible for an electric current to flow in any direction through a conductor without having circular lines of magnetic flux set up around it in concentric circles in planes at right angles to the axis of that conductor. In other words, an electric discharge necessarily produces magnetism, and this is true whether the discharge is from a voltaic battery, whether it is from the dynamos that are producing elec- tricity for arc and incandescent lamps, from the generators that are producing the electricity for moving trolley cars, or whether it be a lightning flash passing between the clouds and the earth. WHAT OERSTED DISCOVERED 117 Remembering what has been said about' magnetic in- duction; i. e., that when magnetisable substances such as soft iron or soft steel filings are brought into a magnetic field, so that the magnetic flux can thread or pass through them, there is necessarily produced a south magnetic pole where the flux enters the fllings, and a north magnetic pole where it leaves them, you can understand that if iron fllings are scattered over the surface of the glass plate or card represented in the preceding flgures, and the plate or card Fig. 28. — Magnetic Flux Paths Produced by Conducting Wibe be gently tapped, they will be arranged in the directions of the circular flux paths. It is in this way that the magnetic flgure or magnetic field represented in Fig. 28, surrounding a conducting wire through which a current is flowing, has been obtained. The dark circle in the centre shows the place where the conductor is passing through the centre of the plate, while the concentric circles surrounding the conductor consist of masses of magnetised iron filings that have arranged 118 THE WONDEE BOOK OP MAGNETISM themselves in closed paths with their north and south poles in contact when the plate was gently tapped. You can now also understand from the examination of Pig. 26, already referred to, what would happen if a mag- netic needle were brought near the conductor DD, for of course the lines of circular flux are produced along the entire length of the active conductor. No matter, there- fore, where the needle is placed, provided it is placed near the active conductor, there will happen to it just what happens to a compass needle placed in the earth's magnetic field. The unseen attendants; i. e., the mag- netic flux, will gently push the needle around until the flux enters at its south pole and comes out at its north pole. As soon as the needle gets in this position, it will come to rest. Or, putting this matter in another way, a magnetic needle if brought near an active conductor comes to rest when it points in the direction in which the flux is moving, and, since direction around a circle is constantly changing, the compass needle must of course take up different posi- tions on different sides of an active conductor. Scientific men were quick to see the many important properties possessed by active Fig. 29.— Deflection of conductors, by reason of their Needle by Active power of changing the posi- onductob tion of a compass needle. If a small magnetic needle NS, be placed as shown in the centre of the rectangle Eig. 29, each portion of the circuit will act so as to deflect WHAT OEESTED DISCOVEEED 119 it in the same direction. Shortly after the announcement of Oersted's discovery, Schweigger, a German physicist, showed that if an insulated conducting wire were bent into the form of a hollow rectangle with many turns, the power of the conductor to deflect a magnetic needle would be greatly increased or multiplied. The form generally given to Schweigger's multiplier, as the device was called, is shown in Fig. 30. Here, as can be seen, the needle is placed at the centre of a rectangle that consists of many turns of insulated wire, so that the deflecting power of the circuit is greatly multiplied. A variety of electrical instruments depend for their Fig. 30. — Schweiggeb's Multiplibb operation on this principle. These instruments are called galvanometers, voltmeters, and ammeters, and are em- ployed in the measurement of electricity. Ampere, who may fairly be regarded, at this time, as one of the foremost of French physicists, almost immediately after hearing of Oersted's discovery, began an extended series of investigations, and was soon able to demonstrate that not only was an active conductor able to deflect a compass needle, but that a compass needle or magnet was able to deflect an active conductor, and, moreover, that an active conductor, suitably coiled and supported, possessed 120 THE WONDER BOOK OP MAGNETISM directive powers like a compass needle, and had poles at its ends, like those of a compass needle or a bar magnet. Ampere made many curious experiments among which were the following : In order to show how one active con- ductor was able to attract or repel another conductor ac- cording to the directions in which the current was flow- ing, he arranged matters so that one of the conductors was fixed and the other movable, as represented in Pig. 31. Fig. 31. — ^ampebe's Expeeiment Here two metallic pillars, connected respectively at + ^nd — with the positive and negative poles of an electric source, were placed, as shown, on a stand. Horizontal arms, at- tached to the top of these poles, were provided with agate suspension cups, in which were placed a few drops of mercury. The conducting wire, bent in the form of a rectangle. A, has its upper extremities provided with points, so that, when dipped in the mercury cups, they are not only WHAT OEESTED DISCOVEEED 131 suspended so as to be free to move, but readily permit the electricity to flow through the circuit. In this manner the conducting rectangle is suspended at the extremities of the pillars so as to be free to turn. Now when a current of electricity flows through this circuit, the streams of circular magnetic flux that are produced around both the fixed and movable conductors act on each other magnetically and cause the movable conductor to be deflected. & Fig. 32. — Ampebe's Experiment In order to show the action of a magnet on a movable conductor, the following apparatus was used. A movable rectangle was placed over a straight bar magnet, NS, with its plane parallel to the plane of the bar magnet as repre- sented in Eig. 32 by the dotted line. As soon as an electric current was caused to flow through the rectangle in the direction of the arrows, a mutual attraction occurred be- tween the flux of the bar magnet and the circular flux of 133 THE WONDER BOOK OF MAGNETISM the conductor. This resulted in the movement of the mov- able conductor in a direction that, provided the current is strong enough, will tend to place it at right angles to the axis of the magnet. By means of these and other experiments, Ampere proved that parallel electric currents attract each other when they flow in the same direction, and repel each other when they flow in opposite directions. Fig. 33. — Roget's Oscillating Sptbat, Since a conductor wound in the form of a hollow spiral will have the current in its neighbouring parallel spirals flowing in the same direction, and since parallel currents attract each other, if the spiral is suspended from the top of a vertical stand as shown in Fig. 33, so that its lower end dips in the mercury in the cup p, then, when a power- ful current is passed through the circuit, the attraction between the neighbouring coils of the spiral lifts the spiral out of the cup and thus breaks or opens the circuit. The WHAT OEESTED DISCOVEEED 123 attraction no longer existing, the spiral lengthens by its weight, and the lower end, again dipping in the mercury in the cup, again closes the circuit, when the spiral is again drawn out of the cup, breaking the circuit. These succes- sive breakings and makings of the circuit result in rapid to-and-fro motions of the spiral, and, since at each break a bright electric spark is produced, the result is quite brilliant. The apparatus above described was devised by Eoget, an English physicist, Professor of Physiology in the Eoyal Institution of London, and is called Eoget's oscillating spiral. It was one time employed as an automatic make- and-break for rapidly opening and closing the circuit of a piece of apparatus, known as the Euhmkorff induction coil, that will be described in " The Wonder Book of Elec- tricity." But by far the most important result of Oersted's great discovery was that it showed how a variety of magnets called electro-magnets can be made. This will be dis- cussed in a subsequent chapter. CHAPTER XII ATTEMPTS TO EXPLAIN THE CAUSE OF MAGNETISM Many attempts have been made to account for the phe- nomena of magnetism. While it is not my intention to discuss these attempts fully here, since I do not think such a discussion would pay, yet I will briefly tell you some of the remarkable ideas the ancients had. Both Thales and Anaxagoras believed that a magnet possessed a kind of soul, intelligence, or immaterial spirit; that the movements of the magnet either towards the north or south poles of the earth, or towards or from the poles of other magnets, were due to the action of this soul or intelligence. Epicurus, the famous Grecian philosopher, who taught that the supreme good of life consists in enjoying one's self, by eating good things and in doing things that cause bodily pleasure, gravely asserted that the ability of pieces of soft iron or steel to hold on to the poles of magnets was due to the fact that' they were provided with hooks. An- other man, a follower of Epicurus, declared that iron was the natural food of the magnet; that the peculiar motions of iron, when drawn towards a magnet, occurred while the iron was being eaten or devoured. I am not quite sure Just what this man meant, but I suppose he intended to say that these movements were merely the squirmings or death agonies of the iron. If a magnet possessed but one pole only, it would, per- 124 ATTEMPTS TO EXPLAIN MAGNETISM 135 haps, have been easier to account for magnetic phenomena, but since it possesses two poles, and these poles have oppo- site properties, it is more difficult to explain the cause of magnetism. One of the early explanations was that mag- netism is due to the presence in the magnet of two differ- ent kinds of fluids. In this, as in all explanations, in order to satisfactorily account for magnetism, an endeavour was made to explain what happened to a bar of steel that possessed no magnetic properties whatever until it was brought under the influence of a magnetising force, as for example when touched by the pole of a magnet. The double-fluid theory of magnetism claimed that there ex- isted in all magnetisable substances two fluids, a north or boreal fluid, and a south or austral fluid; that when a body was magnetised, these fluids, which were ordinarily combined and, therefore, neutralised or masked each other's presence, were separated and driven to the opposite ends of the bar, thus producing a north pole at one end, and a south pole at the opposite end, with no magnetism what- ever between them or at the magnetic equator. I may say that this explanation of magnetism was generally accepted until the experiment was made of cutting a bar magnet in two at its equator. If the double-fluid theory was true, it should be possible to obtain a magnet that possesses north polarity or south polarity only. But when, as al- ready explained, this was found to be impossible, the double- fluid theory of magnetism was necessarily abandoned. It was not long after the great discovery by Oersted, referred to in the preceding chapter, that a series of in- vestigations were begun (September, 1830) by the French physicist. Ampere. Starting from the principle that an 126 THE WONDEE BOOK OP MAGNETISM active circuit possesses all the properties of a magnet, Ampere set about trying to discover how it would be pos- sible to account for magnets on the assumption that they were caused by electric currents. At that time it was generally believed that the very small particles of magnets naturally possess magnetic prop- erties. Ampere, therefore, attempted in different ways to account for the cause of magnetism by assuming that the very small particles of matter known as molecules were naturally magnetic, and that the cause of their magnetisa- tion was the presence of minute electric currents flowing through them. At last he succeeded in framing a hypothe- sis that seemed to meet in a remarkable manner the require- ments of the case. During these investigations, as soon as Ampere thought of a possible explanation to account for magnetism, he tried to construct apparatus to test its probability, and when the apparatus failed to work, as it often did, he threw it away. In this manner he was obliged to throw aside as useless many pieces of apparatus. You see, therefore, he thought' of explanations one day, only, after actual trial, to throw them aside the next day. At last, however, he thought out an explanation which, in a modified form, is to-day generally accepted as the cause of magnetism. Let us, therefore, examine Ampere's explanation of the cause of magnetism. Ampere asserted that the molecules of a magnet are naturally magnetic because they have currents of elec- tricity flowing through them in what are known as closed circuits; that is, where the electric current, after starting from some point in each molecule, flows through or around ATTEMPTS TO EXPLAIN" MAGNETISM 137 the molecule until it again comes to that point, when it again goes on flowing and continues this practically for- ever. As soon as Ampere had framed this theory, seeing that the magnetised molecules could easily be represented by short conducting wires bent on themselves in the shape of circles, he began trying to devise some plan by which copper Fig. 34. — Ampere's Solenoid or other electrical conducting wires might be so bent as actually to form a number of closed circuits. In Fig. 34 a conducting copper wire has been wound around the surface of a cylinder in eqid-distant coils, and its ends, led back along the length of the coil, have after- wards been bent so that the coil can be suspended in mer- cury cups. When a current of electricity flows through such a suspended system, previously placed in an east and west direction, it will, as soon as the current begins to flow, tend to point, approximately, towards the north of the earth. In other words, the coiled wire acquires all the 128 THE WONDEE BOOK OP MAGNETISM properties of a magnet, and, like a magnet, tends to place itself approximately due north and south. Ampere called this coil a helix or electric solenoid. This was, indeed, a beautiful experiment. Whether the theory was true or not, it showed how a series of parallel circuits through which a current of electricity was flowing x5 1 ^ ^ N Fig. 35. — Dibection of Ciboulab Flux in Rectangulab Cntcurr could be made to acquire all the properties of a bar magnet. You may possibly think that Ampere's electric solenoid did not consist of a number of parallel active circuits only, since, besides such circuits, it had a straight wire passing in the general direction of the length of the solenoid, and, moreover, it contained a number of short pieces of wire connecting the contiguous circular coils. I acknowledge this is true. If you were older, I could make you under- ATTEMPTS TO EXPLAIN MAGNETISM 129 stand that these two extra conductors completely neutralise each other, since their currents are flowing in opposite di- rections, so that the solenoid does consist practically of the separate parallel circuits that Ampere declared existed in all magnets. Now, as Oersted's discovery has shown that a con- ductor through which an electric current is flowing has N Fig. 36. — Dibection op Cieculae Flux in Eectangulae Ciectjit flux produced that flows around it in concentric circles in planes at right angles to its axis, it follows that, in Am- pere's electric solenoid, there would be produced circular magnetic flux around the conducting wires of each of the separate circles. There would, therefore, be produced in the solenoid flux that passes or threads through its coils at one end, and emerges or passes out at the other end. Now the end of a solenoid at which flux enters acquires a 130 THE WONDEE BOOK OF MAGNETISM south magnetic polarity, and the end at which it passes out, a north magnetic polarity. Consequently, such a solenoid must possess all the properties of a bar magnet whose poles are situated at the end of its axis. If, therefore, a bar magnet has one of its poles brought near to the end of a solenoid, it will attract or repel the solenoid according to whether its pole is of the same or opposite polarity to that of the solenoid. Again examining Fig. 32 (see page 131), where a small rectangular circuit is conveying an electric current, there will be no difficulty in understanding why such circuit tends to place itself at right angles to the straight bar magnet placed below. Suppose, as represented in Pig. 35, this rectangle has an electric current flowing through it in the direction represented by the straight arrows, then there will be produced magnetic flux that, as represented by the small arrows, enters this face of the rectangle, and thus produces a south magnetic pole. But if, as in Eig. 36, the direction of the current through the rectangle be changed, so that the circular flux paths will take the direction shown, the flux will appear to pass out of this face of the rectangle, thereby producing a north pole. Now, if the rectangle is readily movable, it will be moved by the mutual action of its flux and the flux of the bar magnet until the flux from the magnet passes through its south face or end; and to do this the rectangle must place itself at right angles to the length of the bar magnet. You will naturally ask what produces the currents of electricity that Ampere assumed are constantly flowing through the molecules of either magnetised or unmagnet- ised iron or steel, so as to endow them naturally with mag- ATTEMPTS TO EXPLAIIST MAGNETISM 131 netic properties. It is well known that all circuits with which we are acquainted possess a resistance that opposes the passage of the electric current, and that, in order to cause an electric current to flow through any circuit, energy must be expended. It would, therefore, seem impossible for electric currents to flow continually day after day, year after year, through the molecules of substances capable of being magnetised, unless there was some source of energy to keep them flowing. This difficulty has caused Ampere's theory of magnetism to be generally rejected by scientific men, although it must be confessed, now we have learned more about the properties of atoms and molecules, that it does not seem so improbable. Indeed, Sir Oliver Lodge, one of the foremost of living English electricians, says concerning this matter : " To all intents and purposes, certainly atoms are infi- nitely elastic, and why should they not also be infinitely conducting? Why should the dissipation of energy occur in respect to an electric current circulating wholly inside an atom ? There is no reason why it should." In accordance with Ampere's explanation as to the cause of magnetism, it is assumed that the molecules are natu- rally magnetised by the constant passage of an electric cur- rent through them in a closed circuit. You may, therefore, ask why there should be any necessity for magnetising them if they are already magnetised. The answer is simple. They must be subjected to some process of magnetisation in order to insure that all their many poles are turned in the same direction. When unmagnetised, the molecules and, consequently, their poles point in different directions, so that the opposite poles neutralise each other, just as they 133 THE WONDEE BOOK OF MAGNETISM did in the tube filled with iron filings by De Haldat that lost all its magnetism when shaken so as to prevent their like poles from all pointing one way. The theory of magnetism that is most generally accepted to-day may be regarded as a modification of Ampere's theory. A magnet is supposed to have its ultimate particles of matter, possibly its molecules, inherently or naturally magnetised ; each molecule possesses north and south polar- ity, and this without attempting to explain how this polar- ity has been caused. This polarity may be caused by the presence of closed electric circuits or it may be caused in some other way. It is simply assumed that the ultimate particles are naturally magnetic. ISTow, this being the case, the act of magnetisation consists in simply so turning these particles that their like poles shall all face in the same direction. As soon as this has been done, the sub- stance will be magnetised as powerfully as it possibly can be; or, in other words, the magnet is saturated with magnetism. If this theory be accepted as true, one can easily see why it is much more difficult to magnetise a bar of hard steel than a bar of soft iron or soft steel, since the particles of hardened steel are more difficult to turn round than are those of soft iron. Then, too, a bar of hard steel having once been magnetised, tends to retain its magnetism, after the removal of the magnetising force, by reason of the very fact that it is so hard to turn these molecules. The theory of magnetism that is most generally adopted to-day assumes, in order to account for the flux produced by every magnet, that its separate molecular magnets pos- sess the power of producing either whirlings in the sur- ATTEMPTS TO EXPLAIlsr MAGNETISM 133 rounding ether, or an actual forward motion of the ether whereby it is expelled or blown out of their north poles. I have purposely assumed the forward or blowing motion of the ether in preference to the vortical or whirling motion, not only because the blowing motion is easier for you to understand, but especially because, in a certain sense, so far as the effects they produce, the two motions are practically the same. It will be noticed in the above that I have referred to the ether as constituting the imponderable substance that is blown out at the north pole of the magnet and sucked in at its south pole. Without going into an explanation here I will say that the universal ether is an imponderable and extremely tenuous form of matter that fills, not only the spaces between the atoms and molecules, but also the greater spaces between the stars and other heavenly bodies. It is through the universal ether that the light and heat of the sun are propagated by means of waves. CHAPTEE XIII MAGNETS THAT CAN READILY FORGET THEY HAVE BEEN MAGNETISED I REMEMBER, when I was a boj', that every bow and then the volunteer fire companies of the city of Philadelphia had good-natured competitions in order to see which of their engines could throw the best stream of water. The question was not only how far these streams could be thrown, but also how much water they contained. In a similar manner, it was only during last year that the Paid Fire Department in Philadelphia had trials in order to find the height streams of water could be thrown from the high- pressure mains that have recently been located in the central portions of the city, in order the better to protect the sixteen, eighteen and twenty-story skyscrapers that have been erected. If a somewhat similar trial were made between perma- nent and electro-magnets I would back the electro-magnets to win every time, both as regards the quantity of mag- netic flux they can produce, as well as the distance to which they are able to blow it. Let us, therefore, try to under- stand the peculiarities of electro-magnets; for, on account of their great importance, I especially wish you to learn their construction and operation. In the first place as to how electro-magnets are made: I have already explained how the turns of conducting wire of an electric solenoid through which an electric current is 134 MAGNETS THAT FORGET 135 passing have circular magnetic flux produced around them, and how this flux is able to set up strong north and south poles as it is blown out of the coils at one end and sucked in again at the opposite end. You can, therefore, under- stand why it was, when Arago, a French physicist,, held a small length of copper wire, through which a powe;rful electric current was passing, in a mass of iron filings, that the filings clung to it in parallel circles corresponding to the circular lines of magnetic flux that are always set up around an active conductor. You probably know that electrified bodies draw or attract towards them small particles of dust or other light bodies in their neighbourhood. If a glass rod or a hard copper comb is electrified by rubbing it briskly against the coat, it will attract light bodies such as feathers, powdered charcoal, dust, etc. Arago, of course, knew this and thought that, perhaps, the iron filings had been caused to cling to the copper wire by means of electric attraction. To test this he plunged the copper wire in powdered glass, and also in finely divided copper dust. There was no attrac- tion, so he came to the conclusion that the iron filings clung to the copper wire, through which an electric current was passing, simply by reason of the attraction of its mag- netic poles. But it was not only Arago and Ampere who were thus experimenting in the direction in which Oersted had pointed out. Sir Humphrey Davy, in England, also began trying to find out how steel could best be magnetised when brought in different ways near an active conductor. In this way he soon discovered that an active conductor could produce the best results, in magnetising straight bars of 136 THE WONDEE BOOK OF MAGNETISM hardened steel, when the bars were placed inside a hollow coil through which the current flows. Since the magnetic polarity produced by an electric current depends on the direction in which the current is flowing, it is easy to change the position of the poles produced when a bar of steel is placed inside a hollow coil by changing the direction in which the current flows through the coil. If, for example, the current has been flowing into the end of the wire from the top to the bot- tom of a coil or solenoid, or from its right-hand side to the left-hand side, then the polarity will be changed by Fig. 37. — Right-handed Helix causing the current to flow in from the bottom to the top, or from the left-hand to the right-hand side. But there is another way in which the polarity of a coil or solenoid can be changed, and this is by changing the direction in which its wire is wound or coiled. There are only two ways in which wire can be coiled on a solenoid; namely, as a right-handed solenoid or helix, or as a left- handed solenoid or helix. In a right-handed solenoid or helix the winding is clockwise; that is, the wire is wound in the direction in which the hands of a watch or clock move over its dial face. Now this is by no means an idea that is easy to grasp, and yet you must grasp it if you would understand what I am about to tell you. Hold a lead pencil in a horizontal position so that its blunt end points directly towards you. Now take a piece of wire and begin winding it on the end nearest you in the Lifting Pig Iron with an Electro-Magnet The magnetism is instantly produced on the closing of the circuit and lost on its opening-. By courtesy of P . F. Collier &f Son MAGNETS THAT FOEGET 131/ same direction as that in which the hands of a watch move. Such a spiral would be wound clockwise, or would be a right-handed helix. If the winding is in the opposite direction, it will be a left-handed winding, or counter- clockwise. A right-handed helix is represented in Fig.' 37, where, as you can see, the windings are clockwise, and a left-handed helix is represented in Pig. 38, where the windings are counter-clockwise, provided in both of these cases you are looking at the helix when its left-hand end is turned towards you. If you examine the above drawings carefully you will see that the direction in which the magnetic flux encircles the wire of the helix varies with the direction in which the ilP^«^M«liiiPl|«g@P^j^f^ Fig. 38. — Left-handed Helix current is flowing through it, and that, moreover, the direc- tion in which the magnetic flux will thread through the helix from one end to the other will depend not only on the end of the helix at which the current begins to flow, but also on the direction of its winding. You can easily convince yourself of this fact. Take, for example, the right-handed winding shown in Pig. 37, when looked at from its left-hand end. Holding this next to you, it is clear that the coil is wound in the same direc- tion as that in which hands of a watch move over its dial. If, however, you hold the right-hand end next to you, and note the direction in which the wire winds around the helix to the other end, you will see that it is left-handed, or winds in the opposite direction to that of the hands of a 138 THE WOXDEE BOOK OF MAGNETISM watch. In other words, a winding that is right-handed when regarded from one end of a helix is left-handed when regarded from its other end. This is the reason the polarity of the helix is reversed by causing the current to enter at its opposite end. It is easy to produce consequent poles; i. e., two free poles of the same name at any point intermediate between the ends of a helix. To produce such poles it is only neces- sary to change the direction of the winding. Again examining the figures that represent the direction in which the flux passes through the conducting loop, it Fig. 39. — Polarities of Solenoids will be seen that in order to produce a north pole at one end of the solenoid, it is necessary to cause the magnetic flux to come out of the solenoid at that end ; and, similarly, to produce a south pole, it is necessary to cause the flux to pass into it at that end. If, then, you remember the direc- tion assumed by the circular flux that is produced by an electric current passing vertically through the card already shown in Pig. 36, you may be able to reason out for your- self the direction in which the current must flow through the circuit in order to cause the flux to pass in the re- quired direction through the solenoid. MAGNETS THAT FOEGET 139 As the prediction of the polarity of a solenoid requires considerable thought, endeavours have been made to sim- plify the matter. For example, it has been suggested that if, as shown in Pig. 39, N and S represent the ends of a bar of soft iron around which coils have been wrapped, and the current flows through the coils in the directions indicated by the arrows on the outside, the polarities produced will be north and south respectively. For example, in the left-hand figure, if the current, as indicated by the curved arrows, flows in a counter-clockwise direction, a north pole, N, will be produced at the end fac- ing the observer. Or, printing the letter N in the manner shown, with its two vertical lines consisting of arrows pointing in the directions indicated, these directions will show how the current must flow through the wire in order to produce a north pole at this end. In the same way, at the right figure, in order to produce a south pole on the end of the bar facing the observer, the direction of the current through the coils must be clockwise, or the same as the curved arrows forming the top and bottom of the letter S. The different kinds of windings that are capable of pro- ducing north and south poles, and the directions the cur- rents through these windings are required to have to produce the polarities marked at the ends of the soft iron bars, are shown in Fig. 40. We are now ready to examine the construction of an electro-magnet. Strictly speaking, an electro-magnet diifers from an active coil or helix only in that, in an electro-magnet, the coil or helix has been provided with a soft iron core. A coreless helix or solenoid cannot prop- 140 THE WONDEE BOOK OP MAGNETISM erly be called an electro-magnet. It is simply a magnetic solenoid or helix. I especially wish you to remember this since, otherwise, we should properly regard Ampere as the inventor of the electro-magnet; for he was the first who showed that a rectangular circuit, consisting of a single wire, acquires all the properties of a magnet both as regards its powers of attraction and repulsion, as well as its power of pointing, approximately, to the north of the earth. And even if it be urged that a single coil of this character should not be regarded as a magnet, then we might still point to Ampere's solenoid and call it a magnet, or we Fig. 40. — Windings of Solenoids might point to Schweigger's multiplier with its closely wound coils and call that a magnet. But if we limit the term electro-magnet to a helix or solenoid provided with a soft iron core, then we must give the credit for this great invention to William Sturgeon, who was the first to intro- duce the core of soft iron. I acknowledge that this use of a soft iron core does not at first sight seem to be a great invention. The solenoid had been produced before Sturgeon's time. It was known that a bar of steel introduced into a solenoid becomes strongly magnetised, and I think it more than probable that straight soft iron cores had also been introduced into solenoids before Sturgeon's time. Nevertheless, it seems MAGNETS THAT FOEGET lil that Sturgeon was the first purposely to bring these two things together; i. e., the coil and the soft iron core, to form a fixed piece of apparatus, the core of which was not afterwards intended to be withdrawn. You have probably often regretted that your memory is not more reliable, and have pictured to yourself how great' an advantage you would possess if you could calmly store away facts in your memory with the absolute certainty that you could promptly recall them whenever needed. You may be certain that you have facts stored away some- where in your brain. You are sure they are there. Indeed, you prove this by afterwards remembering them; but, try as hard as you may, there are times when you find it im- possible to recall these facts. Now I do not doubt that you will be surprised when I tell you that one of the great advantages possessed by electro- magnets over permanent magnets is that they are able almost instantly to forget they ever were magnetised. An electro-magnet differs greatly in this respect from a permanent magnet. A bar of hardened steel, when prop- erly magnetised, appears practically never to forget that it has been magnetised. Its magnetic retentivity, or, as it is also called, its magnetic memory, is exceedingly tena- cious. It can remember for years that it has been mag- netised. Now do not understand me to say that a good magnetic memory is to be despised. On the contrary, it is an ex- cellent thing in such a device as a compass needle, since, as you can see, it would be very awkward if a navigator, going some day to examine his compass needle, should find it had forgotten it had been magnetised, and had, there- fore, ceased to point, approximately, to the north. 142 THE WONDER BOOK OP MAGNETISM Suppose, however, you wish to use a magnet for the pur- pose of ringing a bell, or for sounding an alarm, where it is desired that the bell shall receive a number of strokes rapidly following one another. For such purposes a per- manent magnet would be useless, since if the magnet was very strong and the steel very hard, a considerable time would be required both before it became a magnet or before it lost its magnetism. There are two reasons why electro-magnets are so valuable : 1. Because they possess the property of both instantly acquiring their magnetism on the closing of an electric circuit through the magnetising coils, and of instantly losing their magnetism on the opening of such circuit. 2. Because they possess enormously greater strength; that is, are capable of producing a much greater quantity of magnetic flux than any other form of magnet. To give you some idea how rapidly a well made electro- magnet with a core of soft iron is able to acquire and lose its magnetism, I will say that in a form of small electro- magnetic engine invented by Page,, its magnetism can be made, lost and reversed 4000 times a minute, and in other pieces of apparatus even this great speed is increased. You will be interested in examining the form of electro- magnet first constructed by Sturgeon. As shown in Fig. 41, it was of the simple horseshoe type, the soft iron core of which was wrapped with insulated wire. When the electric current flows through the coil by inserting the terminals of an electric source in mercury cups, north and south poles are instantly produced at the extrem- ities of the soft iron core. MAGKETS THAT FOEGET 143 I think an electro-magnet is, beyond doubt, one of the most important if, indeed, not the most important electric and magnetic device ever produced. I am sure all of you who have read about the wonderful things in both elec- tricity and magnetism that electro-magnets are capable of accomplishing will understand that this means very much. The great value of the electro-magnet consists in the won- derful rapidity with which it can be magnetised and can Fig. 41. — Sturgeon's Elbctbo-Magnbt afterwards lose its magnetism ; for, instantly when the cur- rent flows through its coils it becomes a magnet, and in- stantly when the current ceases to flow, by opening the circuit, it loses its magnetism. Moreover, the places or points where the electric circuit can be made or broken may be situated thousands of miles from the electro-magnet. Without electro-magnets it would be impossible to have the electro-magnetic telegraph, the telephone, burglar alarms, or annunciator calls. 144 THE WONDEE BOOK OF MAGNETISM But this is not all. The enormous increase in the strength of electro-magnets, due to the introduction of a soft iron core, renders it possible to use electro-magnets for such apparatus as dynamo-electric machines and electric motors. Without electro-magnets, therefore, arc and in- candescent electric lighting would be impossible; the driv- ing of mills and machinery or trolley cars by electric motors would be impossible, as would also be the very im- portant principle of the transmission of power by means of electric energy. Indeed, I think you would find it diffi- cult to conceive how a greater calamity could be brought on the world than would befaU it were it deprived of all its electro-magnets. Before closing this brief chapter on the general con- struction and operation of electro-magnets, I wish to tell you what Professor Henry of Princeton University did for the electro-magnet. Henry devised a form of in- strument called a high-resistance electro-magnet, the construction of which is shown in Fig. 43. Here a number of separate coils are placed, as shown, over the surface of a horseshoe-shaped core of soft iron. The advantage was thus obtained of being able to connect the separate cir- cuits in different ways so as to vary the strength of the magnet, as well as to make it suitable for use with dif- ferent electric sources. Fig. 42. — Heney's Electeo-Maqnet CHAPTER XIV DIFFERENT KINDS OF ELECTRO-MAGNETS The electro-magnet, with, its soft iron cores wrapped with coils of insulated wire, is of such simple construction that it can readily be made in a number of different forms or shapes. We will describe a few of these. The simplest form of an electro-magnet consists of a straight cylindrical bar of soft iron covered with coils of insulated wire. These coils must necessarily be insulated since it is necessary that the electric current shall pass in Fig. 43.^Cobe and Bobbin of Electeo-Magnet at one end of the wire and come out at the other end with- out taking any short cuts, as it would if the coils of wire touched one another at any point. In this, as in almost all forms of electro-magnets, the coils of wire, instead of being wrapped directly on the surface of the soft iron cores, are generally wound on a bobbin, or spool, consisting of some form of insulating material such as wood, hard rub- ber or pasteboard soaked in shellac varnish. The spool is provided with eods that project beyond the top layer of 14,5 146 THE WONDER BOOK OP MAGNETISM ■wire. This is necessary, since, unless the ends of the core are provided with some such device, the coils are apt to slip off. Such a core and bobbin are represented in Fig. 43. In practically all electro-magnets, the core is covered with more than a single layer of wire. In such cases the wire is wound in turns, placed close together, from one end of the core to the opposite end, say from the top to the bottom. It is necessary that the direction of the winding be maintained throughout ; that is, it must all be either right- handed or left-handed. The second layer is wound over Fig. 44. — Horseshoe Electro-Magnet the first, returning in the opposite direction ; i. e., from the bottom to the top, and in this way the coils of wire are placed, layer after layer, on top of one another, the same direction of winding being maintained throughout. It makes no difference in the polarity because the cur- rent flows through the separate layers in opposite directions when they are wound in this manner; provided the same right-handed or left-handed method of winding is pre- served in all the layers, each separate layer will tend to produce north polarity at one end and south polarity at the ELECTEO-MAGFETS 147 opposite end; for, as we have seen, a coil whose winding is right-handed when observed from one end, is left-handed when observed from the opposite end. The commonest form given to the electro-magnet is that of a horseshoe. The wire may be wound directly on the core, beginning at one end and continuing to the other end, keeping the same direction of winding on each of the legs throughout; that is, seeing that the winding shall be either clockwise or counter-clockwise. If, as is usually the case, more than a single layer is wound on the core, the separate layers are placed on one another, as has been described in the case of the straight bar electro-magnet. A horseshoe electro-magnet so wound is represented in Fig. 44. Here the direction of winding has been main- tained throughout just as if the core or bar had been straight when wound, and had afterwards been bent in the form of a horseshoe. For small electro-magnets, it is generally more con- venient to wind two separate coils on bobbins or spools and then slip them on two separate cores, one set of the ends of which, as shown in the above iigure, the lower set, are firmly connected together by a soft iron yoke. The ends of the coils are then so connected as to preserve the same direc- tion of winding on each core. This leaves the two free poles. It is customary in all electro-magnets to leave the poles projecting a short distance beyond the ends of the spools, in order to ensure a freer play to the armature as it moves to-and-fro. Fig. 45 shows the form of electro-magnet that consists of a bar of soft iron on the two legs or poles of which two spools of insulated wire have been placed. In this form. 148 THE WONDEE BOOK OF MAGNETISM the soft iron core, instead of consisting of two separate straight iron bars and a yoke, is formed of a single bar bent into the horseshoe form. The magnet, suspended as shown from the top of a frame, is provided with a suspen- sion plate for the reception of weights. A cylindrical form of electro-magnet devised by Joule, an English physicist, represented in Eig. 46, is of the Fig. 45. — A Fobm of Electro-Magnet, horseshoe type. When its a'Tmature is in place against the poles this magnet has the form of a cylinder. The core consists of a hollow cylinder of wrought iron, planed off on one side so as to make a gap or opening of about a third of an inch wide between the two sides. The armature con- ELECTEO-MAGNETS 149 sists of a piece of soft iron shaped so as both to fit in close contact with these poles, as shown, and to complete the cylindrical outline. The larger piece of the cylinder is covered with insulated copper wire. When an electric cur- rent is sent through the coils, opposite poles are produced at the openings that attract the armature with consider- able force. In the cylindrical horseshoe magnet there is such an extent of polar surfaces that the armature is held to the magnet poles with considerable force. Another curious form of electro-magnet is known as an iron-clad electro-magnet. This magnet, as represented in Fig. 46. — Joule's Ctxindeical Electeo-Magnet Eig. 47, takes its name iron-clad from the fact that its magnetising coils are completely surrounded by iron. A soft iron cylinder, that has been brought into firm con- tact with the lower pole, surrounds the magnetising coil and reaches up to the level of the upper pole. In this manner two concentric poles are produced that are capable of exerting a considerable attraction on a suitably shaped soft iron armature brought near them. A still more curious form of electro-magnet consists of what is known as the zigzag-magnet, also devised by Joule. This magnet, as represented in Pig. 48, is wound as shown 150 THE WONDER BOOK OP MAGNETISM so as to produce a number of alternate north and south poles, and is employed in some forms of alternating-cur- rent dynamo-electric machines. The powerful electric currents that are now produced in nearly all the large cities of the world, either for the supply of systems of arc and incandescent electric lighting, or for the driving of electric motors or trolley cars, make it easy to produce electro-magnets enormously stronger than the great magnetic battery produced by Knight that was Fig. 47. — Ieonclad Electeo-Magnet justly regarded in its time as one of the world's great mag- netic wonders. A brief description of a large electro-magnet that was made at the United States torpedo station at Willet's Point on Long Island Sound will interest you. Here a huge steel cannon, some sixteen feet in length, weighing"^ 50,000 pounds, was wrapped with 5250 turns of insulated wire. When a powerful electric current was sent through the coils, the quantity of magnetic flux that was blown out from its north pole and sucked in again at its south pole made this huge electro-magnet one of the greatest magnetic blowers and suckers the world had ever seen. ELECTEO-MAGNETS 151 Let me tell you some of the many wonderful things this magnet was capable of doing. It could support five cannon balls hanging one under another, something like the needles or pins you may have seen suspended from the poles of a small bar magnet. How great, however, the difference in weight! Each of these Fig. 48. — Zigzag Magnet balls weighed 350 pounds, so that a total weight of 1250 pounds was suspended by only the small contact existing between the spherical surface of the ball and the magnet pole. Had a flat armature been placed on such pole a much heavier weight might have been supported. Indeed, when an armature was so placed against the plate it re- quired the united strength of sixteen able-bodied men. 152 THE WONDER BOOK OP MAGNETISM tugging at a rope and tackle, to pull it from the magnet. Contrast this with the 100 pounds that was sufficient to separate a bar of soft iron from the poles of Knight's great magnetic battery. This was truly a wonderful cannon and many interesting things were done with it. A soldier stood with his back towards the north pole of the huge magnet ; the flux blown out from this pole passed almost as freely through his body as if it were not there, and its presence was shown by the clinging of heavy iron spikes to his forehead and chest. The flux passing through the man produced powerful south poles where it entered his body and powerful north poles where it passed out. It is these north poles that caused the iron spikes to hang on to him. You may inquire whether the soldier experienced any inconvenience from this passage of the magnetic flux through his body. I will discuss this matter in another chapter, but so as to remove any doubts you may have, I will say that no injurious effects could be detected. There is another form of electro-magnet that differs from any already described, in that the soft iron core is movable instead of being fixed. This magnet is known as a sucking magnet. It consists of a hollow coil containing many layers of insulated wire and a core of soft iron, so much smaller in diameter than the opening of the coil that it can move through it without touching at any place. As soon as an electric current is sent through the coil, the movable core, if brought near it, is sucked or drawn into the coil, in which it remains suspended, as it were, in mid-air. There is a tradition that in the Mohammedan mosque ELECTRO-MAGNETS 153 where the body of Mahomet the Prophet is buried, the iron coffin containing his body is sustained in mid-air between the roof and the floor of the mosque by some invisible force. I do not know whether this is true or not, but I think it very improbable, although the soft iron core of the suck- ing magnet described in the preceding paragraph could and did remain suspended in mid-air when a sufficiently pow- erful magnetising current passed through its coils. Pro- vided cost were a matter of indifference, such a phenome- non might easily have been actually produced, but the amount of electric current that would be required to oper- ate a sucking magnet coil, hid in the walls of the mosque, would be so great that a very large and conspicuous elec- tric plant would have to be established somewhere in the neighbourhood. If, as I have read in other accounts of Mahomet's coffin, the coffin was merely suspended at a point in the ceiling of the mosque without any other attachment, the wonder would cease; for if the coffin and its contents only weighed a few hundred pounds, the suspension of such a weight even from practically a single point would be but a trifle when compared with the 1250 pounds of iron cannon balls suspended from a single point on the pole of the huge electro-magnetic gun magnet already referred to. It is in the construction of the dynamo-electric machines employed for producing the electricity required for the lighting of large cities and for driving machinery and motor cars that electro-magnets are especially useful. It may interest you, therefore, to hear a few of the things pointed out by Sylvanus Thompson in his " Dynamo- Electric-Machinery " that must be carefully borne in 154 THE WONDEE BOOK OP MAGNETISM mind in producing electro-magnets for this purpose. Briefly, these are as follows: To produce powerful electro-magnets, plenty of iron should be provided for their cores. When the cores are cylindrical in shape, the strength of the magnetism in- creases as the square of the diameter of the core; or, other things being equal, a magnet that is twice as thick as another (that is, whose diameter is twice as great) will be four times as strong. The iron should be as soft as possible, not so much because soft iron is readily magnetised and de-magnetised, but because soft iron is not so quickly saturated as hard iron. It is especially important, wherever there are joints in the iron of electro-magnets, as, for example, in the pole pieces or yokes, that these joints be as close fitting as pos- sible and, therefore, free from air-gaps, so as to make the magnetic resistance or reluctance of the circuit as small as possible. CHAPTBE XV THE MAGIC WAND AND PURSE OF FORTUNATUS Have you ever thought what a wonderful thing a fairy's wand is ? The fairy has only to touch a thing with it to bring about a great change. Sometimes the wand changes a man or a woman into a wild beast. Sometimes it leaves them alone and only changes their clothes; sometimes it changes neither the appearance nor the clothes but only endows them with new qualities or powers. How nice it would be to have a fairy's wand, so that we could go about touching people or things so as to change them. I sup- pose this is the idea of the wand employed by the modern magician when he does his tricks of legerdemain, pretend- ing that it is the magic powers of the wand that enable him to perform them. I imagine that some of you, remembering how I have employed fairy stories to impress certain facts on your mind, are saying to yourselves: Now I wonder what Dr. Houston can possibly find in magnetism, or in a magnet, that in any way resembles a fairy's wand? If you will wait a few moments, I am sure I will be able to prove that it is indeed possible to find such a wand in a magnet. Probably all of you have heard the fairy story of For- tunatus ; how a fairy whose good feeling he had won made him the present of a famous purse, or — more correctly — brought about a change in his own purse that greatly sur- 155 156 THE WOXDER BOOK OF MAGNETISM prised him. Disguised as an old woman clothed in rags, the fairy appeared to Eortunatus and asked for help. Now I am sorry to say that Fortunatus had been " burning his money up," or in other words squandering it, so that there were only ten pieces of gold left in his purse. He was a generous fellow, however, and when the old woman, who seemed to know everything, asked Fortunatus to give her ten pieces of gold to save herself and her large family from starving, he answered: " I have but ten pieces of gold left in my purse, but take them, my good woman. I'll manage somehow or other." No sooner had the old woman taken the ten pieces of gold Fortunatus offered her than she turned into a beau- tiful fairy. Smiling at the surprise that was seen in the face of Fortunatus, she said : " Fortunatus, you will never regret the kind deed you have just done in giving me all you had left in your purse. Let me see that purse again " ; and when Fortunatus held it towards her she touched it with her wand and said : " Never while the purse remains in your possession shall you fail to find in it ten pieces of gold. No matter how often you may take the ten pieces of gold from that purse, you shall find ten pieces still remaining in it." Fortunatus thanked the fairy for the manner in which by a single touch of her wand she had given him such wonderful resources. He found that, as the fairy had promised, he could never empty his purse, although he tried his best to do so. Of course with so great a fortune at his command, he travelled over different parts of the world and had great times. THE PUESE OF FOKTUNATUS 157 Now here are two wonderful things. A magic wand and an inexhaustible or, perhaps, as you might call it, an un- emptiable purse. With the iirst, one can instantly change any object the wand touches. With the other, one can keep on drawing something out of a receptacle in which it has been kept, without decreasing the quantity that re- mains. You might say that you would willingly take a long walk to be able to see anything that possessed either of these wonderful properties. Well, you will not have to go far to see such a thing; for, an ordinary magnet, or a little piece of magnetised hardened steel or lodestone, actu- ally possesses both of these properties. Of course you have a pocketknife or penknife. If so, try to get it magnetised. This is simple, if you can find anyone who has a strong electro-magnet. You need only place the knife across the two poles and let it remain there for a few moments, when it will become thoroughly mag- netised. Now what I am about to tell you of the magic powers of a magnetised knife-blade is true only if the knife has been made of good and properly hardened steel. If it is a cheap knife, the blades of which are of soft steel, it will lose its magnetism almost as soon as it is removed from the magnet poles, and will, therefore, be worthless for what I am about to describe. If it so happens that you live somewhere at a distance from an electro-magnet, or are unable to get permission to place your knife across its poles when the current is flowing, you can magnetise your knife-blade by placing it against the poles of a permanent magnet, although, gener- ally speaking, an electro-magnet is preferable because, as 158 THE WONDEE BOOK OP MAGNETISM we have seen, an electro-magnet is generally more powerful than an ordinary steel magnet. After you have magnetised your penknife, in order to convince yourself that it possesses magnetic properties, open one of the blades and bring it near a small piece of steel, say a sewing needle that is resting on a table. Instantly the point of the blade will draw or attract the needle towards it, so that if the knife is removed gently from the table it will carry the needle with it, and you can see the very curious sight of the needle swinging to-and-fro and hanging at one end from the sharp point of the knife-blade. You cannot see what holds it, but you know it is the mag- netic flux that is either being blown out or sucked in at that end of the penknife blade. But the magnetised knife-blade can do more than merely pick up the needle. The point of the needle, which has been touched by the knife-blade, has instantly been en- dowed with a new property. Acting as a magic wand, the knife-blade has given the needle properties it did not be- fore possess ; for, if the needle be now floated on the surface of water, it is able to point out or show the way to the north or south pole of the earth. It has been magnetised by the mere touch of your knife-blade and can now act as a small compass needle. In other words, your knife has actually acted as a magic wand. I wish to say here that in so small a piece of steel as a sewing needle, with so weak a magnet as a penknife blade, the strength of magnetism imparted to the needle is apt to be quite small, so that unless you are skilled in experi- menting you may have some trouble in making the floating needle point readily to the north. If you have such trouble, THE PUESE OP FOKTUNATUS 159 don't conclude that what I have told you is untrue, but try- something larger than a sewing needle. Try the open blade of a penknife that has never as yet been magnetised, and, bringing the blade of your magnetised knife so as to touch or come in contact with the blade of the unmag- netised knife, draw the blade over the other blade, begin- ning, say, at the thick end where it turns in the knife handle, draw it towards the point, and then, returning the blade through the air, again draw it towards the point, repeating this ten or fifteen times. ISTow turning the blade over, do the same thing with the other side, and also with the back of the blade. When you have thus completed these motions you will find that the blade of the other knife is now magnetised and will be able to pick up needles and pins nearly as well as your own knife does. If you dip the blade of your magnetised penknife in a quantity of iron filings, the filings will collect on the ends in the same feathery tufts that you read about being pro- duced when a bar magnet was rolled in iron filings. If you have properly magnetised the other knife-blade it will also be found able to collect iron filings at its point. But your magnetised penknife blade, which as above described is able like a fairy's wand to endow a thing touched with new properties, is more than a magic wand. It is also like the wonderful purse of Portunatus. You can take out from it a certain quantity of magnetism, and yet it will have as much magnetism remaining in it as it originally had. Indeed, if you are successful, and go through the strokings carefully as I have explained to you, you can take out from your penknife blade apparently all the magnetism it originally contained; for you can make 160 THE WONDEE BOOK OF MAGNETISM the blade of the magnetised knife as strongly magnetic as your own, and yet, when you come to examine your knife- blade, perhaps naturally expecting to find it empty or free from magnetism, you will find it as full of magnetism as before. ISTow what makes the matter still stranger is that it makes no difference how many penknives you may mag- netise from your knife, if the process has been properly carried on; especially, if you have never tried to magnetise a blade that has been already magnetised, it will make no difference. You will find that the strength of magnetism in your knife is apparently as great as it ever was. Indeed, if the truth were known, it would most probably be stronger than it ever was before. Is it not like the won- derful purse of Fortunatus, in which ten gold pieces taken from the ten in the purse leave ten? One knife-blade full of magnetism, given to an unmagnetised knife-blade, and, therefore, taken from the magnetising knife-blade, leaves one knife-blade full of magnetism in the knife. Now I shall not be surprised if you should say : " I will not believe any such ridiculous statement no matter who makes it. It is contrary to all common sense, and, more- over, I never have had such an experience." As regards the matter being contrary to common sense, I hope to be able to prove to you that, so far from this being true, it is exactly what you should expect, if you thoroughly understand what is taking place when your knife-blade is giving magnetism to the other knife-blade. As regards the thing being contrary to your experience all I have to say is, try it for yourself. But let me see if I can show you that what has hap- THE PUESE OF FOKTUNATUS 161 pened to the knife is strictly in accord with common sense. Some people have the idea that in the case of a per- manent steel magnet, in order to prevent the magnet from losing its magnetic power, the soft iron armature placed over its poles when removed from them should be taken off gently. Such people say, never remove the armature from a magnet suddenly — if you do you will ruin its mag- netism ; that is, will cause the magnet to become demagnet- ised. Now, if this were true, since during the numerous strokings of your knife-blade against the blade of the knife you are magnetising, your knife-blade would be suddenly drawn from the other blade, it would lose its magnetism. But it can be shown, as Professor Sylvanus P. Thomp- son has done, that, so far from a magnet being weakened by suddenly pulling its armature away from its poles, it is actually strengthened. But you may say : " How can my knife-blade have as much magnetism left in it as it first had if it is con- stantly giving magnetism to another penknife ? " Now, let us look at this matter and get it clear. When you stroke the unmagnetised blade of a penknife with the magnetised blade of your knife, the other knife- blade certainly acquires magnetism. It looks as if its mag- netism comes from your knife-blade. But it does nothing of the kind. In discussing the theories of magnetism, I told you that, in accordance with modern ideas, it is now generally believed that the very small particles of steel in a magnet, say the molecules, are naturally magnetic; that when such a bar is magnetised, the magnetism is not de- rived from an external source, the magnetising magnet, but from its own particles, which are naturally magnetic ; that 163 THE WONDER BOOK OP MAGNETISM is, which were magnetic before they were brought into the neighbourhood of the magnetising magnet. Before you brought the magnetised knife-blade in contact with the blade that is unmagnetised, the latter was already mag- netic, but this magnetism was hidden because the little magnets were arranged in all sorts of positions, with their opposite poles together. Their magnetic flux, therefore, instead of passing outside the magnet, flows in little closed circuits through these tiny magnets. All your knife- blade does, when it is drawn over the surface of the un- magnetised blade, is to catch hold of the little separate magnets and, swinging them around, cause them to point in the same direction. Their flux is thus united so as to flow in a general stream through the bar, coming out at one end, and entering again at the other end. As soon as this is done the bar is magnetised. Your knife-blade has given up none of its magnetism to the other blade, but has simply pulled the molecular magnets around so as to cause them to point in the same direction. But you may ask what has happened to the little mag- nets of which your knife-blade consists. Do they all con- tinue pointing in the same direction ? I can see no reason why they should not. Indeed, on the contrary, I can see that since, when the other knife-blade has been magnetised, all its little magnets are pointing in the same way, neces- sarily all the little magnets of the magnetising knife are similarly pointing in the same way; for, otherwise, they could not have been able to turn all the little magnets of the magnetised blade in the same direction. Before leaving this matter, it may be well to present it to you in a somewhat different light. Not that what I am THE PUESE OP -POETUNATUS 163 about to tell is exactly the same as what I have told you about magnetism. It is simply what may be called an analogous case. Suppose a baseball player has thoroughly mastered the art of pitching a ball so as to puzzle the batter with his outshoots, inshoots, or drops; or suppose he has learned how to deliver that very perplexing form of ball called a " spit " ball, that so often fools the batter as it comes moving in a straight line directly towards the plate, and just as the batter, sure of its direction, strikes at it, it sud- denly drops or rises, thus escaping the bat. Now, I need hardly tell you, because one has a full knowledge of how to deliver any kind of ball that he will lose this knowledge either because he keeps on practising the delivering of such balls, or because he is willing to impart his knowledge to another pitcher in order to teach him to deliver them. You know, as well as I do, that the more he hands out, as it were, such knowledge, the more he will continue to possess it. It is the same with any kind of knowledge. The fact that you give or impart this knowledge to others only makes it all the more cer- tain that you will keep it yourself. CHAPTEE XVI DIFFERENT WAYS IN WHICH BODIES MAY BE MAGNETISED Althodgh there are various ways in which magnetism may be imparted to magnetisable bodies, yet all of them consist in causing magnetic flux to pass through the body, so that it shall enter it at one point and pass out from it at another point. The simplest method of magnetisation is by touch. The body to be magnetised is simply touched by one of the poles of a strong magnet; or, since this is not apt to pro- »«'»«■*. duce a very strong magnetisation, the hskliiiiiliwL.,. . ^jo^y ig repeatedly touched or stroked by drawing the magnetising pole a number of times over its surface. In all such cases, the body that is mag- netised acquires a polarity opposite to that of the pple by which it has been touched or stroked. It is a matter of considerable im- portance, in the case of any magnet, that its magnetism be so distributed ;; as to cause the poles to be located at il the opposite ends of straight bars or ■ ' at the free ends of horseshoe or TJ- FiG. 40.-EKROB gjj^ . magnets. When the bar to be FROM Position of ^ " . , , i • ^ Poles magnetised is a straight bar, and is to be employed as a compass needle, it is especially important 164 WAYS OF MAGNETISM 105 that its opposite poles shall come exactly at the ends or extremities of the needle. Suppose, for example, it is desired to magnetise the needle N"S, Fig. 49, for use either as a compass needle, or as the needle of a galvanometer, so that in either case the exact amount of the deflection of the needle can be readily measured. Now, unless the poles are situated exactly at the extremities N and S, an error will be made in reading the direction in which the needle is apparently pointing. Since it is assumed that the poles of a compass needle actu- ally occupy the positions at its opposite ends, the amount the needle is turned out of its position of rest when under the influence of the magnetism is estimated on this assump- tion, so that if the poles are located elsewhere an error will result. Suppose, for example, in the needle represented in the preceding flgure its N and S magnetic poles, in- stead of being located at its extremities N and S, are situ- ated at N' and S'. In other words, instead of lying in the direction of the line NS, they lie in the direction of N'S'. Then, while the north pole of the needle is in reality pointing towards 38° on the scale shown in the figure, it appears to be pointing to 31°, thus causing a serious error of 3°. The magnetisation of a needle obtained by stroking it from end to end with a single magnet pole is known as " magnetisation by single touch." This method is not apt to bring the opposite poles exactly at the extreme ends of the needle. Sometimes, in magnetisation by single touch, the needle is magnetised by rubbing one pole of the magnet- ising magnet against one end of the needle, and the other pole against its other end. Perhaps, however, the best 166 THE WONDEE BOOK OF MAGNETISM method of magnetising by single touch is to place the pole of the magnetising magnet in contact with the needle, drawing it repeatedly along the needle in the same direc- tion. Ko method of magnetisation by single touch pro- duces very strong magnetisation, and is very apt to produce consequent points. In what is known as " magnetism by divided touch," the opposite poles of two separate magnets of exactly equal strength are placed at the middle of the bar to be mag- netised, and are then drawn simultaneously in opposite directions towards the ends of the bar. On reaching these Fig. 682 Fig. 50. — Magnetisation by Divided Touch ends, the magnets are returned through the air, again placed at the centre of the bar and the same motion continued a number of times. The bar is then turned and the other side subjected to similar movements. The magnetising bars may either be held vertically, or may be inclined. In this way, a much more uniform distribution of magnetism is produced than by single touch. The best results are ob- tained when the ends of the bar to be magnetised are rested on the poles of two separate magnets, as represented in Fig. 50. In another method, known as " magnetisation by double touch," two magnets are placed with their opposite poles WAYS OF MAGNETISM 167 close together, but not touching, at the middle of the bar to be magnetised. Instead, however, of moving them in opposite directions towards the two ends of the bar, they are firmly fixed together and simultaneously moved first towards one end of the bar to be magnetised and then to- wards its other end, without removing them from the bar that is being magnetised. In order to obtain the best re- S 1 Fig. 51. — ^Magnetisation by Double Touch suits it is necessary to see that each half of the bar to be magnetised receives the same number of strokes; or, in other words, that the process be stopped when the two mag- netising magnets are at the middle of the bar. You can get a better idea of magnetisation by double touch by examining Fig. 51. In this method, instead of employing separate magnets in order to obtain the two opposite poles, a horseshoe magnet may be employed, the poles of which are brought rather closer together than usual. While the method of magnetisation by double touch produces powerful magnets, it is very apt to produce consequent points. 168 THE WONDEE BOOK OP MAGNETISM In the magnetisation of compass needles by the electric current, the needle is placed inside a hollow coil or sole- noid, the length of which is preferably equal to, or slightly in excess of, the length of the needle that is to be mag- netised. Bar magnets are also magnetised in this manner. Fig. 52 represents a hollow coil suitable for magnetisa- tion by electric currents. The coil is placed over a hole in the top of a table or other support. When the magnetising current is passing through the coil, the bar to be magnet- ised is passed up and down through the hollow coil an equal number of times. It will be remembered that Knight constructed his famous magnetic battery for the purpose of magnetising Fig. 52. — Magnetisation by Electeic Cuerents compass needles. This was done by placing the needles on the ends of the opposite magnet poles of two magnetic magazines, the needles being magnetised by a single touch ; for it was only necessary to rest the needles for a moment on these poles to magnetise them to saturation. As we have seen in one of the preceding chapters, bars of iron or steel are frequently magnetised by long contact with the earth; for the earth acts as a huge magnet. Sometimes lightning discharges passing through bars of iron or steel permanently magnetise them. In such cases, of course, the magnetisation is due to the magnetic WAYS OF MAGNETISM 169 flux produced around the circuits through which an elec- tric current is flowing. In such cases, where the bars are of hardened iron or steel, the magnetisation is permanent. In a similar manner, lightning discharges passing in the neighbourhood of compass needles may either completely demagnetise them, or may reverse their magnetism. Such a thing occurring to the compass needles of a ship, might easily cause serious losses. There are some curious phenomena connected with the magnetisation of hardened steel or iron by lightning flashes. A lightning flash differs in no respect from the discharge of a Leyden jar battery except in its strength. Early in the history of magnetic science, while Arago, Davy and others were permanently magnetising hardened needles of steel by placing them in the hollow coils of the solenoids, and passing the current from a voltaic battery through the coils, a trial was made to see whether the discharge from a Leyden-jar battery could be employed in place of the voltaic battery. The experiment was tried and the bar was found to be magnetised apparently in the same way as it would have been had a steady or constant current from a voltaic battery been employed. When, however, this bar was more carefully examined it was found, to the great surprise of those making the experiment, that the polarity was not the same from the outside to the inside of the bar. On the contrary, there had been produced al- ternate layers of opposite magnetism at different distances from their surfaces. This discovery greatly puzzled scientiflc men. They tried, and for a long time unsuccessfully, to explain this curious variety of magnetisation. It was because they were 170 THE WONDER BOOK OF MAGNETISM unable to explain it that they called it anomalo-us magnet- isation and the magnet so produced an anomalous magnet. As you will remember, this name has already been given to magnets that apparently possess three, five, or any num- ber of odd magnet poles greater than one. I think it would be better if the name anomalous magnet were re- stricted to a magnet produced by a Leyden jar discharge and the magnet containing three, five, etc., odd poles were called a magnet with consequent poles. In 1842, long after the above experiments were made. Professor Henry, already referred to, pointed out the cause for the peculiar magnetisation produced by Leyden jar dis- charges. He showed that the discharge of a Leyden jar, like that of a lightning flash, instead of continuing to pass in one and the same direction, like the current from a vol- taic battery, rapidly changes its direction a very great number of times a minute, first flowing in one direction and then in the opposite direction. Of course, this would result in the magnetising current flowing in different di- rections through the magnetising helix, say first from the lop to the bottom, and then from the bottom to the top, so that at one moment a north magnetic polarity would be produced, and at the next moment a south magnetic polar- ity at the same end. Instead, therefore, of a single mag- netic pole being produced at the end of a needle, there would be a number of thin shells of opposite magnetic polarities, as they were actually discovered to be, either by carefully turning off or by eating away one shell by acids, until the next shell was reached. No little excitement was produced in the year 1813 by the assertion made by Morichini, that if a steel needle is WAYS OP MAGNETISM 171 merely exposed to the violet rays of the sun, the needle was thereby magnetised. A Mrs. Mary Somerville, who had written a book on natural philosophy, claimed that she re- peated these experiments and succeeded in permanently magnetising a steel needle by exposure to the violet light of the sun for a period of two hours. It may be interesting if I quote what Playfair says con- cerning this experiment: " The violet light was obtained in the usual manner, by means of a common prism, and was collected into a focus by a lens of sufficient size. The needle was made of soft wire, and was found, upon trial, to possess neither polarity nor any power of attracting iron filings. It was fixed horizontally upon a support, by means of wax, and in such a direction as to cut the magnetic meridian at right angles. The focus of violet rays was carried slowly along the needle, proceeding from the centre towards one of the extremities, care being taken never to go back in the same direction, and never to touch the other half of the needle. At the end of half an hour after the needle had been exposed to the action of the violet rays, it was care- fully examined, and it had acquired neither polarity nor any force of attraction, but after continuing the operation twenty-five minutes longer, when it was taken off and placed on its pivot, it traversed with great alacrity, and settled in the direction of the magnetical meridian, with the end over which the rays had passed turned to the north. It also attracted and suspended a fringe of iron filings. The extremity of the needle that was exposed to the action of the violet rays repelled the north pole of a compass needle. This effect was so distinctly marked as 172 THE WONDER BOOK OF MAGNETISM to leave no doubt in the minds of those who were present that the needle had received its magnetism from the action of the violet rays." It seems that there must have been some mistake in these experiments; for, when such able experimenters as Michael Faraday and Sir Humphrey Davy went to Italy the next year and repeated the experiment in Morichini's laboratory, they were unable to magnetise a needle. In connection with magnetisation, it may be interesting to note that when a bar of iron is suddenly magnetised a change occurs in its length. While this may seem aston- ishing, yet a little thought would seem to show that it should be expected, since, if the very small magnets of which an unmagnetised bar consists are longer in one direction than in another, necessarily the turning around of these small magnets, so as to cause them all to point in the same direction, must cause a change of length. If you would like to surprise some of your friends by invisible writing of a wonderful character, obtain a thin sheet of hardened steel, which you might get from the manufacturers of steel pens. Then take your magic wand, the blade of your penknife, or, since this experiment re- quires a rather stronger magnet pole, bring either pole of a bar magnet in contact with the surface of the steel plate and slowly write on it any word, say your name. You will then magnetise the plate wherever you touch it with the bar, so that a permanent record will be left on it wherever the magnet pole has been moved over it. You cannot see the writing, nor can your friends, but if iron filings be sprinkled over the plate, they will collect on the portions that have been magnetised, so that, if you after- WAYS OF MAGNETISM 173 wards tip the plate and at the same time strike it gently with a lead pencil so as to get rid of the surplus filings, the writing can be distinctly seen by the collection of filings that remain clinging to the magnetised portions. As we shall see in a subsequent chapter, " The Talking Newspaper," a somewhat similar method is employed for permanently recording on a wire or ribbon of hardened steel the message received by a telephone. CHAPTEE XVII HOW THE FLUX CAN BE STRENGTHENED AND RETAINED IN MAGNETS But the power of permanent steel magnets is greatly increased by placing separate pieces of soft iron or steel, called " pole pieces," in contact with each pole. Pole pieces may, therefore, be regarded as the soft iron or steel prolongations of the poles. Armatures or keepers consist of a single piece of soft iron or steel placed near or across the two poles and capable of motion towards them, so as to come in contact with both of them. Armatures are sometimes formed of hardened steel. When made of soft iron or soft steel, they are called " non-polarised armatures." If made of hard iron or steel, they are called " polarised armatures," or armatures that possess a polarity independent of that imparted to them by an electric current flowing through the magnetising coils of the magnet near whose poles they are placed. An armature is sometimes incorrectly called a keeper. But an armature is a keeper only under certain circum- stances. There is always a tendency for a magnetised bar of hardened steel to lose its magnetisation gradually, and a keeper is employed only in order to prevent this loss. Now, while an armature, as long as it is in contact with the magnet poles, does the same thing and can, in this sense, be called a keeper, yet an armature has other work to do. It must be so placed that it can move towards or from the 174 HOW FLUX CAN BE STEENGTHENED 175 magnet poles, in which case it is entirely different from a keeper. Pole-pieces are employed for the purpose of increasing the strength of the magnet to whose poles they are per- manently fixed. They are of especial nse in compound magnets, which, as you remember, consist of numerous separately magnetised pieces of steel. Pole-pieces, keepers and armatures, therefore, possess different properties. But there is one respect in which they agree: they all tend greatly to increase the amount of magnetic flux that is blown out from the north pole of the magnet, and sucked into its south pole. I have frequently called your attention to the marked increase in the amount of magnetic flux that passes through a magnetic circuit on the introduction into it of soft iron or soft steel. This is especially marked in the case of a magnetic solenoid when a bar of soft iron or steel is placed inside it. Since the cause of the increase is the same in solenoids as it is in magnets, I will explain the matter to you by taking the case of a magnetic solenoid that has within its hollow coils a mass of air or, what is the same thing, a mass of wood; for, as far as magnetic flux is concerned, wood acts practically the same as does air. Afterwards, this wood is replaced by a core of soft iron of the same size and dimensions as the wooden core. Suppose a wooden ring is uniformly wrapped with a coil of insulated wire, and an electric current is caused to flow through this coil. Magnetic flux will be set up within the interior of the ring, and will flow through it in par- allel streams. All this flux is confined to the interior of the ring. If now another ring, of precisely the same di- 176 THE WON"DEE BOOK OP MAGNETISM mensions, be wrapped with precisely the same kind of wire and the same number of turns, but over a core of very soft iron instead of wood, and the same electric current is caused to flow through the coils, parallel streams of mag- netic flux will flow through the interior of the iron core just as they flowed through the wood core. There is, how- ever, this difference : that, provided it is made of very fine soft iron, the amount of magnetic flux in the iron core will be, perhaps, as much as 1000 times greater than in the wood. This is certainly wonderful. It seems at first almost impossible to explain, and, in point of fact, it has not been until a comparatively recent date that the true ex- planation was discovered. It is a difficult matter to under- stand, but I think I can explain it to you if you try your best to follow me. Of course you know what causes magnetic flux to flow through the solenoid. It is the electric current flowing through the wire. Now in order to produce magnetic flux, there must exist a force called a magneto-motive-force; or, as we have agreed to abbreviate it, an M. M. P. In the case of any magnetic circuit, the quantity of this flux that passes depends on two things; namely, on the value or strength of the M. M. P. and on the value of the magnetic resistance or reluctance of the circuit. In other words, the quantity of magnetic flux that passes through the inside of the electric solenoid will increase with the increase of the strength of the M. M. P. and with a decrease in the magnetic resistance of the circuit. This being understood, let us consider the case of the two solenoids, one provided with a wooden core, and the HOW FLUX CAN BE STRENGTHENED 177 other with a core of soft iron. Since the current strength is the same in each, and the number of magnetising turns of wire is the same in each, the strength of the M. M. F. will be the same. Unless, therefore, the magnetic reluc- tance of the iron be different from that of the wood, the same amount of magnetism or flux must pass through each circuit. Now it is well known that the magnetic reluctance or resistance of soft iron is much smaller than that of wood, so that when scientific men first thought about this matter, they came to the conclusion that a greater amount of mag- netism flowed through a solenoid provided with a soft iron core than through one provided with a wooden core, because the soft iron core offered a smaller resistance and, therefore, permitted a given M. M. F. to force more flux through the iron ; and for a long time this explanation was accepted as the true one. For many years the scientific world was satisfied with the above explanation. Now, without wishing to ridicide the legal profession, for lawyers are generally able men, I may be forgiven if I point out to you that science is a somewhat different thing from law. Scientific men are not always satisfied with the truth of a statement because men regarded as authorities in science declare their belief as to its correctness. If I understand the matter cor- rectly, this is different among lawyers ; for, if a matter has once been what I believe the lawyers call adjudicated, — that is, has been passed on by some learned judge, — they feel that one of the strongest arguments they can use, if, indeed, not the strongest, is to call attention to this former decision, and if a number of decisions have been made in 178 THE WONDEK BOOK OP MAGNETISM the same direction, they regard the matter as practically settled, especially if an appeal has been taken against such a decision and the matter has been referred to some higher court, it may even be the Supreme Court of the land. A reference to such a decision is generally regarded as final. In other words, lawyers are apt to believe in what is called precedent. Now I wish to submit, with due respect to the bench, that this is not the kind of evidence a scientific man can properly regard as settling a question. If it were, scientific progress would be hopelessly barred in many directions. I do not deny, however, that if an opinion has been expressed by an able scientific man as to the correctness of a certain explanation, such an opinion should be given due respect. Moreover, if this decision has been referred to some other scientific authority, the highest known at that time, or, as it were, the Supreme Court of that particular branch of science, and his decision has been to confirm that of the other authority, then one would be justified in leaning towards the ruling of the higher scientific court. But no matter who may have ruled as to the correctness of a scientific principle, whether a lower or higher court, or even the Supreme Court, it will not do in science to follow that belief too closely. In physical science a thing is true, not because one or more authorities have declared they believe it to be true. It is only true because it is true, and herein I think a scientific man differs markedly from a lawyer. A scientific man is, as it were, a free lance, and, though willing to give due respect to authorities, he will never receive a statement as true because some other HOW FLUX CAN BE STEENGTHENED 179 one, even much better and greater than himself, has de- cided in its favour. It was exactly this way with the world's belief as to why a given M. M. E. should cause more magnetism to flow through an iron circuit than through a circuit con- sisting entirely of air or wood. Up to a certain date, the decision by practically all the scientific courts had been that the increased magnetism that follows the introduction of soft iron into a magnetic circuit is due to the iron per- mitting more flux to flow because it oSered a smaller resistance to its passage. At last, however, fortunately for the advance of science, one who might properly be called a kicker took up the question. Now, I have a great respect for kickers, at least when they kick intelligently. Much of the world's progress has resulted from this very valuable, though I think generally too little appreciated, class of people. One of this class, and he deserves great credit for what he did, noticed that, when the current flowing through the coils of the magnetic solenoid ceased to flow on the opening of the circuit, the magnetism of the iron core did not promptly disappear, as it should have done if the gen- eral belief had been correct. On the contrary, it remained in the core in the form of what is known as " residual magnetism." Then our kicker began to get in his work. He reasoned, and, as I think you will see, reasoned correctly, that the decisions of the court in this case had been clearly wrong. If the great increase in the flux flowing through an iron core is caused only by the M. M. E. of the current flowing through the coils of the solenoid, then, as soon as this current ceases to flow, no magnetic flux should remain in 180 THE WONDER BOOK OF MAGNETISM the core; for, if the flux were only set up by the electric current, how could it continue to exist when the current causing it ceased to flow? Having come to this conclusion, which I may tell you is the correct conclusion, he properly determined to pub- lish a statement of his belief. His friends came to him, urging him to bear in mind the eminent scientific men who declared that the old explanation was correct. But he published it, nevertheless, and the result was that, on care- fully reading it, practically all scientific men came to the conclusion that the old explanation was wrong, so that in this case the decision of the Supreme Court was reversed. You will say — at least I hope you will — " I can under- stand that, but was this all the man did? Did he only show that the old explanation was wrong? Did he also offer a better explanation ? " He certainly did, and that is the point to which I am now coming. It is evident, as I hope you can see, that in the case of a magnetic solenoid containing soft iron or steel, there must be two separate and distinct causes that set up or produce magnetic flux. One of them is the M. M. P. of the current flowing through the coil. Suppose we agree to call this the prime M. M. F. It increases with the increase of the strength of the electric current. The other was an M. M. F. that was set up, or, more correctly, set free, in the little molecular magnets of the iron or steel. These molecules are naturally magnetic. They have flux flowing through them that is due to a separate and distinct M. M. F. given to them when they are called into existence. But this natural M. M. F. was unable to produce any effect on the core so as to cause the magnetic flux to pour out HOW FLUX CAN BE STEENGTHENED 181 of the north end of the iron core and to enter it again at its south end. The streams of magnetic flux were confined to the circuit of the molecules and produced no effects on circuits lying outside of them. Now under the influence of the prime M. M. F. — that is, of the M. M. P. produced by the electric current — all these little molecular magnets are turned or aligned so as to have their poles point in the same direction; i. e., the length of the bar, so that there are set up a number of magnetic circuits parallel to each other and extending from one end of the bar to the other end. In this way, all their small M. M. E.'s are added, thus producing a separate and powerful M. M. P., which causes an increased quantity of flux to flow through the core. In order to distinguish this M. M. P. from the prime M. M. P. it is called the " aligned for structural M. M. P." You are now able to see how pole-pieces, keepers and armatures are able to increase the amount of magnetic flux that flows through a circuit. Let us begin with the pole-pieces. Suppose we take for this purpose a magnetic battery, such as that of the com- ■ pound horseshoe magnet of Jamin's, referred to in a pre- ceding chapter. (Eepresented by Pig. No. 10.) These pole-pieces consist of fairly large and heavy masses of soft iron, so attached to the ends of the magnets as to leave no air-gap at the joints. The result was a marked increase in the amount of flux that passed through the circuit of the magnet. This, of course, must be so, since we now have besides the M. M. P. of the permanent magnet, which in this case may be regarded as a prime M. M. P., the very much stronger M. M. P. produced by the alignment of the 183 THE WONDEE BOOK OF MAGNETISM separate molecules of the soft iron of the pole-pieees. You can now understand why in a Jamin horseshoe mag- net, consisting of three separate magnets only, the arma- ture of the magnet that was only able to sustain a weight of 4 kilogrammes (8.82 pounds avoirdupois), without pole- pieces, was able to sustain a weight of 140 kilogrammes (308.8 pounds avoirdupois), when pole-pieces had been placed on the ends of the magnet. The advantage gained by the use of a keeper or an armature of soft iron is the marked increase in the total amount of flux or magnetism passing through the magnetic circuit, as has been above explained. When an armature of soft iron or steel is employed on an electro-magnet, the magnet will attract the armature no matter in what direction the current is made to flow through its magnetising coils. It makes no difference if the direction of the magnetising current is reversed; for the armature of soft iron or steel will have opposite poles set up in it by induction, no matter what the polarity of the poles may be. When the current flows through the magnetising coil, and the electro-magnet thereby acquires polarity, the armature is attracted, so that in order to enable this armature to do work, some device must be provided to pull the armature away from the poles as soon as the current ceases. The first opportunity you have in a telegraph station to see the working of the Morse sounder, you will be able to understand how this is done by means of a spring placed so as to pull the armature away from the magnet poles. When the operator depresses the key, so as to close the circuit and permit the electricity to flow through the coils of the electro-magnet in the sounder, the HOW FLUX CAF BE STKENGTHENED 183 strength of the magnetic attraction being greater than that of the spring, the armature is drawn towards the magnet, thus producing a clicking sound; while, as soon as the key is raised, and the circuit is thus broken, the electro- magnet loses its polarity and the spring pulls the armature back, thus producing another clicking sound. These click- ing sounds following one another in different order and at different intervals produce the Morse characters or alphabet. In some pieces of electrical apparatus, it is necessary to cause the armature to move either towards one side or an- other, or towards one pole or another, of an electro-magnet. In this case the armature differs from an ordinary arma- ture in that it does not come to rest when touching both poles, but only when it comes in contact with the pole to which it had been attracted. The spring attached to the armature is adjusted so as to cause the armature to take up a position midway between the two poles when a weak current of a certain strength is passing. If, now, the strength of the current flowing through the electro-magnet is decreased, the armature will be pulled by the action of the spring against one of the poles, while if it is increased, it will be pulled by magnetic attraction towards the other pole. But an armature can be caused to move towards either pole in a much better manner by the use of what is called a "polarised armature," or an armature that is mag- netised independently of the electro-magnet. This arma- ture is placed, as in the case of the armature above re- ferred to, midway between the poles of an electro-magnet. When an electric current is sent through the coils of the 184 THE WONDER BOOK OF MAGNETISM electro-magnet in a certain direction, its armature, being already magnetised, will move in a certain direction. While if the direction of the flow of current through the magnetising coil be reversed, it will move in the opposite direction. Such armatures are called polarised armatures. As will be explained in " The Wonder Book of Electric- ity," polarised armatures are employed in systems of duplex and quadruples telegraphy; that is,, in systems of telegraphy called duplex telegraphy where two messages can be transmitted in opposite directions over a single wire at the same time, or in systems of telegraphy called quad- ruples telegraphy where four separate messages, two in one direction and two in the opposite direction, can be transmitted simultaneously over a single wire. CHAPTBE XVIII PARAMAGNETISM AND DIAMAGNETISM There have been so many references to magnetic flux passing through iron and steel that I fear you may think these are almost the only substances through which flux is able to pass. Now, while it^is true that magnetic flux passes through iron and steel far more readily than through other substances, yet there are very many other substances through which it can also -pass. Indeed, as Faraday has proved, there are no substances through which some mag- netic flux cannot pass, provided they are subjected to a very powerful flux. It follows, therefore, that there are no substances that cannot have magnetic poles produced in them, south poles at the place where the flux enters, north poles where it leaves them, although in most cases these poles are too weak to allow ready detection. Faraday's discovery that magnetic flux can pass through all kinds of substances was a great achievement, and natu- rally attracted much attention in scientific circles in dif- ferent parts of the world. Faraday found that, if different substances were placed in a sufficiently powerful magnetic field, the flux passes through them; but they were not all affected in the same manner. On the contrary, they acted in a way that led him to divide them into two great classes. Before going any further, I wish to say that when I used the word substances in the above sentences, I do not wish you to understand me 185 186 THE WONDEE BOOK OP MAGNETISM as meaning solid substances only; for, Faraday found that not only all kinds of solid substances, but also all kinds of liquid or gaseous substances, when placed between the two poles of a powerful electro-magnet, either set themselves directly across such poles, as would a piece of iron, or in other words, were attracted to the poles; or they placed themselves midway between the poles in a line at right angles to the direction in which the flux was passing, as would a bar of bismuth. In other words, it seemed to Faraday that they acted as if they were repelled or driven away from the poles instead of being attracted. Faraday, to resume, concluded that, as regards the action of magnetic flux, all substances could be divided into two classes; i. e., those like iron, that are attracted, and those like bismuth, that are apparently repelled. In order to examine this action of magnetism on differ- ent solid substances, Faraday cut the solid into slender needle-shaped pieces and suspended them by means of a delicate thread between the opposite poles of a powerful horseshoe magnet. When experimenting with liquid or gaseous substances, he placed the liquids or gases in small needle-shaped glass tubes similarly suspended. Now, in every ease, as soon as the magnetic flux passed between the opposite poles of the horseshoe magnet, the needle-shaped pieces of solid substances, or the small cylindrical glass tubes filled with liquids, came to rest either when extended directly across the poles with their greatest length in the direction in which the flux was passing, or with their great- est length midway between the poles, or at right angles to the direction in which the flux was passing. I think you will have no difficulty in seeing that, where PAEAMAGNETISM AND DIAMAGNETISM 187 a needle-shaped substance came to rest, as would a needle of iron, directly across the poles or in the same direction as that in which the flux was passing, there was an un- doubted magnetic attraction. The needle was pushed around by the invisible attendants of magnetism until it came to rest in such a position that the flux could enter in at its south pole and pass out at its north pole. Fara- day called substances that would come to rest in this posi- OIA MAGNETIC Fig. 53. — Paeamagnetic and Diamagnetic Substances tion "paramagnetic substances," or substances that are magnetised like iron. In the other class of substances, where the needle-shaped pieces came to rest with their greatest length midway be- tween the poles or at right angles to the direction in which the magnetic flux was passing, instead of there being an attraction there was apparently a repulsion; for, as the flux passed through the substances, instead of producing a south pole where it entered and a north pole where it came out, it seemed to produce a north pole where it en- 188 THE WONDEE BOOK OP MAGNETISM tered and a south pole where it came out ; for, the slender needle-shaped masses came to rest only when their poles had apparently been driven as far as possible from the in- ducing pole. Faraday called such substances " diamag- netic substances." I am so desirous that there shall be no doubt about your understanding the different positions in which paramag- netic and the diamagnetic substances come to rest, when placed between the poles of a powerful horseshoe magnet, that I have shown in Fig. 53 the position assumed by a paramagnetic substance, and that assumed by a diamag- netic substance. The apparatus actually employed in such experiments consists of a powerful electro-magnet with means for suspending between the poles the substances to be experimented on. Since, as I have already explained, paramagnetic sub- stances, when placed between two strong magnet poles, are attracted to the poles because of the unlike poles pro- duced in them by magnetic induction, it was believed to be necessary to suppose that diamagnetic substances when similarly placed had like poles produced in them by in- duction and would, therefore, be repelled by the inducing poles. Indeed, Faraday assumed that diamagnetic sub- stances possessed a polarity called " diamagnetic polarity " that was the reverse or opposite of ordinary or " paramag- netic polarity." One wonders how so able a man as Faraday could have been willing for a moment to acknowledge the existence of a diamagnetic polarity; for, it necessitated a belief that, instead of the ilux producing a south pole where it en- tered a substance and a north pole where it passed out of PAEAMAGNETISM AND DIAMAGNETISM 189 it, in certain substances a north polarity was produced where the flux entered, and a south polarity where it came out. It would certainly seem there could be no reason why flux should act so differently in different kinds of matter. In the following table you will find the names of some of the most important paramagnetic and diamagnetic sub- stances arranged in the order of their relative strengths : Paramagnetic. Diamagnetic, Iron Bismuth Nickel Phosphorus Cobalt Atitimony Manganese Zinc Chromium Tin Cerium Cadmium Titanium Mercury Palladium Tjead Osmium Silver Many ores of the above Copper metals Gold Oxygen gas Arsenic Liquid oxygen Water Ozone Alcohol Selenium Sulphur Hydrogen As you will see in the above table, iron, nickel, cobalt and manganese possess the most powerful paramagnetic properties, while bismuth, phosphorus, antimony, and zinc are among the most powerful diamagnetic substances. 190 THE WOKDEE BOOK OE MAGNETISM You will notice that bismuth heads the list of diamag- netic substances. This was the substance in which the phenomenon of diamagnetism was first observed, long be- fore the time of Faraday. What Faraday did was to show that all substances when placed between the poles of a magnet acted either as paramagnetic or diamagnetic sub- stances. Coming back to the matter of bismuth, I would say that in the year 1778, Brugmans of Leyden, while experi- menting with a needle of bismuth suspended between the poles of an electro-magnet, observed that the bismuth was Fig. 54. — Pabamagnetic (a) and Diamagnetic (b) Liquids apparently repelled; and that at a much later date, 1837, the French physicist, Becquerel, obtained similar results with a mass of bismuth suspended in practically the same manner. In the above table I have only mentioned the metals as diamagnetic substances. There are, of course, besides those named in the preceding table a great number of other sub- stances that are also diamagnetic, such as rock crystal or quartz, alum, some varieties of glass, sulphur, sugar and bread. So also besides the paramagnetic substances named PAEAMAGNETISM AND DIAMAGNETISM 191 in the table, there are many kinds of paper, sealing-wax, some kinds of glass, fluorspar, graphite, charcoal, etc. When, in order to determine the character of liquids, they were placed in glass tubes, in order to prevent the glass, which is sometimes paramagnetic and sometimes diamag- netic, from influencing the result, the tubes were made as thin as possible. Tests were also made with the empty tubes so as to determine the influence they would exert when filled. Sometimes, however, instead of placing the liquids in tubes, they were placed in shallow watch crystals placed Fig. 55. — ^Diamagnetic Chabactee of Candle Flame between the magnet poles as shown in Fig. 54. Under these circumstances, if the liquids were paramagnetic, such as are solutions of the salts of iron or cobalt, then on the passage of the electric current the mass instantly assumed a curious change in shape, placing itself in an elongated mass as if endeavouring to come to rest across the poles in the direction in which the flux was passing; or, as repre- sented at A, in the above flgure. If, however, the liquids were diamagnetic, the change in shape was such as to elongate the mass in the opposite direction, or so as to place the greatest amount of its mass at right angles to the direction of the magnetic flux. 193 THE WONDEE BOOK OF MAGNETISM Since most gases are invisible, when experimenting with gas it is necessary to make them visible bj^ the admixture of a small quantity of smoke or iodine vapour. It is not necessary to place gases in glass tubes since, if no air is blowing between the magnet poles, gas issuing from a small orifice placed below the pole at a point midway be- tween will rise in a thin, vertical stream. Some exceedingly curious and wonderful results can be obtained with streams of gas flowing between opposite Fio. 56. — Davy's Expeeiment magnet poles. For example, a candle flame consists of highly heated streams of gas containing red hot particles of carbon. If there is no air stirring in the room, the candle flame when permitted to burn between the magnet poles will point vertically upwards when no flux is passing, but when the magnetising circuit is closed, and the mag- netic flux, therefore, is passing between the poles, a curious change instantly occurs in the shape of the candle flame. PAEAMAGNETISM AND DIAMAGNBTISM 193 One might think that the flame would be blown from the north towards the south pole by the magnetic flux as it is blown out of the north pole. But we must not forget that the flux which comes out of the north pole of a magnet is an exceedingly curious kind of material. It is weight- less or imponderable, and, probably, consists of that very tenuous imponderable material before referred to as form- ing the luminiferous ether. As soon as the flux passes, the candle flame shows that it is diamagnetic by taking a posi- tion at right angles to the direction in which the flux is passing, or in the position shown in Fig. 55. Indeed, so violently does this change of position occur that if the mag- netic flux is strong the candle flame will be instantly extinguished. You have all seen the powerful electric arc lights that are employed for the illumination of the streets of our cities and towns. If you examine this light through a piece of smoked glass, you could see that, when the cur- rent is turned on, a luminous flame in the shape of an arc or bow is produced between two carbon rods placed near together. Now, this arc or bow consists of highly heated carbon vapour, and carbon vapour is a diamagnetic substance. It was shown by Sir Humphrey Davy that if a carbon arc is placed, as represented in Fig. 56, between powerful magnet poles, A and B, as soon as the flux passes, the voltaic arc or bow is suddenly increased greatly in length and is shot out at right angles to the direction in which the flux is passing, as shown at C. As you can see from examining this figure, the flux causes the voltaic arc to assume the appearance of a blowpipe. Indeed, an are so deflected has actually been employed for the purpose of 194 THE WOFDEE BOOK OF MAGNETISM fusing metals that only melt at exceedingly high tempera- tures. At the present time scientific men no longer believe that there is any difference between paramagnetic substances and diamagnetic substances. In other words, there is no such thing as a diamagnetic polarity, the reverse of ordi- nary polarity. It is true that such distinguished scientific men as Weber in Germany and Tyndall in England were great believers in the existence of the diamagnetic polarity, and Tyndall, who has written a large book on diamagnet- ism, apparently continued to believe in its existence up to the time of his death. It will interest you to know Earaday afterwards acknowl- edged his mistake, and agreed that there was no diamag- netic polarity. Now you may say : " Did you not tell us that diamag- netic substances in the shape of needles came to rest between the poles of powerful electro-magnets in the direc- tion in which the flux was passing, and that paramagnetic substances came to rest at right angles to this direction ? " Quite correct, and if you try to follow me I think I can explain why diamagnetic substances come to rest in a posi- tion at right angles to that in which the flux is passing and that this is by no means caused by repulsion. A feeble magnetic substance, or let us say a substance possessing weak paramagnetic properties, if suspended be- tween powerful magnet poles when surrounded by a para- magnetic liquid, acts as if it were actually diamagnetic. For example, a small glass tube filled with a weak solution of a certain salt of iron (ferric chloride), if suspended between powerful magnet poles, will come to rest in a PARAMAGNETISM AND DIAMAGNETISM 195 position at right angles to that in which the flux is passing; or, in other words, it is paramagnetic. If, however, it is surrounded by a stronger solution of the same salt, or by a solution that is more powerfully paramagnetic, it will point equatorially. Or, in other words, it will then be diamagnetic. This experiment, therefore, shows that when substances point at right angles to the direction in which the magnetic flux is passing, it does not prove they possess a diamagnetic polarity, but simply that they are less para- magnetic than the medium surrounding them. What has taken place is that, under the influence of the magnetic flux, this medium has come to rest with its greatest mass extending directly across the poles, and this has made the so-called diamagnetic substance appear to point at right angles to the direction in which the flux is passing. Since oxygen is a paramagnetic substance, or a substance magnetic after the manner of iron, when a substance is suspended in the shape of a slender needle that is much less magnetic than the oxygen of the air, the air surround- ing the needle will point directly across the poles and thus cause the needle to come to rest with its greatest length midway between the poles. But there is no repulsion. There is simply a stronger attraction of the opposite poles on the air magnet that extends between the poles of the electro-magnet. When a needle of bismuth is suspended in air between opposite magnet poles, as soon as the flux passes, the air between the poles becomes by induction a strong air mag- net. So, too, the flux which passes through the bismuth makes it a bismuth magnet. In both of these the point where the flux enters becomes a south pole, and where it 196 THE WONDEE BOOK OP MAGNETISM comes out, a north pole. But the induced polarity of the air magnet is so much stronger than that of the bismuth magnet that its poles are attracted and the two magnets are swung around so as to produce the least magnetic resistance in that part of the magnetic circuit lying between the poles. In order to do this the bismuth, which possesses an enormously greater magnetic resistance than the air, is placed with its least dimensions in the direction in which the flux is passing, or in other words, it is placed at right angles to the direction of the flux. CHAPTEE XIX THREE PECULIARITIES OP THE EARTH'S MAGNETISM We, who are living in the beginning of the twen- tieth century, are, perhaps, apt to think too highly of our wonderful success in interpreting the laws of nature, and are, possibly, too prone to hug ourselves in delight at the great things that have been accomplished. And, indeed, to a certain extent, I am not sure but that we are justified in this self-conceit ; for the achievements that may properly be called recent are certainly very wonderful. But we must not forget that all the great discoveries in physical science have by no means been made during the twentieth century, or, for the matter of that, during the nineteenth or eight- eenth centuries. Indeed, one of the most wonderful gen- eralisations in physical science was made near the close of the seventeenth century; i. e., in 1690, by Dr. Gilbert, who wrote a wonderful book on magnetism that will be referred to in the last chapter of this Wonder Book. The generali- sation to which I refer was that the entire earth acts as one huge magnet. This, indeed, was a most important discovery. Once assured that our earth is a huge magnet, many things otherwise extremely difficult of explanation become almost matters of course. Por example, the reason the compass needle points, approximately, to the earth's north and south becomes practically self-evident. It is due to the 197 198 THE WONDEE BOOK OP MAGNETISM attraction of the earth's opposite magnetic poles in the Northern and Southern Hemispheres. The natural lode- stone magnets, found in the earth, or the bars of hardened steel, that have rested for many years on its surface, have become magnetised by induction from. the earth. It is be- cause our earth acts as a huge magnet, with its magnetic poles not exactly at the ends of the earth's axis but only in their neighbourhood, that compass needles point only ap- proximately to the earth's north and south. You remember the experiments made with iron filings scattered over the surface of glass plates resting on magnet poles in order to show the direction taken by the flux as it passes out of the north pole of a magnet into its south pole. I have often thought how wonderful a picture would be seen on the earth's surface, or more correctly, in the lower layers of its atmosphere, if both magnetic flux and the oxygen of the air were visible. Now, although both these things are invisible to the bodily eye, yet one might see them by the eye of the imagination. Let me, there- fore, endeavour to show you this wonderful picture, as I would behold it, if I could get far enough from the earth to see one-half of its surface, and if I were at the same time able distinctly to see both the magnetic flux stream- ings and the almost innumerable molecules of oxygen scat- tered through the earth's atmosphere. Out of the earth's north magnetic pole in the Southern Hemisphere I see a huge stream of magnetic flux blown with such force that it passes through the lower layers of the atmosphere until it reaches the earth's south magnetic pole in the Northern Hemisphere; and, on the supposition that I could also look through the earth, I see the flux PECULIARITIES OF EAETH'S MAGNETISM 199 returning to the Southern' Hemisphere where, again being blown out of the earth's north pole, it again completes the great magnetic circuit of the earth. In this description I have limited the path of the earth's flux through its atmosphere to the lower layers. I have no doubt, however, that it passes practically through all the layers, but possibly with greater intensity in the case of those lying comparatively near the surface. As to the arrangement of these magnetic streamings, as they pass from the Southern to the Northern Hemisphere, I imagine I can see that, although for the greater part they are almost equally distributed through the air, yet they by no means move directly towards the north. Nor are they everywhere horizontal or parallel to the earth's surface. They are variously inclined to one another. More- over, in certain regions near the earth's surface, where the resistance to their passage has been decreased by the pres- ence of large quantities of magnetic oxides of iron, or fairly great masses of metallic iron or steel, they crowd together in passing through these substances as if only too pleased to find a road that ensures easy travelling on account of the decreased resistance. There are two kinds of places on the earth's surface where the flux streams are especially apt to crowd together. These are either where the huge skyscrapers or tall build- ings have been placed with their great quantities of struc- tural steel, or where the iron or steel rails of the railroad or trolley tracks have been laid. The flux streams are especially numerous where they pass through tracks that extend for great distances almost due north and south, such, for example, as the Cairo-to-Cape Eailroad in Africa, 200 THE WONDEK BOOK OF MAGNETISM that extends from Cairo on the Mediterranean Sea almost uninterruptedly to the Cape, at the extreme southern part of the continent. I regret it will be impossible fully to treat the subject of the earth's magnetism in so small a book as this. I will, therefore, take up some of the more important of its peculiarities only. By far the most important property possessed by the magnetic flux that passes continuously between the earth's poles is its directive power, by which it causes the ends of a compass needle to point towards the earth's poles. It is this power that has permitted the mariner to find his way over the pathless ocean, and an almost continuous com- merce to be carried on between all navigable ports of the world. You cannot have failed to notice how careful I have been in speaking of the pointing of the compass needle always to say that it pointed " approximately " to the earth's north and south. It would point exactly to the true north and south if the magnetic poles of the earth were situated exactly at the ends of its axis, but since they are situated at some little distance from the geographical poles, the poles of the compass needle, except in a few places, point either to the east or the west of the true north. Generally speaking, people who have not given much study to the earth's magnetism believe that the compass needle points exactly to the north. They are apt to say, therefore, when they wish to find a figure for the constancy with which a certain thing occurs, that it is as "true as the needle to the pole star-." The two magnetic poles of the earth are situated at PECULIAEITIES OF EAETH'S MAGNETISM 301 points in the Northern and Southern Hemispheres at some distance from the ends of the earth's axis. Moreover, they are not situated at equal distances from the earth's geo- graphical poles. The magnetic pole in the earth's Northern Hemisphere has been accurately located; that in the Southern Hemi- sphere has only been approximately located. Humboldt speaks as follows concerning the history of the discovery of the earth's magnetic poles: (In reading this quotation, it should be remembered that it was written during the time when the earth's magnetic north pole was regarded as being situated in the Northern Hemisphere, and its south magnetic pole as being situated in the South- ern Hemisphere.) " It is due to the bravery and scientific keenness of one single navigator that we know the exact position of these poles. Sir James Clark Eoss determined the position of the north magnetic pole during the second expedition of his uncle, Sir John Eoss, in 1829 to 1833. The south magnetic pole he determined during his own expedition to the Antarctic regions in 1839 to 1843. " The north magnetic pole is situated about latitude 70° 5' and 90° 5' west longitude; being about five degrees more distant from the north geographical pole than the south magnetic pole is from the south geographical pole. The south magnetic pole is situated about latitude 75° 5' and 151° 48' east longitude. The longitudinal distance between the poles is 109°. The North Pole is situated in the Isle of Boothia Felix, near to the part of the American Continent called North Somerset by Captain Parry. It is situated a short distance from the west coast of the isle not far from 202 THE WONDEE BOOK OP MAGNETISM Cape Adelaide, which is between King William's Sea and Victoria Strait. The south magnetic pole has not been directly reached as the North Pole has. On February 17th, 1841, the Erelus was in latitude 76° 13' south and 161° 40' east longitude. The dip was then only 88° 40', so that it was concluded that they were not more than 160 nautical miles from the south magnetic pole. A great many observations of declination made with extreme care to determine the intersection of the magnetic meridians lead to the supposition that the south magnetic pole is situated in the great Antarctic polar Continent South Victoria Land, to the west of the Albert Mountains, which include the active volcano Erebus, which is about 11,000 feet high." It is quite possible that the magnetic poles of the earth are situated at somewhat different positions than those fixed by Eoss, not only by reason of possible errors of observa- tion, but also because the positions of such poles change slightly from time to time with the changes in the earth's magnetism. This failure of the magnetic needle t'o point to the true north and south of the earth is called its " declination." You can remember this word by imagining the needle say- ing to itself, " I decline to point always to the true north and south, and, therefore, at times point to the east and west of these points." The magnetic declination is said to be east or west according to whether the needle points to the east or west of the true geographical poles of the earth. It appears that although, as we have seen, the compass needle was in use for many years, it was not until the time PECULIARITIES OP EARTH'S MAGNETISM 203 of Columbus, in 1492, that it was actually known that in many places it does not point to the true north. The reason for this of course was that until the time of Columbus no navigator had gone very far from the part of the world in which he lived. I can imagine how astonished the sailors of ' Columbus must have been when for the first time they noticed their compass needle was pointing so far from the North Star in the heavens, which, as you know, is practically always directly over the earth's north pole. The thought that the needle was losing its mysterious power of guiding them caused great alarm, especially since sailors, as a class, are exceedingly superstitious. Washington Irving gives the following description of the amazement occa- sioned by this 'fact: " On the 13th of September, 1493, he (Columbus) per- ceived about nightfall that the needle, instead of pointing to the north star, varied about one-half a point, or between 5° and 6°, to the northwest, and still more on the follow- ing morning. Struck with this circumstance, he observed it attentively for three days, and found that the variation increased as he advanced. He at first made no mention of this phenomenon, knowing how ready his people were to take alarm ; but it soon attracted the attention of the pilots, and filled them with consternation. It seemed as if the laws of nature were changing as they advanced, and that they were entering into another world, subject to unknown influences. They apprehended that the compass was about to lose its mysterious virtues ; and, without this guide, what was to become of them in a vast and trackless ocean? Columbus tasked his science and ingenuity for reasons with 204 THE WONDEE BOOK OF MAGNETISM which to allay their terrors. He told them that the direc- tion of the needle was not the polar star, but to some fixed and invisible point. The variation was caused not by any failing in the compass, but by the movement of the North Star itself, which, like the other heavenly bodies, had its changes and revolutions, and every day described a circle around the pole. The high opinion that the pilots entertained of Columbus as a profound astronomer gave weight to his theory, and their alarm subsided." There are three peculiarities of the earth's magnetism that I wish especially to impress on your minds. The first relates to the failure of the magnetic needle to point to the earth's true north and south as above described. Now, since, as we have seen, a compass needle comes to rest only when it lies with its greatest length in the direction of the earth's flux, so that the flux can enter it at its south pole and pass out at its north pole, it is evident that the earth's flux does not extend due north and south in most parts of the regions between the north and south poles, although it is passing between them. Another peculiarity of the compass needle that was first observed in 1576, by a practical London optician named Eobert Norman, was that, if free to move in a vertical as well as in a horizontal direction, the compass needle would not only come to rest when pointing to the earth's poles, but, instead of remaining in a horizontal position, was in most parts of the earth's surface inclined towards the earth. It seems that Norman, who was an excellent work- man and, therefore, properly took pride in making a good piece of work, was preparing an unusually long magnet as a compass needle. He did his best to so place the pivot PECULIAEITIBS OP BAETH'S MAGNETISM 205 employed for its support that when resting on its suspen- sion point it would assume a true horizontal position. Therefore, after adjusting and readjusting it, he at last succeeded in obtaining a needle that came to rest in an exact horizontal position. But there was a great surprise in store for Norman. Having properly magnetised the needle and placed it on Fig. 57. — Angle of Magnetic Dip its vertical support, one end seemed heavier than the other. The north pole of the needle inclined or pointed towards the earth. Thinking that some mistake had been made and that, after all, the northern half of the needle was heavier than its southern half, Norman attempted to adjust it by cutting off a small part of the southern end. This, however, did not remove the difficulty, and at last he made so many changes in the needle that I am sorry to say. 206 THE WONDEE BOOK OP MAGNETISM unlike the philosopher he was, he lost his temper and broke the needle and threw it away. Fortunately, both for Norman and physical science, he described the matter to one of his customers, an able physi- cist, who induced him to make a series of experiments. This was done and resulted in the discovery that the difficulty was not caused by any defects in the needle, but by a peculiarity in the earth's magnetism that had not before been observed. It was found that, when the needle is so suspended as to be free to move in a vertical as well as in a horizontal direction, its north pole would dip or incline towards the earth on almost all parts of its surface. This peculiarity is called the "dip" or "inclination" of the needle. A second peculiarity, therefore, of the earth's magnetism is to be seen in the fact that a compass needle, when suspended so as to be able to move in a vertical as well as in a horizontal plane, inclines or dips towards the earth. In the earth's Northern Hemisphere it is the north pole of the needle that dips or inclines, and in its Southern Hemi- sphere it is the south magnetic pole that inclines. The angle of magnetic dip or inclination is measured, as shown in Pig. 57, by the amount of its deviation from the horizontal line OB. If you are familiar with geometry, you can see that it is measured by the angle BOC. The dipping needle, represented in the preceding figure, gives only a rough measurement of the value of the dip or inclination of the compass needle at different parts of the earth's surface. When it is desired to make more careful measurements a form of dipping needle invented by Biot is employed. The general appearance of this needle PBCULIAEITIBS OF EAETH'S MAGNETISM 207 is shown in Pig. 58. In its construction, as indeed in the construction of all magnetic instruments, it is necessary that no iron or steel be employed except that of the mag- netised needle. All other parts must be made of some substance such as brass that is not readily magnetised, like the metals iron and steel. In the Biot needle the mag- netic needle, la, is suspended on knife edges in such a Fig. 58. — Biot's Dipping Needle manner as to be able to move over a vertical circle, such as shown, that is graduated in each direction from a horizontal line from 0° to 90° in a vertical position. A little thought will enable you to understand the causes of the dipping of the compass needle. There is no inclina- tion or dip on the earth's magnetic equator; that is, on a line that extends around the earth everywhere equidistant between the earth's magnetic poles in the direction of the geographical equator. The amount of inclination increases 208 THE WONDEE BOOK OF MAGNETISM from the equator to the poles, the needle pointing verti- cally, downward or with an angle of inclination or dip of exactly 90°, when the needle is brought directly over either pole. You will understand this better by an examination of Pig. 59. The circular line represents the earth, N, S its north and south poles; or, more correctly, the circle is a section of a wooden sphere that represents the earth, and N, S is a permanent magnet placed inside the sphere as Fig. 59. — Cause of Magnetic Declination. shown. Then, if a number of dipping needles, a, h, c, d, e, f, and g, be placed in different positions around the wooden sphere, their opposite poles will incline or dip towards the poles N, S, as shown. The needle d, which is on the magnetic equator, or exactly midway between the poles N", S, of course will show no dip, but will come to rest in a horizontal position, since the ends are equally attracted towards N and S respectively. The needles a and g, that are directly over the poles, will have an angle of dip of 90°, while the amount of dip in the remaining PECULIARITIES OP EARTH'S MAGNETISM 209 needles will be greater the nearer they are to the poles of the earth ; that is, the dip in the needles i and / will be greater than in the needles c and e. But there is still another peculiarity of the earth's mag- netism. If a compass needle that has come to rest with its north pole pointing approximately to the earth's north is moved out of this position for a short distance, it will, before again coming to rest, swing to-and-fro like a pendu- lum. Now if you count the number of oscillations or swings the needle makes at a certain part of the earth in a given time, say in a minute, and do the same at some other part of the earth, if these places are far enough apart, the chances are that the number of swings or oscilla- tions will not be the same. The following simple experiment, that you can easily make for yourself, will enable you to understand the cause of this peculiarity. Place a bar magnet in a horizontal position on any suitable support. Bring this compass needle within, say, half a foot of one of the poles of the bar magnet, and when it has come to rest pointing towards this pole move it out of its position of rest and carefully count the number of swings it makes in a given time, say one second. Now move the compass needle nearer to the end of the bar magnet, and again count the number of oscillations the needle makes during the same time. You will find the number of oscillations to be greater than in the former case, and, moving it still nearer without, of course, permitting it to touch, you will find that the needle will make a greater number of oscillations in a given time, say a minute, or a quarter of a minute, the nearer it is to the magnet pole. 210 THE WONDEE BOOK OP MAGNETISM Without' attempting to explain to you just why it is so, I can yet assure you that the number of oscillations the magnetic needle makes under these circumstances, like the number of oscillations a pendulum would make when under the influence of gravity, vary with the strength of the magnet pole near which the needle has been brought, just as the number of oscillations a pendulum makes in difPer- ent parts of the earth varies with the difference in the force of gravity. The strength of the earth's magnetism, therefore, varies at different parts of its surface, and since the intensity of a magnetic field at any point depends upon the amount of flux at that point, there must be a greater quantity of flux crowded in some portions of the earth's field than in others. These three peculiarities of the earth's magnetism; i. e., the magnetic declination, the magnetic inclination, and the magnetic intensity, are called the earth's "magnetic ele- ments." In order to understand the peculiarities of the earth's magnetism, it is necessary carefully to study the distribution of its flux as regards these peculiarities : that is, the direction in which the needle points, the way the needle inclines or dips, and the number of oscillations it makes before coming to rest when disturbed at different points of the earth's surface. CHAPTEK XX DISTRIBUTION OF THE EARTH'S MAGNETISM If you wish to observe the peculiarities of the earth's magnetism at any part of its surface, such as its intensity and the direction of its path as it passes out of the north magnetic pole in the Southern Hemisphere and enters the south magnetic pole in the Northern Hemisphere, it is only necessary to note the number of times the needle swings to-and-fro in a given time when it has been dis- turbed from a state of rest while pointing to the earth's magnetic pole, as well as changes in its declination or in- clination; for the number of oscillations will tell whether the flux paths are crowded together or have moved away from one another, and the declination and inclination will show the path taken by the flux. Now one of the most important discoveries ever made concerning the earth's magnetism is that its three magnetic elements do not possess the same values but vary to a greater or less extent from time to time. These variations can be divided into four distinct classes: (1) Diurnal or daily variations that occur at different hours of the day or night. (2) Annual or yearly variations that occur at different seasons of the year. (3) Secular variations extending over many hundreds of years or those that occur during great cycles or periods of time. (4) Irregular variations that accompany unusual out- 211 212 THE WONDEE BOOK OP MAGNETISM bursts of activity on the sun, or that occur at irregular intervals during the prevalence of auroras. I will now tell you about the great discovery that was made concerning the four kinds of variations to which all the magnetic elements of the earth are subject. The great physicist and traveller, Baron Alexander von Humboldt, suggested, in the year 1836, that an extended series of simultaneous magnetic observations be made over widely different parts of the earth. He persuaded the governments of the principal civilised nations to send out expeditions for the purpose of making such observations of the magnetic declination, inclination, and intensity as would enable them to obtain more accurate ideas as to these peculiarities of the. earth's magnetism. It was a great undertaking, and required the expenditure of large sums of money, such as great governments only are able to expend. In the first place it was necessary to send with each expedition a number of trained observers. Moreover, at each of these points it was necessary to build a magnetic observatory, and, since if such observatories have iron or steel anywhere in their construction the indi- cations of their magnetic instruments would be affected thereby, giving false values for the declination, inclination and intensity, no iron or steel could be used in the construc- tion of the building. Iron girders or beams were, therefore, necessarily ruled out. Even the use of iron hinges, bolts or locks for the doors, or nails or screws, were tabooed, mate- rials like brass or similar substances being used in their stead. This greatly increased the expense. Then, moreover, after an observatory had been erected, the observers were naturally obliged to remain there for a long time, since, in DISTEIBUTION OF EARTH'S MAGNETISM 313 order to get true values of the magnetic declination, inclina- tion, and intensity at any place, it is necessary to make ob- servations extending over long periods of time. This again increased the expense ; for even scientific inen must be fed and clothed, kept warm and paid for their labours. All these things cannot be done without expending considerable money. Then, too, the apparatus costs something, less, per- haps, then than it would now, for the more recent apparatus employed for such observations is somewhat complicated. Nowadays magnetic needles automatically make permanent records of their to-and-fro movements on sheets of photo- graphic paper that are accurately moved by expensive clockwork. To give you some idea of the great extent of the work as thus planned and superintended by Humboldt, I would say that the expeditions were sent to and magnetic observa- tories erected at different places within the Arctic and Ant- arctic Circles, as well as in different parts of Africa, North and South America, on different islands of the Pacific Ocean and in various parts of India, Asia, Europe and Anstralia. Extremely important results were obtained by these simultaneous observations, and much was learned concern- ing the peculiarities of the three elements of the earth's magnetism. But the most astonishing fact discovered was that every one of the four variations in the declination, the inclination, or intensity of the earth's magnetism that were observed at any one of these stations was simultaneously observed at practically all the other stations. In other words, the diurnal, annual, secular, or irregular variations occurred simultaneously over all parts of the earth, thus 214 THE WONDER BOOK OF MAGNETISM showing the wonderful connection and coincidence of the causes producing the magnetism of the earth. Of course this was only what might have been expected ; for, if the earth acts as a single huge magnet, then, neces- sarily, a disturbance either in the direction or amount of its magnetic flux at any point on its surface must aflEect the distribution and quantity of flux in all other parts of its surface. Let us now examine some of the variations in the three elements of the earth's magnetism. Pirst, as to the diurnal or daily variations. It has been observed, in certain parts of the earth, that between eight and nine o'clock in the morning the north pole of the compass needle begins to move slowly towards the west and continues this motion until about noon, when it begins moving in the' opposite direction, or towards the east. The total amount of range of this motion is small, being only about the sixth of a degree. These diurnal variations of the needle appear to have been first observed in the year 1722 by a London optician named Graham. The annual variations of the compass needle were first observed in the year 1780, by Cassini. These variations were also quite small in value. They are greatest, however, in the Northern Hemisphere in March, at the time of the vernal equinox, when the sun is shining vertically on the equator; and least, about June 20th, the time of the summer solstice, when the sun's rays fall vertically on the Tropic of Cancer. The secular variations of the needle are those in which the compass needle during greater periods of time fails to point to the earth's true north and south. DISTEIBUTION OF EAETH'S MAGNETISM 215 Of course you can understand that, since the secular rariations occur so slowly, it is necessary to compare the value of a great number of difEerent observations made during many different years. A single case will be suffi- cient to explain what is done. Observations have been made in the city of Paris since the year 1580 as to the exact value of the declination of the magnetic needle. During this year the north pole of the compass needle pointed 11° 30' east of the true north. The needle then began to move slowly towards the north, until the year 1663, when it pointed due north and south. In other words, during the year 1663 there was no magnetic declina- tion at Paris, but after this year the needle began to move towards the west, and, continuing this motion, its declina- tion during the year 1700 was 8° 10' west; during the year 1780, its declination was 19° 55' west, the needle moving further and further towards the west until the year 1814, when it reached its greatest western declination, and then began moving towards the east. The declination in 1835 was only 22° 22' west; in 1865 it was 18° 44' west, with the needle still moving nearer to the true north. The irregular variations in the compass needle are due to a variety of causes, such as the appearance of an unusual number of spots on the sun, the prevalence of auroras, and even, it is claimed by some, the occurrence of volcanic eruptions. These irregular variations are sometimes known as " magnetic storms." As we shall see in a subsequent chapter on the aurora, the presence of electric currents in the atmosphere at these times would naturally produce variations in the magnetic flux of the earth, and thus result in variations in the compass needle. 216 THE WONDER BOOK OF MAGNETISM It is an interesting fact that the presence of huge cyclonic or rotary storms in the highly heated gaseous at- mosphere of the sun, by tearing away the luminous atmo- sphere and so permitting the relatively dark body of the sun to be seen through the opening as sun spots, is invari- ably followed by magnetic storms on the earth. This has been proved beyond any doubt by Wolf by means of a care- ful comparison of the times of occurrence of the appear- ances of spots on the sun and the prevalence of unusual magnetic storms on the earth between the years 1773 and 1880. When, so as to prevent injury to the eyes by the intense light and heat, the body of the sun is observed through a telescope by the aid of properly screened glasses, the peculiar appearance the sun's atmosphere presents dur- ing the prevalence of a sun spot can be carefully studied. The portion of the sun's surface that is ordinarily visible consists of a part called the " photosphere," a word mean- ing the sphere of light. When examined through a tele- scope of small magnifying power, the sun's disc appears darker at its edge than at the centre. The rest of the sur- face, however, is not uniformly bright, but is mottled, presenting somewhat the appearance of rough drawing paper. Now, during the prevalence of a sun-spot, the pho- tosphere presents an appearance as if a great disturbance was taking place in the sun's luminous atmosphere. Ac- cording to Professor Charles Young, from whom this de- scription has been condensed, it appears probable that during this disturbance a whirling motion takes place in the photosphere, so that a hole or crater is produced through which one can look down on the relatively darker body of '"1 •J e i. ' *|i ss V'i 5 "w E-i "/; 2 02 ■3 -g t'i tA >. % ■< a E in,. o t3 " '' &4 ■s^ >* c 3 / H s fe '^ 2 ■=t •6.S " ee o ;- THE GEEAT TWIN BEOTHEES 259 same time; that is, each are exactly of the same age. But when electricity is produced from magnetism, since it is only while the strength of an electric current passing through a conductor is rapidly increasing or decreasing, so that the circular lines of flux produced around the con- ductor, cut or thread through a neighbouring conductor, that electricity is thus born in that conductor, then elec- tricity is slightly older than its twin brother magnetism, since a small fraction of a second is required for the flux to reach and pass through' the conductor in which the elec- tricity is born. But in each of the above cases, electricity and magnetism, or magnetism and electricity, are born at practically the same time, or are twin brothers. Let us now see when and how the discovery of the pro- duction of electricity by magnetism was made. You re- member that Oersted's great discovery of the production of magnetism by electricity was made towards the close of 1819. Faraday's analogous discovery of the produc- tion of electricity from magnetism was not made until the year 1831. It was long suspected that some relationship existed be- tween electricity and magnetism, but this relationship was only established by Oersted when he closed the circuit of an electric source and brought a portion of the conducting wire forming a part of this circuit over and in the imme- diate neighbourhood of a magnetic needle. In a similar way, believing that the passage of electric flux should produce an electric current somewhere or some- how, but how he did not exactly know, Faraday began a series of experiments in which he caused magnetic flux to flow in various ways in the neighbourhood of electrical 260 THE WONDER BOOK OP MAGNETISM conducting circuits. Eor a long time Faraday had no suc- cess. Indeed, if he had not known from long experience how extremely difficult it often is to be successful in a scientific investigation, he would probably have stopped experimenting in disgust long before he had reached any results. But Faraday kept on experimenting, with all his senses on the alert to note the slightest hints telling him what to do. At last a faint hint came which set him on the right road. I am sure you will be interested in knowing just what this hint was, as well as what Faraday was doing when he received it. Faraday might have used a perma- nent magnet for the production of the flux from which he would cause an electric current to be born. In point of fact, however, at this particular time he was employing an electric current to produce magnetic flux Just as Oer- sted had done, and he caused this electric flux to pass through another circuit and cause electric currents to be born or set up in it. Suppose now, for the sake of convenience, we call the first of these circuits the primary circuit — i. e., that through which the electric current is flowing, and thereby producing magnetic flux — and that in which the electric current is produced by the magnetic flux of the primary circuit, the secondary circuit. Faraday discovered that, no matter what position the wires of the primary and sec- ondary circuits occupied as regards each other, he was never able, by causing an electric current to continue to flow through the primary circuit, to produce even the smallest current of electricity in the secondary circuit. On one happy day, however, the 39th of August, 1831, THE GEEAT TWIX BEOTHEES UGl while his whole attention was directed to these two circuits, he noticed that, during the very short time existing between the moment he made or completed the primary circuit by closing the poles of a voltaic cell and the time required for the current to reach its full strength in this circuit, a slight movement occurred in the needle of a galvanometer he had placed in the secondary circuit to aid him in de- tecting the nresence of a current flowing through it. The needle of the galvanometer moved from its position of rest when such a current was set up. He also noticed at the moment the primary circuit was opened, during the time the electricity from the voltaic battery was disappearing or dying out in the primary circuit, that a current of electricity was produced in the secondary circuit. But the current so produced was now evidently flowing in the op- posite direction, for the galvanometer needle in the sec- ondary circuit now moved in the opposite direction to what it did at the moment the current began to flow in the primary circuit. Pondering no little as to the meaning of these phenom- ena, and remembering that while the strength of an elec- tric current flowing through a conductor is increasing, the circular lines of magnetic force surrounding it increase in number and expand or move outward, and that, when the strength of such current is decreasing, the circular lines of flux contract or move inward, he saw that under these circumstances the flux cuts the conductors of the secondary circuit in one direction while increasing, and in the oppo- site direction while decreasing. It, therefore, occurred to him that the condition necessary to cause magnetic flux to produce electricity in the secondary circuit was to cause 262 THE WONDEE BOOK OP MAGNETISM it to pass across or cut its conductors. In this way another of nature's great secrets was discovered. An entirely new means for producing electricity was now made possible. In order to permit an electric current to flow through a circuit, the circuit in which it is induced must be closed ; that is, a continuous path must be provided for the induced current. In this sense, therefore, the secondary circuit can be regarded as consisting of a single loop; so, instead of saying Faraday discovered that, in order for the flux of the primary circuit to induce electricity in the sec- FiG. 6G. — Electro-Dynamic Induction ondary circuit, it must move so as to cut or cross the con- ductors of such circuit, we can say that the flux must be caused to thread or pass through the conducting loop of which such circuit may be regarded as consisting. The production of electricity in this manner by causing magnetic flux to cut or pass through conducting circuits is called electro-dynamic induction. There are a number of different ways in which electric currents can be pro- duced by electro-dynamic induction. Suppose, as is represented in Fig. 66, that a hollow cylindrical spool, B, is wrapped with two insulated wires THE GEEAT TWIN BEOTHEES 363 placed side by side and constituting the primary and sec- ondary circuits before referred to. Suppose, moreover, that the terminals or ends of the primary circuit or wire be connected with the terminals of a voltaic cell, P, through the two mercury cups, g and g', and that the terminals of the secondary circuit be connected as shown with the coils of the galvanometer, G. Now, while a constant electric current is flowing through the primary circuit from the voltaic cell P, the gal- vanometer needle remains at rest, thus showing the absence of any current in the secondary circuit. If, however, one of the terminals of the voltaic cell P be raised from the mercury cup g, thus breaking the circuit and causing the current strength in the primary circuit to suddenly fall from its full strength to nothing or zero, the needle of the galvanometer will be deflected in a certain direction, and the amount of this deflection will be greater the more sud- denly the circuit has been opened. This deflection of the galvanometer needle, however, is only momentary. As soon as the current has ceased to flow in the primary circuit the needle comes to rest, after swinging to-and-fro for a few moments, thus showing that the induced current is no longer flowing through the sec- ondary circuit. If, however, the current through the primary be again made to flow by inserting the wire of the voltaic cell in the mercury cup at g, the galvanometer needle is again de- flected but now in the opposite direction. I think you can now understand what has taken place in the appa:ratus represented in the above figure either on the opening or the closing of the primary circuit. 364 THE WONDER BOOK OF MAGNETISM When the circuit through the primary was suddenly opened the magnetic flux that had been set up around the coils of wire in the primary circuit and had, therefore, spread through those coils, suddenly dying out, contracted and, moving closer to the primary wire, cut through the coils of the secondary, or threaded through the hollow helix in a certain direction, and in so doing, and only while so doing, induced an electric current in the secondary coils flowing in the direction indicated by the deflection of the needle of the galvanometer. In a similar way, when the circuit of the primary was again closed by dipping the wire in the mercury cup g, the magnetic flux around the primary coils increasing or expanding again cut through the coils of the secondary or threaded through the hollow helix in the opposite direction, and while, and only while, the current was increasing from nothing up to its full strength, or while the circular flux around the primary coils was increasing was the current produced. The introduction of a soft iron core inside the hollow helix greatly increases the amount of magnetic flux passing and therefore greatly increases the strength of the current induced in the secondary circuit. The preceding experiments show: (1) That a current is induced in a secondary circuit either at the moment of the breaking of the primary cir- cuit, or at the moment of closing that circuit. (3) That the direction of the current induced in the secondary circuit on breaking the circuit in the primary is opposite to that induced on making that circuit. (3) That the currents induced by the secondary circuit continue for very short periods of time; i. e., only during THE GEEAT TWIN" BEOTHEES 265 the time it takes the current to entirely cease on the open- ing of the circuit, or to reach its full strength on the closing of the circuit. In order to produce currents by electro-dynamic induc- tion, that is by Faraday's invention, the primary circuit is often opened and closed thousands of times in a minute. Currents so produced are called alternating currents be- cause they flow in opposite or alternate directions through the circuits in which they are set up. When it is desired to change alternating electric currents into direct currents, or those that flow continually in the same direction, de- vices called commutators are employed. These will be described in " The Wonder Book of Electricity." But it is not necessary to employ an electric circuit in order to produce the inducing magnetic flux. As we have already seen, flux is continually flowing out of the north pole of a magnet and constantly entering its south pole, so that if a magnet is moved past a conducting circuit, or a conducting circuit is moved past a magnet, there will be produced electric currents in that circuit. If, for example, a bar magnet be thrus't into a hollow coil, the ends of which are connected with a galvanometer, electric currents as shown by the deflection of the gal- vanometer needle will be set up in the coil, in one direction when the magnet is thrust into the coil, and in the oppo- site direction when it is drawn out of it. It can, moreover, be shown that the direction of these currents is what would have been produced by magnetic flux flowing in the direc- tion in which, according to Ampere's theory of magnet- ism, it must flow in order to produce such a magnet pole. It would seem, therefore, that we have not yet exhausted 266 THE WONDER BOOK OP MAGNETISM all the wonderful magic that exists in your magnetised knife-blade ; for, if you thrust an open blade into any hol- low coil, it will produce electricity, and when you draw it out of the coil it will also produce electricity, but now in the opposite direction from that which resulted when it was thrust into the coil, and this you can. continue to do without the knife losing any of its magnetism. The great importance of Faraday's discovery of the production of electricity by magnetism will be better un- derstood when I tell you it was this discovery that rendered possible the invention of the dynamo-electric machine, thus making it possible to produce those enormous currents of electricity that are employed every day in different parts of the world for lighting streets and houses at nights, for the driving of machinery, for the moving of trolley cars, or for the manufacture of the many products that are now produced by electric furnaces. It also led to the invention of the electric motor. CHAPTER XXV MAGNETISM AND LIGHT The discovery by Faraday of the relations between mag- netism and electricity, when taken in connection with the discovery by Oersted of the relations between electricity and magnetism, practically brought these great branches of physics under a single head. It now remains to tell you how another discovery by Faraday, when taken in connection with the studies of two English physicists. Clerk Maxwell and Lord Kelvin, resulted in showing that such close relations exist between electricity, magnetism and light as to result in the creation of a new branch of physics called electro-optics, or more correctly, electro- magneto-optics. Since these phenomena are connected with the flow of magnetism, as well as with the flow of electricity, they will be briefly discussed in this Wonder Book. Faraday evidently gave considerable thought to Oersted's discoveries, as I shall prove to you. Before doing this, however, I must tell you that there are two entirely differ- ent conditions in which free electricity may exist; i. e., as an electric charge and as an electric current. In order to be able to produce magnetism, electricity must be in mo- tion. An electric charge, by which is meant free electricity in a state of rest, on the surface of an insulated conductor, is unable to produce magnetism. It is only when the electricity is moving or flowing — that is, when in the con- 267 368 THE WONDER BOOK OF MAGNETISM dition of an electric current — ^that it is able to produce, as first observed by Oersted, the circular lines of flux in planes at right angles to the plane of the conductor. Now, Faraday, who understood this thoroughly, argued that if a conductor possessing an electric charge were given a sulficiently rapid motion, it should acquire the properties Fig. 67. — Luminous Brush of an electric current, and would, therefore, be able to produce magnetism. It does not seem that Faraday ever made such an ex- periment, probably owing to want of time and the proper means for giving the charged conductor a sufficiently rapid motion. This, however, was afterwards done in 1876, by Eowland, an American physicist, who showed that a charged disc when rapidly rotated acquires the power of deflecting a magnetic needle as would a feeble electric current. But there are much better ways of proving that a rapidly MAGNETISM AND LIGHT 269 moving charge acquires the properties of an electric cur- rent : ways that are both simpler and far more convincing. There are numerous instances where streams of electrified particles of matter thrown off from electrified surfaces at very high velocities are capable of acting like electric currents in being deflected by magnetism. As this is an important matter, and as wonderful effects are produced by such streams, I will describe some of the most inter- esting of them. When a powerful frictional electric machine is operated in cold dry air, with its rubbers in good order, there can be seen in a darkened room faintly luminous brushes on the projecting portions of its conductors. If a large con- ducting plate is held before a pointed conductor connected with the positive conductor of the machine, and held at such a distance as will prevent the discharge from passing as a single spark, a brush, such as represented in Pig. 67, will be seen. This discharge consists of a short luminous stalk or stem with a great number of rays diverging, as represented, from the stalk, so as to give it the appearance of a luminous brush. The discharge is therefore known as the brush discharge. Wheatstone has shown that this discharge does not consist of a continuous discharge, as it appears, but of a great number of partial discharges that follow one another with great rapidity, thus producing a faint musical tone. What especially interests us here in this form of electric brush discharge, is that it consists of streams of molecules of air together with exceedingly small particles of metal that have been torn from the conductor by the discharge. All these molecules and particles possess charges and move 2r0 THE WONDEE BOOK OF MAGNETISM with great rapidity as they are thrown off from the con- ductor. They should, therefore, be capable of acting as an actual electric current. In point of fact, such streams are deflected by means of magnetic flux just as movable conductors carrying electric currents would be deflectetd. Suppose, for example, a soft iron rod. A, covered with an insulating ma- terial, Fig. 68, is sealed air-tight in a glass vessel containing a high vacuum. A metallic ring B is placed as showJi so as to surround the iron rod at its lower edge without touching it. Now if the terminals of an electric machine or a Euhmkorff coil are connected at the terminals of two platinum wires that are hermetically fused into the glass of the tube, a luminous sheaf of light will be seen pouring between the top of the iron rod A and the ring surrounding the iron. If, while this light is passing, the soft iron rod is strongly magnetised, the luminous sheaf will slowly rotate around the rod in a direction that de- pends on the direction in which the magnetic flux is pass- ing in the electro-magnet'. While speaking in a previous chapter of diamagnetism, reference was made to Davy's experiment as to the action of magnetic flux on a carbon voltaic arc placed directly between the poles of a powerful electro-magnet. When no flux was passing the arc occupied an approximately vertical position between the upper and lower carbon rods. As Fig. 68 — Deflection of Chakges by Magnetic Flux MAGNETISM AND LIGHT 371 soon, however, as the flux was passed by sending an electric current through the magnetising coils of an electro-magnet, the flux caused the flame of the arc to be deflected in a surprising manner; for, instead of continuing as a vertical flame, it was at once driven out and caused to place itself at right angles to the direction of the magnetic flux, as would any diamagnetic substance. There were, therefore, in this experiment, two different forces at work. Carbon vapour is diamagnetic and was, therefore, placed in a posi- tion at right angles to the direction of the flux. But the charged, vapourised molecules of carbon contained powerful electric charges. Indeed, the entire arc was a powerful movable electric current, and was, therefore, moved by the magnetic flux, and, being moved, placed itself in a position in which the flux could pass through it with the least magnetic resistance; that is, in a position at right angles to the path of the flux. When an electric discharge passes through a rarefied gas, so as to produce the well known luminous discharge, this discharge consists of molecules of the residual gases, being thrown off with great velocity as they pass between the poles. Now since these streams consist of electrified molecules of the residual gas and the still smaller metallic particles torn off from the metallic electrodes, and since, moreover, they move with great velocities, they possess the properties of an electric current and are, therefore, displaced or moved by the magnetic flux. In fact, in all cases where light is produced in the residual atmospheres of a high vacuum by means of electricity, the paths of the luminous streams can be changed in curious ways by causing magnetic flux to pass through them. 273 THE WONDEE BOOK OF MAGNETISM This fact makes it comparatively easy, in the case of certain discharges ; e. g. X-rays, or Becquerel rays, that will be referred to in a subsequent chapter of this book, to tell whether they consist of streams of electrified particles, or are merely pulses or vibrations of the ether. If they consist of electrified particles they must necessarily be deflected by Faraday's Magneto-Optic Appaeatus the approach of a magnet; whereas, if they consist of pulses of the universal ether, they are not necessarily so de- flected. Many strange effects are produced on the paths of lumin- ous discharges in vacuum tubes by the passage of magnetic flux. This subject will be discussed at length in " The Wonder Book of Electricity." There are, however, other instances of the effects pro- duced on light by the passage of magnetic flux that we will discuss here. One of the most interesting of these was first MAGNETISM AND LIGHT 273 observed in 1845, by Faraday, who noticed that a beam of light, when in the condition known as polarised light, in which the vibrations of the ether take place in a single plane only, has, under certain conditions, its plane of polar- isation shifted by the influence of magnetism. The apparatus employed by Faraday for the purpose of showing this influence of magnetic flux on polarised light consisted of the powerful electro-magnet MN, of the shape and construction represented in Fig. 69. The soft iron cores of the electro-magnet were hollow, so as to permit the light to pass directly through them. A table for the support of a plate of peculiar transparent glass, 0, was so placed that when the electric current was caused to pass through the electro-magnet, the magnetic flux was passed directly through the glass plate. A full explanation of just what took place here would necessitate a knowledge on your part of how polarised light differs from ordinary light, and this is so difficult that I think it unwise to attempt an explanation of it. But it is suffieient to say that, when light is polarised, the vi- brations in the ether causing the light take place in a single plane, while in ordinary light, the plane in which the- vibrations occur is constantly changing. For this reason, polarised light possesses many properties that differ in a marked manner from ordinary light. For example, while a beam of ordinary light can pass through transpar- ent substances in any direction, a beam of polarised light can pass through some of these substances in certain direc- tions only. Now in Faraday's experiment, the plate of glass was placed at 0, so that not only the light from the candle 374 THE WOKDEE BOOK OP MAGNETISM but also the flux from the electro-magnet MIST could pass through the glass. I must confess I am somewhat in a quandary as how best to explain to you what took place. There would be no diflBculty whatever if you were familiar with the phenome- non of polarised light, which I am quite sure but few of you are. I must, therefore, content myself with saying that the hollow cores of the legs of the electro-magnet MK were not open at a and h, but were closed by optical devices that possess the power not only of permitting com- mon light to pass through them, but also of polarising such light. If a candle ilame were placed in front of i, its light could readily pass through 6, and, provided no flux were passing between the poles of the electro-magnet, could also pass through the plate of heavy glass at 0. More- over, if the optical device at a were placed in a certain position the light could also pass through it, so that an eye at a could distinctly see the candle at &. Now note what a curious effect was produced when mag- netic flux was also permitted to pass through the plate of glass. The magnetic flux is invisible and therefore trans- parent. The plate of glass was also transparent as long as the magnetic flux was not passing. As soon, however, as it flows through the plate of glass, although neither you nor I could see any difference in the appearance of the glass, yet it has suddenly become opaque, for, to the eye placed at a, the candle flame disappears as completely as if it had been extinguished, or as if a sheet of black rubber had been placed in the gap between M and N. Here then is a re- markable instance in which magnetism has profoundly affected polarised light. MAGNETISM AND LIGHT 275 Wittout going into a detailed explanation, it is enough to say that what has actually happened is that the flux, while passing through the plate of transparent glass, has altered its optical structure. When the light from the candle flame passed through the device at b, it was polar- ised. An optical change in the glass, caused by the flux passing through it, has been such as to enable the glass to twist, turn, or rotate the plane in which the light has been polarised by b, so that when it reaches the optical device at a, it can no longer pass through a, unless it is turned to such an extent as to compensate or make up for the amount of the turning that has been caused by the plate of glass. In 1877, this curious phenomenon was observed by Dr. Kerr of Glasgow: that a ray of polarised light when reflected or flung off from the polished surfaces of a mag- net is rotated by the powerful magnetic whirls existing at the poles of a magnet. Now this is practically the same phenomenon that occurs in the light passing through a transparent substance ; for, although a thick mass of iron appears to be absolutely opaque, yet a very thin slice of the iron will permit light to pass through it. Now the light that is reflected from the polished surface of a magnet actually penetrates the iron a very short distance before it is flung off, and although this distance is small, yet ow- ing to the great density of the flux within the mass of iron of a powerful magnet, is nevertheless sufficient to twist or rotate the plane in which the light has been polarised. Let me now give you a brief statement of the manner in which a theory known as the electro-magnetic theory of light was framed. It is interesting to note that this 276 THE WONDER BOOK OE MAGNETISM theory had its origin in certain mathematical investiga- tions, of great perplexity, that were begun in the year 1864, by a young, but exceedingly talented English mathemati- cian. Clerk Maxwell. By means of these investigations. Maxwell, from an entirely theoretical standpoint, declared there could be no doubt that electro-magnetic waves could be set up in the universal ether, either by the rapid oscilla- tions of an electric discharge, or by the rapid changes in the intensity of magnetism passing through a magnetic circuit, and that such waves would be found to possess all the properties of light. Maxwell died while quite young, and was, therefore, unable to complete the details of his theory. His assumed electro-magnetic waves remained as theoretical waves only until Professor Hertz, of the University of Bonn, by means of investigations begun in the year 1883, was able before the close of the year 1888 not only actually to set up such waves, by spark discharges passing through an air gap between two polished metallic globes, but also to prove by a beautiful series of experiments that they possessed all the properties of ordinary light. He showed that they could be reflected, refracted, transmitted, polar- ised, etc., like ordinary light. For example, he showed that when the electro-magnetic waves produced in space by the discharge either of a EuhmkorfE coil or a Leyden jar, between two polished metallic balls separated by a small air space, were permitted to fall on various sub- stances possessing reflecting powers, they would, like ordi- nary rays of light, be flung off or reflected from the sur- faces at an angle equal and opposite to that of the angle at which they fell on the surface. He showed that when MAGNETISM AND LIGHT 377 the waves fell on substances that were transparent to them they underwent a change in direction at the bounding sur- face between the medium through which they had been passing and the new medium on which they had fallen, and they would turn them out of their original direction in accordance with what is known as refraction. He, there- fore, constructed lenses by means of which he could bring these rays to a focus just as I do not doubt you have often done with rays of sunlight by means of a burning glass. B Fig. 70. — HIeetz's Electric Eesonatob He also constructed large prisms and so turned the rays out of their course that, since the rays consisted of a num- ber of different wave lengths, they were differently re- fracted. Each separate ray emerged from the prism in a separate path, thus producing a spectrum of electro-mag- netic light. Hertz got the best results by the use of sparks produced by the discharges of Leyden jars, and found that waves so produced were more satisfactory than those obtained by the use of the Euhmkorff coils. This was, probablj^, because the oscillations produced by the discharge of a 378 THE WONDEE BOOK OP MAGNETISM Leyden jar change their direction much more rapidly and, therefore, produce shorter wave lengths than those pro- duced by the discharge of the Euhmkorff coil. It will be impossible here to do more than give a brief explanation of the means employed by Hertz to make these waves, which he called electro-magnetic waves, visible to the eye; for, although as has been proved, they possess all the properties of ordinary light, yet they are too long, or fol- low one another too slowly, to permit them to affect the human eye as ordinary light. Hertz was, therefore, obliged to provide a device, sometimes called the electro-magnetic eye, for detecting their presence. This device, as shown in Eig. 70, consists of an open circuited conductor A, gener- ally rectangular in shape, as shown, with an air gap left at the middle, B, of one of its sides. The ends of the cir- cuit at this gap were provided with small polished brass balls. The length of the open rectangular wire is such that the electro-magnetic waves passing through it were able to flow once through the circuit in exactly the same time as would the electro-magnetic waves passing through space in its neighbourhood. In other words, if this cir- cuit, generally called by Hertz an electric resonator, and also known often as the electro-magnetic eye, is of such dimensions that it was tuned or in sympathy with the electro-magnetic waves in its neighbourhood, sparks would be seen between the polished balls at B. But if the waves in the neighbourhood were not in sympathy or accord with the resonator no sparks would be seen. There is a simple experiment with sound waves that will probably help you in understanding what is meant by sym- pathetic vibrations. You can try this experiment with a MAGNETISM AND LIGHT 379 piano. Raise the lid at the top of the piano, so as to see the strings; place a foot on the loud pedal, bring your mouth near the opening, and, in a strong tone, sing any- single note into the piano and then listen. You will hear this note thrown back by the piano. Now sing another note in the same way. This note will also be flung back ; and so, too, if you sound as many differ- ent notes as you may be able to sing. The explanation is simple when you once know it. There are a great num- ber of strings in a piano, each of which is capable of pro- ducing a single definite note. One note differs from an- other because one string differs from another, principally because one string is of different length or is stretched more or less tightly than another; or because it consists either of a simple wire, or of a wire wrapped with another wire. It swings to-and-fro, or makes what is called a vibra- tion, with a rapidity varying according to the above named circumstances, and this results in the same string always producing the same tone. The placing of the foot on the loud pedal simply removes a damper from the strings, thus permitting all of them to vibrate freely. The note you sing into the piano has a definite pitch because something in the vocal organs of your throat, corresponding to a piano string, has been purposely stretched by you when you have made up your mind you are going to sing a certain note. Since sound waves consist of to-and-fro motions of the air, when the waves fall on the strings of the piano they give them all a slight push or to-and-fro movements. Very shortly afterwards, however, the air particles again give the string another push and- again move away from 380 THE WONDBK BOOK OF MAGNETISM them, and this continues a number of times, dependent on the pitch of the note. In some cases, for example, they move to-and-fro at the rate of 256 times a second. Now consider carefully what will happen in the case of the particular string that is able to move to-and-fro exactly 256 times a second, or, in other words, of that string which is exactly in tune or in sympathy with the note you sing into the piano. The waves or to-and-fro motions of the air that' produce this note strike the string and give it a little push or shove away from them. The string moves a short distance in this direction and then moves back towards the approaching sound waves. Since the rapidity of this to-and-fro motion is exactly the same as that of the sound waves you sang into the piano, at the moment the string has swung as far in this direction as it can and is just beginning to move back again, it gets another slight push from the sound waves, and this continues for 256 times in every second. By the end of a second, therefore, although each sound wave may have given the string a slight push, yet, having received 256 of these pushes, it will have acquired a perceptible motion. Not so, however, with the other strings, whose rates of to-and-fro motions are not exactly 256 per second. Al- though at the first these strings are pushed away by sound waves practically as much as the particular string, still, moving as they are, to-and-fro at a different rate, they will not receive these faint pushes or impulses in the most favourable directions; for sometimes they are pushed away from the sound waves when they are moving in the opposite direction towards them, so that this push or im- pulse instead of adding its motion to the moving string MAGNETISM AND LIGHT 281 actually takes away some of the motion it already pos- sesses. One string, and one string only, will be set into motion sufficiently great to affect the ear, and that is the string whose rate of vibration is exactly that of the note sung into the piano. In a similar way the electro-magnetic waves that may be passing through space in the neighbourhood of an electro- magnetic resonator can set up sympathetic vibrations only in such a resonator that is tuned exactly to their rate of motion. The principle of a tuned resonator is the principle on which the receiver of a wireless telegraph operates. In wireless telegraphy electro-magnetic waves are produced in various ways, as by the discharge of the Leyden jars or in- duction coils from the top of a mast reaching some distance above the surface of the earth. These waves move outward in all directions, with a velocity equal to that of light. Whenever they pass in the neighbourhood of a tuned cir- cuit they will set up surgings in that circuit that are caused in various ways to affect delicate instruments, so that if the waves follow one another in a definite order, as for example in the order of the characters of the Morse tele- graphic alphabet, despatches can be sent across the Atlantic Ocean from America to Europe, or even further. I will tell you no more about wireless telegraphy here since this will be fully discussed in " The Wonder Book of Elec- tricity." I have already told you not to suppose that electro- magnetic waves affect all substances just as ordinary light waves do. It is true that substances behave towards elec- tro-magnetic waves as light does, so as to readily pass 383 THE WONDER BOOK OF MAGNETISM through them ; or, in other words, the siibstances are trans- parent to the waves. But on account of the greater length of the electro-magnetic waves, they can readily pass through substances that are absolutely opaque to ordinary light. For example, in some of his experiments, as I have already mentioned. Hertz made lenses and prisms of sub- stances transparent to electro-magnetic waves. I am sure you will be surprised when I tell you that some of these lenses and prisms were made of pitch, which, as you know, is quite black and absolutely opaque to ordinary light. Nevertheless, the pitch lenses or prisms were as trans- parent to the electro-magnetic waves, if, indeed, not more so, as pure glass or pure water is to light. One of the most remarkable facts that has been discov- ered respecting electro-magnetic waves, is the fact that they travel through free space with a velocity that is exactly the same as that of ordinary light. As I have explained, it is by electro-magnetic waves that wireless telegraphic despatches can be sent from station to station, although at distances of many thousands of miles. Professor Henry, already referred to for his inves- tigation in electro-magnets, came very near discovering this kind of telegraphy. Speaking of some experiments he was making with the discharges of Leyden Jars, Henry said : " In extending the researches relative to this part of the investigations, a remarkable result was obtained in regard to the distance at which induction effects are pro- duced by a very small quantity of electricity; a single spark from the prime conductor of a machine, of about an inch long, thrown on to the end of a circuit of wire in an MAGNETISM AND LIGHT 383 upper room, produced an induction suiSciently powerful to magnetise needles in a parallel circuit of iron placed in the cellar beneath, at a perpendicular distance of 30 feet, with two floors and ceilings, each 14 inches thick, inter- vening. The author is disposed to adopt the hypothesis of an electrical plenum " (in other words, of an ether), " and from the foregoing experiment it would appear that a single spark is sufficient to disturb perceptibly the elec- tricity of space throughout at least a cube of 400,000 feet of capacity ; and when it is considered that the magnetism of the needle is the result of the difference of two actions, it may be further inferred that the diffusion of motion in this case is almost comparable with that of a spark from a flint and steel in the case of light." I have endeavoured in this way to show you the manner in which the magnificent theory of light has been built up by the mathematical researches of Clerk Maxwell. It cer- tainly shows that this young physicist possessed mental power of a very unusual character. Listen to the splendid eulogy pronounced by Sir Oliver Lodge at a lecture he gave in England a year after Maxwell's death : " On November 5 last year there died at Cambridge a man in the full vigour of his faculties — such faculties as do not appear many times in a century — ^whose chief work has been the establishment of this very fact, the dis- covery of the link connecting light and electricity ; and the proof — for I believe it amounts to a proof — that they are different manifestations of one and the same class of phe- nomena : that light is, in fact, an electro-magnetic disturb- ance. The premature death of James Clerk Maxwell is a loss to science which appears at present utterly irreparable. 284 THE WONDEE BOOK OF MAGNETISM for he was engaged in researches that no other man can hope as yet adequately to grasp and follow out; but for- tunately death did not occur till he had published his book on " Electricity and Magnetism," one of those immortal productions which exalt one's idea of the mind of man, and which has been mentioned by competent critics in the same breath as the Principia itself. " But it is not perfect like the Principia; much of it is rough hewn, and requires to be thoroughly worked out. It contains numerous misprints and errata, and part of the second volume is so difficult as to be almost nnintelli- gible. Some, in fact, consist of notes written for private use, and not prepared for publication. It seems next to impossible now to mature a work silently for twenty or thirty years, as was done by Newton two and a half cen- turies ago. But a second edition was preparing, and much might have been improved in form if life had been spared to the illustrious author. . . . " The main proof of the electro-magnetic theory of light is this. The rate at which light travels has been meas- ured many times, and is pretty well known. The rate at which an electro-magnetic wave disturbance would travel, if such could be generated, can also be determined by calculation from electrical measurements. The two veloci- ties agree exactly. This is the great physical constant known as the ratio ' v,' which so many physicists have been measuring, and are likely to be measuring for some time to come." CHAPTER XXVI HAVE MAGNETS HEALING POWERS? I BELIEVE it has always been found that charlatans and quacks, and, indeed, at times, fairly reputable physicians, are apt to claim that every mysterious or not yet thor- oughly understood force possesses wonderful curative powers. It is scarcely necessary to tell you that electricity is especially a force that has again and again been thus claimed as capable of curing all the ills ilesh is heir to. Now it must be confessed that in electricity we find a force that is capable of doing much to cure diseased conditions of the human body, as well as to maintain and ensure its health and vigour. In the hands of skilled physicians, electricity can work wonders, but in the hands of quacks and charla- tans it is perhaps more apt to do harm than good. 'Not, in such cases, do certificates of wonderful cures count for much, since the patients who are killed give no certificates. It is only the people who are cured who testify to its curative powers, and these are probably far from being in the majority. Wonderful curative powers have also been claimed for magnetism. I am not at all surprised at this, since the mystery surrounding this force has naturally led the igno- rant to claim all kinds of impossible things for it. Indeed, there has arisen, although, I am glad to say, to but a slight extent, a variety of medical treatment known as 385 286 THE WONDER BOOK OP MAGNETISM Magneto-Therapy. I shall show you in this chapter that magnetism is incapable of producing any effects whatever on the health of the human body. It has for many years been believed by some that mag- netism possesses the power of curing human diseases. I remember some time ago reading in an old English patent specification a description of a device for ensuring a prolonged healthy life. This so-called invention consisted in a bed that as far as I could see differed from other beds in no respect save that it was placed with its head and foot-board in a due north and south position, the inventor gravely informing all who used the bed that in order to secure its advantages they must be careful to sleep with the head to the north and the feet to the south. Just what idea the inventor had, if, indeed, he had any, I do not know. Probably he had a vague notion that there was some wonderful potency about the north pole of a magnet, and, therefore, advised all who used his bed to sleep with their heads to the north and their feet to the south. Since, however, neither the inventor nor the users of the patent bed knew that the north magnetic pole of the earth is situated in its Southern Hemisphere, those who obeyed his directions had been sleeping all this time with their feet and not their heads to the north. They, therefore, had been happily ignorant of the fact that the invention was working exactly opposite to the way in which it was supposed to act. Happily, however, no harm was done ; for we are certain that magnets possess no power whatever over the human body. I also remember reading about another patented bed that also depended for its operation on the passage of mag- HAVE MAGNETS HEALING POWEES? 387 netic flux throtigh the body. Here, evidently with a view of preventing the magnetism escaping from the bed, the inventor had rested the bed posts on heavy glass balls that were to act as insulators. Now, since, as we have seen, magnetic flux can pass through glass almost as well as it can through air, the absurdity of such an invention is evident. Still another inventor went even further in this ridicu- lous idea; for he wound huge spirals of insulated wire around his bed in such a manner that the sleeping person would be inside the helix. He then insured the passage of flux through the sleeper by causing electric currents to flow through the helix as long as the person was in the bed. Now if it could be shown that the passage of magnetic flux through the body is absolutely able to produce any beneficial effect whatever on its condition of health there might have been some sense in this invention. But since, as I will show you beyond the slightest possibility of doubt, magnetic flux produces no effect whatever on the health, its absurdity becomes self-evident. Many years ago, Mr. Thomas Edison made the follow- ing interesting experiment in his laboratory. Wishing to see what, if any, influence is produced by the passage of strong magnetic flux through the brain, he kept a boy for a long time inside a huge electro-magnet with his head placed between the poles, so that the flux passed directly through his brain. If now a magnet is capable of produc- ing any effect whatever on the body, it should certainly have done it in this case. But, as far as could be seen, no effect whatever was felt by the boy. When asked privately how he felt he replied confidentially : " The experiment is 288 THE WONDER BOOK OF MAGNETISM bully. I am all right in the magnet. I like to be here, for I do not have to work while the experiment is going on and I can take a nap occasionally. But don't tell Mr. Edison. I hope he will keep me here for a long time." You will remember that, while telling of the huge gun electro-magnet that had been constructed at the United States torpedo station at "Willets Point, Long Island Sound, I described an experiment in which magnetic flux was passed through the body of a man from the back to the front. This flux was of a strength sufficient to rigidly hold heavy iron spikes both against the breast and forehead. Now since in these experiments no ill results whatever were produced in the man's health, it can be positively asserted that magnetic flux possesses absolutely no effects on the human body, and that Magneto-Therapy has no foundation in fact, and must, therefore, be regarded as a delusion. Many of you have probably heard of the magnetised iron ring that is often worn on one of the fingers of the hand as a cure for rheumatism. It certainly seems im- possible that such a ring can possess any curative poVers. In the first place, the magnetic flux the ring produces is ridiculously small. In the second place, even if it were large, it could hardly affect the body. It is indeed strange that such an unlikely portion of the body as one of its flngers should have been selected as the preferable place for the application of magnetism as a curative agent. Even when applied in the shape of a magnetic belt worn around the body no effect can be produced, since the strength of the magnetism so produced is also ridiculously small. HAVE MAGNETS HEALIFG POWEES? 289 The ancients entertained ideas of the curative powers of magnets that were equally ridiculous. Gilbert in his great book, " De Magnete, or the Lodestone," says that Dioscorides believed that if a piece of lodestone is finely ground, mixed with water, and swallowed in doses of about half a grain, certain disordered conditions of the blood will be greatly aided. He acknowledges, however, that, according to others, it is exceedingly dangerous to take magnetic medicine of this character even in small doses, since it may produce a melancholic condition of the mind and even death. The discordant views concerning the effects of lodestone as a medicine are reconciled by others by the fact that there are many different kinds of lode- stones, some of which are beneficial and others highly dan- gerous. For example, it has been claimed that some varie- ties of lodestone^ produce a purgative effect not unlike castor-oil, while others produce entirely opposite effects. It is claimed by still others that small quantities of ground lodestone when taken internally have the effect of preserving youthfulness and thus prolonging life. Perhaps one of the most curious things about the lodestone medicine taken internally is the power it was believed to possess of drawing iron arrow heads from the body. Here a strange confusion of ideas existed as to the well known attractive power of the solid lodestone for soft iron, and its assumed power when taken internally of causing the arrow head to be silently absorbed. Indeed, Gilbert claims that Paracelsus, one of the earliest of the medical quacks, who nevertheless did much to forward the study of medicine, concocted one of his most famous salves or ointments for stab wounds made by iron daggers. 290 THE WONDER BOOK OF MAGNETISM swords, etc., by mixing large quantities of powdered lode- stone in an ordinary ointment. But' it was in the case of a peculiar disease known as hysteria that magnetic treatment was claimed to produce the best results. In his " Philosophy of Magic," as trans- lated by Thompson, Salverte makes the following state- ment concerning cures which it was claimed could be effected by what was called " The Eoyal Touch." Salverte is speaking about a well known quack named Greatracks, who cured his patients without the use of any medicine by the mere stroke of his hand. "A more impudent quack than Greatracks has seldom appeared; he flourished in the seventeenth century. The belief in his power was general; from the most highly born and educated, to the most abject and. illiterate mendi- cant, all sacrificed at the altar of Credulity, and relied on the healing touch of Greatracks. In a letter to Lord Con- way, who sent for him from Ireland on account of the health of Lady Conway, this prince of impostors thus expresses himself : ' The virtuosi have been daily with me since I writ to your honour last, and have given me large and full testimonials, and God has been pleased to do wonderful things in their sight, so that they are my hearty and good friends, and has stopped the mouth of the Court, where the sober party are now most of them believ- ers, and my champions. The King's doctors this day (for the confirmation of their Majesties' belief) sent three out of the hospital to me, who came on crutches; but, blessed be God ! they all went home well, to the admiration of all people, as well as the doctors. Sir Heneage Finch says that I have made the greatest faction and distraction be- HAVE MAGNETS HEALmG POWEES? 391 tween clergy and laymen that any one has these thousand years.' Such was his boast; there is retribution in this world as well as in the next : the reputation of Greatracks soon afterwards declined as suddenly as it had risen." Another effect that it was claimed could be produced by magnetism was the so-called "magnetic sleep." In such cases it was only necessary to impress on the imagina- tion of the patient the fact that certain magic ceremonies would beyond doubt produce what was known as the mag- netic sleep, a condition of half-consciousness, during which it was claimed wonderful results could be produced. The alleged marvellous powers of the magnetic sleep and its miraculous effects were first pointed out in 1763, by Swedenborg, who claimed that by its use a man " may be raised to the celestial light even in this world, if the bodily senses could be entombed in a lethargic slumber." Probably one of the most noted cases of belief in the curative power of magnetism was in what was known as the odylic or odic force, a variety of animal magnetism. This force was first described by Baron von Eeichenbach, who claimed that he had discovered an entirely new imponder- able force, produced mainly by magnets, that existed throughout all portions of the universe and differed essen- tially from any other known force. The only manner in which the action of the odylic force, invisible, like magnetic flux, could be seen, was through the curious effects Eeichenbach claimed it produced. It did not, however, affect all people alike, and could produce its mysterious effects on certain sensitive or hysterical per- sons only. You can understand, therefore, how such people could bring themselves honestly to believe they saw many 292 THE WONDER BOOK OF MAGNETISM wonderful people and things that Eeichenbach assured them were produced by the odylic force. Some claimed that they could distinctly see faintly luminous emanations from the poles of a magnet. Others claimed that when the poles of magnets were drawn or slowly moved over different parts of their bodies they could distinctly feel the sensations of heat or of cold. Naturally, since only a small and therefore, as they believed, highly gifted class of sensitives were affected by the odylic force, a rivalry was set up between them, each one claiming to be able to see more than any of the others. Since none of them saw anything save only by the eye of the mind, you can understand how these women (they were most of them women) exaggerated or " batted them out " ; for, as soon as one was ready to claim that she could see a little more than any of the others, there was always somebody ready to go her one better if, indeed, not several better. In this way highly sensitive or hysterical patients claimed they could see faintly luminous flames issuing from the magnet poles. These flames were said to vary both in brightness and colour. Moreover, similar flames, it was claimed, could be seen distinctly issuing from the fingers of the person who was putting them under the influence of this peculiar force. They also claimed they could see flames of fire coming out of the ends of iron nails on the walls of a room on which pictures were hung. You can understand how great a sensation was pro- duced by Eeichenbach's ridiculous claims. Intelligent physicians, however, studying the cures that were claimed HAVE MAGNETS HEALING POWEES? 393 to have been obtained by the odylic force, soon came to the correct conclusion that they were simply due to the influ- ence of the mind over the body ; that, if the patients were only made to believe they would see certain appearances, they would promptly see them, even whether what they were looking at was a natural magnet or simply a piece of wood painted to resemble a magnet. Dr. Carpenter, in his " Mental Physiology," discusses the influence of the mind over the body as seen in so-called cases of influence by the odylic force. In this connection, he quotes from a work entitled " The Power of the Mind over the Body" the following results that were obtained by a Mr. Braid: " A lady, upwards of fifty-six years of age, in perfect health, and wide awake, having been taken into a dark closet and desired to look at the poles of the powerful horseshoe magnet of nine elements and describe what she saw, declared, after looking a considerable time, that she saw nothing. However, after I told her to look attentively, and she would see fire come out of it, she speedily saw sparks, and presently it seemed to her to burst forth, as she had witnessed an artificial representation of the vol- cano of Mount Vesuvius at some public gardens. Without her knowledge, I closed down the lid of the trunk which contained the magnet, hut still the same appearances were described as visible. By putting leading questions and asking her to describe what she saw from another part of the closet (where there was nothing but bare walls), she went on describing various shades of most bril- liant coruscations and flame, according to the leading questions I had put for the purpose of chang- 294 THE WONDEE BOOK OF MAGNETISM ing the fundamental ideas. On repeating the experi- ments, similar results were repeatedly realised by this patient. On taking this lady into the said closet after the magnet had been removed to another part of the house, she still perceived the same visible appearances of light and flames when there was nothing but the bare walls to produce them; and, two weeks after the magnet was removed, when she went into the closet by herself, the mere association of ideas was suffi- cient to cause her to realise a visible representation of the same light and flames. Indeed such had been the case with her on entering the closet, ever since the few first times she saw the light and flames. In like manner, when she was made to touch the poles of the magnet when wide awake, no manifestations of attraction took place between her hand and the magnet; but the moment the idea was suggested that she would be held fast by its powerful at- traction, so that she would be utterly unable to separate her hands from it, such result was realised; and on sepa- rating it by the suggestion of a new idea, and causing her to touch the other pole in like manner, predicting that it would exert no attractive power for the fingers or hand, such negative effects were at once manifested. I know this lady was incapable of trying to deceive myself or others present; but she was self-deceived and spell-bound by the predominance of a preconceived idea, and was not less surprised at the varying powers of the instrument than others who witnessed the results." Another case, also taken from Carpenter's " Mental Physiology," gives some experiments made on a man. This was also a patient of Mr. Braid and has been quoted from the book before referred to : HAVE MAGNETS HEALING POWEES? 293 " I first operated on Ms right hand, by drawing a pow- erful horseshoe magnet over the hand, without contact, whilst the armature was attached. He immediately ob- served a sensation of cold follow the course of the magnet. I reversed the passes, and he felt it less cold, but he felt no attraction between his hand and the magnet. I then removed the cross-bar, and tried the effect with both poles alternately, but still there was no change in the effect, and decidedly no proof of attraction between his hand and the magnet. ... In the afternoon of the same day I desired him to look aside and hold his hat between his eyes and his hand, and observed the effects when I operated on him, whilst he could not see my proceedings. He very soon described a recurrence of the same sort of sensations as those he felt in the morning, but they speedily became more intense, and extended up the arm, producing rigidity of the member. In the course of two minutes this feeling at- tacked the other arm, and to some extent the whole body; and he was, moreover, seized with a iit of involuntary laughter, like that of hysteria, which continued for several minutes — in fact, until I put an end to the experiment. His first remark was, ' Now this experiment clearly proves that there must be some intimate connection between animal magnetism and mesmerism; for, I was most strangely affected, and could not possibly resist laughing during the extraordinary sensations with which my whole body was seized as you drew the magnet over my hand and arm.' I replied that I drew a very different conclusion from the experiment, as I had never used the magnet at all, nor held it, nor anything else, near to him, and that the whole proved the truth of my position as to the ex- traordinary power of the Mind over the Body." 396 THE WONDEE BOOK OF MAGNETISM That the so-called odylic force is merely the result of the wonderful power the mind is able to exert over the body, can be seen from the following quotations that are also either from Carpenter's " Mental Physiology," or are quoted by him from other books: " If a fragment of anything, of any shape, be suspended from the end of the forefinger or thumb, and the atten- tion be intently fixed upon it, regular oscillations will fre- quently be seen to take place. If changes of various kinds are made in the conditions of the experiment, correspond- ing changes in the direction of the movements will very commonly follow. . . . " The public mind was directed to these facts, about the year 1850, by Dr. Herbert Mayo, who, having brought himself to accept Baron Eeichenbach's ' Odyle ' as a ' new force in Nature,' accepted these oscillations as a manifesta- tion of it, and gave to this suspended body the designation of ' odometer.' After varying his experiments in a great variety of modes. Dr. Mayo came to the conclusion that the direction and extent of the oscillations were capable of being altered, either by a change in the nature of the substances placed beneath the odometer, or by the contact of the hand of a person of the opposite sex, or even of the experimenter's other hand, with that from which the odometer was suspended, or by various other changes of the like nature. And he gradually reduced his results to a series of definite Laws, to which he seems to have imagined them to be as amenable as are the motions of the heavenly bodies to the law of Gravitation. — (' The Truths Con- tained in Popular Superstitions,' 3rd edition, 1851, Let- ter XIL) HAVE MAGNETS HEALING POWEES? 29? " Other observers, however, who were induced by Dr. Mayo's earlier experiments to take up the subject, and who worked it out with like perseverance and good faith, framed a very different code; so that it at once became apparent to those who knew the influence which ' expectant atten- tion ' exerts in determining involuntary muscular move- ments, that this was only another case of the same kind, and that the cause of the change of direction in each case lay in the Idea that some such change would result from a certain variation in the conditions of the experiment. Hence the general conclusions which each experimenter works out for himself, so far from being entitled to rank as ' laws of Odylic force,' are merely expressions of what has been passing (though perhaps almost unconsciously to himself) in his own mind. . . . The truth of this rationale was proved by the results of a few very simple variations in the conditions of the experiment. When it was tried upon new subjects, who were entirely devoid of any expectant idea of their own, and who received no inti- mation, by word, sign, or look, of what was anticipated by others, the results were found to have no uniformity what- ever. And even those who had previously been most suc- cessful in this line of performance found all their success vanish, from the moment that they withdrew their eyes from the oscillating body, its movements thenceforth pre- senting no regularity whatever. . . . Thus it lecame obvious that the definite direction which the oscillations previously possessed was due, not to any Magnetic, Elec- tric, or Odylic force, of which the operator was the medium, but to the influence directly exercised by his Ideas over his muscles under the guidance of his visual sense." CHAPTEE XXVII THE TALKING NEWSPAPER Many wonderful things have been accomplished in science. Some of these are so far ahead of what has already been done that we are apt to think, when some- thing uniasually wonderful has been announced, that it certainly marks the limit of human ingenuity; that we cannot expect it ever to be equalled in wonder by any sub- sequent achievement. But again and again what the world has believed to be the limit has been entirely surpassed by some still greater wonder. Now I have told you about many marvellous things in magnetism, but I have kept, for nearly the end of this Wonder Book, something I think you will acknowledge is far more wonderful than anything you have yet heard. When Edison announced to the world his wonderful invention of the phonograph, not in a theoretical condition, but in actual operation, almost every one agreed, when they heard the little cylinder covered with a sheet of in- dented tinfoil talk almost as well as you or I can talk, that this invention was certainly the ne plus ultra of human in- genuity ; that in all probability nothing would ever be able even to equal it. The same belief was expressed on the an- nouncement of the earlier invention of the speaking tele- phone, an invention quite as marvellous as the phonograph, although the presence of an actual living person at the other end of the line seemed, to some, to rob it somewhat of 298 THE TALKING NEWSPAPEE 299 its wonderful nature. But the wonder I have now to describe to you, the talking newspaper, consisting as it does of a combination of a telephone and a phonograph, far exceeds either of these great wonder-workers. The talking newspaper was invented by a Danish en- gineer named Waldemar Poulson of Copenhagen. Since this wonderful instrument depends for its operation on the Fig. 71. — ^Poulson's Talking Kewspaper magnetisation of a hardened steel piano wire, its descrip- tion comes properly in " The Wonder Book of Magnetism." Various names have been given to Poulson's talking news- paper. It has been called the micro-phonograph, the magnetic-phonograph, the tele-phonograph, and the tele- graphophone. It is more generally called the micro-phono- graph in America, but I think the name talking newspaper is one you can understand better, so that we shall there- fore employ it. The construction of the talking newspaper may be un- derstood from an examination of Fig. 71. This apparatus 300 THE WONDER BOOK OF MAGNETISM depends for its operation on the fact that when a steel piano wire is passed between the poles of a peculiarly shaped electro-magnet, called the recording electro-mag- net, through whose coils various currents are passing, per- manent magnetic effects are impressed on the wire. The magnetising coils of the recording magnet are placed in the circuit of an ordinary carbon telephone transmitter and a few cells of a voltaic battery. On speaking into the transmitter, variations in the resistance of the carbon con- tact points of the carbon transmitter are produced that cause corresponding variations in the strength of the OM 37-set. KjBBOrt. vtlKt-l m nt OiA. AraaoM 3AMvtkw/««- A B C D Fig. 72. — Electeo-Magnets Employed in the Talking News- paper currents passing through the magnetising coils of the elec- tro-magnet. These variations in the current strength mag- netise the steel wire as it passes between the magnet poles and leave on it an invisible but permanent record of the variations in the magnetic intensity that are impressed on the record magnet by the sound waves that fall on the transmitting diaphragm. The record wire employed in the apparatus shown in the above figure consists of a length of hardened steel piano wire having the diameter of about the twenty-fifth of an inch. This record wire is wound on the surface of a THE TALKING ISTEWSPAPEE 301 cylinder as in Pig. 71. A small electro-magnet consisting of two separate electro-magnets, electrically connected, as shown at C, Pig 72, is placed so that its opposite poles em- brace the wire as shown at B in the above figure. The cylinder, being revolved by an electric motor, causes the record magnet to move over entire length of the wire from right to left from one end of the cylinder to the other. The magnet poles, therefore, pass over the entire length of the record wire wound on the drum. There is then recorded on it whatever is said or sung into the telephone transmitter. The drum or cylinder is rotated by any suitable means ; such, for example, as an electric motor. By reason of this movement the entire length of the piano wire from right to left, that is, from one end of the drum or cylinder to the other, passes under the magnet. I can imagine your asking what there is about such a device that would justify one in calling it a talking news- paper. As to the talking part, it can talk to you ; and, as to the propriety of its being called a newspaper, I may tell you that it is capable of conveying intelligence of what has been going on in the world; is capable of circulating the news of the day; and of advertising certain articles or commodities offered for sale; that it does in general what the newspaper is intended to do, and can, therefore, very properly be called a newspaper. The talking newspaper is certainly a wonderful improve- ment over the first German newspaper that was published in 1615, or the first English paper, that was published in London in 1622. It cannot only tell the news of the day, but can also speak it into the ear of the listener, who 303 THE WOKDER BOOK OE MAGl^ETISM can, therefore, become acquainted with, what has been going on without even getting out of bed, or of turning on the light so that he can see to read. I will now explain at greater length the manner in which the news is recorded on the piano wire. The mag- netising coils of wire that are wrapped around the core of the electro-magnet B are connected, as shown, with the circuit of an ordinary carbon telephone transmitter. As I shall explain more fully in " The Wonder Book of Elec- tricity," in this form of transmitter an electrical device known as the induction coil is employed, consisting of the usual primary and secondary windings placed close to- gether. The primary winding of the transmitter is placed in the circuit of a small voltaic battery, while the secondary winding is placed in the coils of the electro-magnet B. When one speaks into the telephone transmitter varia- tions in the strength of the current passing through the primary coil are produced, by means of which correspond- ing variations are set up in the secondary coil. It is these variations that transmit to the telephone receiver the speech that has been uttered into the telephone, and thus permits it to be understood. But the variations in the current so produced by speak- ing into the transmitter pass through the coils of the electro-magnet B, and, therefore, magnetise the hardened steel piano wire that is being drawn by the motion of the cylinder between its poles. In this way there is left in the steel wire a record of the variations in the magnetic intensity that have been impressed on the secondary circuit by the sound waves that have fallen on the diaphragm of the transmitting telephone. THE TALKING NEWSPAPER 303 The recordiBg magnet is now placed at the beginning of the record on the wire with its invisible but permanent magnetic record of what has been spoken into the trans- mitter, and its coils are connected with the circuit of an ordinary Bell receiving telephone. The electric motor being set in action, the record wire is made to pass between the poles of the recording magnet. As the magnetised steel wire is drawn between these poles it acts as a magnet or series of magnets that are moved across the coils of wire wrapped on the poles of the recording magnet, and since, as the Fig. 73. — Plan of the Talking Newspapee record wire passes between these poles, it causes magnetic flux to alternately fill and empty the coils of wire on the record magnet, it acts as a dynamo-electric machine and produces currents corresponding exactly in strength, direc- tion and order of sequence to the currents that pass through the magnetising coils of the recording magnet and thus leave their permanent but visible record on the wire. As these currents pass through the wire on the Bell receiving telephone they will reproduce in its diaphragm whatever has been spoken or sung into the transmitter. Considerable difficulty was first experienced in obtain- 304 THE WONDEE BOOK OP MAGNETISM ing an electro-magnet sufficiently sensitive to record the speech that was spoken into the transmitting instrument. If wound in the ordinary manner, the electro-magnet was too slow in its action to give satisfactory results, but with the form of magnet shown at B, in Fig. 72, it is possible not only to transmit speech but even to be able to distin- guish at the receiver the peculiarities of the voice so as to know who is speaking, provided, of course, one is familiar with this voice. The manner in which this invention is applied to the talking newspaper may be understood from an examina- tion of Fig. 73. Here an endless ribbon, or band of hard- ened steel, passing from one rotating cylinder to the other, is made to take the place of the magnetised piano wire. The direction of its rotation is indicated by the arrows. The means for keeping it in motion, in this case a belt passing over the pulley of an electric motor, is also shown. As the record ribbon is thus moved, a transmitting tele- phone, placed as shown at the upper part of the figure, impresses on the ribbon whatever news or speech is uttered into it. These impressions consist of north and south magnetic poles so varying in order and intensity as cor- rectly to represent the directions of the rapidly varying electric currents passing through the coils of the electro- magnet B, in Fig. 73. The record on the steel band is made to reproduce the sounds uttered into the transmitting tele- phone, at any. of the ten separate reproducing magnets placed as shown in the figure. Consequently, a subscriber may listen at any of these receivers. Any number of these reproducing magnets may be employed, even up to a thou- sand or more. THE TALKING NEWSPAPEK 305 Of course, you will understand that the only apparatus placed in the house, room, or ofl&ce of a subscriber is this receiving phone itself. All the other apparatus is placed in a central station, where the news is collected and trans- ferred to the permanently magnetised steel ribbon. CHAPTBE XXVIII MAGNETISM AND MAGIC When a boy sees anything the working of which he is unable exactly to understand he is apt to call it a magigger, or a majigger. I am uncertain as to the exact spelling of this word. While it is possible that it has been derived from " jigger," a general name given to a tool or appli- ance that during its working takes on irregular dancing movements, somewhat resembling the dance called a jig, the motions of which are so irregular as to fool or trick the observer, yet I believe the word is employed by boys rather in the sense of a magic-er. I have, therefore, spelled the word magigger. It is possible that magigger is a new word for you. Indeed, I think you will look in vain for it, even in the largest dictionaries you can find; for it belongs to a class of words included under the general name of " slang." Now, I would earnestly advise you to avoid the use of slang when it consists in the unauthorised popular use of a word of low or illiterate origin, but when, as is sometimes the case, it is merely an odd, grotesque or metaphorical expression that can only be regarded as slang because it has not yet acquired a reputable usage, I think its use can sometimes not only be tolerated but even encouraged. This, I think, is the case with magigger, a word that has been ingeniously coined by some brainy boy to explain a piece of apparatus capable of producing effects so strange and unexpected as to lead to the belief that they are due 306 MAGNETISM AND MAGIC 307 to magic. Now, when you use a new word, you should he able to define it. I will therefore give you its definition: magigger, anything that magigs; that is, any device that produces effects so strange that they appear magical. If you think a moment you will see that, while the word magigger permits of a wide usage, yet this usage necessarily becomes smaller as one's knowledge increases; for, as soon as the causes of any wonderful occurrence closely resem- bling magic are understood, all the magic instantly dis- appears. It has often occurred to me that magnetism is capable of lending itself in a remarkable degree to the production of mechanical devices capable of being employed by the magi- cian. Magnetic flux readily passes through a great variety of substances that are opaque to ordinary light. It would be easy, by suitably placing a magnet inside a wooden box in such a position that the magnet is capable of being moved in a certain direction on the approach of another magnet, to produce many curious effects. The magnetic flux passing through the board could, by moving the mag- net, be readily made to give information to the magician that would enable him to do things that would seem inex- plicable to all who are not in the secret. Since the flux produced by an electro-magnet is in gen- eral much more powerful than the flux of permanent magnets, electro-magnets are, perhaps, more frequently employed by the magician. In this way numerous effects can be produced either by the to-and-fro movements of their armatures or, what amounts to the same thing, by the movements of an electro-magnetic motor. The ad- vantages obtained by employing electro-magnets consist 308 THE WONDEE BOOK OE MAGNETISM not only in their greater strength, but also in the fact that the magnetism can be thrown on or off by the closing or opening of an electric current by a push button or a key placed at any distance from the device it is operating. You all know how the pushing of a button that closes an electric circuit can ring an alarm bell at any distant point. Some time ago, when the knowledge of the practi- cal applications of electro-magnetism was less common than now, a device, first exhibited by the great Houdin, was that known as the magic drum. Here a drum, sus- pended apparently by an ordinary cord midway between the ceiling of the auditorium and the platform, would un- expectedly begin beating a tattoo apparently, at times, for the purpose of preventing something the magician was try- ing to tell the audience or, at other times, in answer to cer- tain questions addressed to the drum. It seemed almost weird that a drum hanging in a big auditorium, forty or fifty feet above the floor, should act as if it understood all that was being said. But the device was very simple. In- side the drum was an electro-magnet, arranged so that the to-and-fro movements of its armature, instead of striking a bell as in the case of the burglar alarm, struck the drumhead on the inside. What looked like an ordinary thread that was employed for the suspension of the drum contained two insulated wires by means of which the electric current was conducted to and from the coils of the electro-magnet. To start the drum it was only necessary to close an electric circuit, and this could readily be done by means of either a contact placed on the stage and operated by the foot of the magician, or by means of a key or push button operated by a confederate out of sight but not out of hearing. MAGNETISM AND MAGIC 309 In an improved form of magic drum two separate elec- tro-magnets are provided; one for beating rolls or tattoo? and the other for producing single raps. This latter electro-magnet is employed by the drum in answering ques- tions in fortune-telling or in detective work. Of course much of the fun will depend on the cleverness of the operator in framing these questions. If sometime, when you are in a large hotel, you will look at the annunciator board in a large frame hung on the wall back of the clerk's desk in sight of the bell-boys, you will hear every now and then a call bell ring which sum- mons a bell-boy, and you will at the same time see some- where on the face of the board a little shutter fall. In large hotels there are many hundreds of these shutters, num- bered consecutively to, say, 900 or more, corresponding to the numbers of the diilerent rooms. When a bell-boy has his attention called to the annunciator board by the ringing of the bell, he looks at the drop that has fallen and seeing on it a certain number, let us say 873, knows that someone in room No. 872, on the eighth floor, wishes some service, and of course goes up immediately. Now what has happened is as follows : The guest in room 873, wishing a pitcher of ice water, one of the evening papers, or some other service, touches a push button, there- by closing a circuit through the coils of a small electro- magnet placed immediately back of shutter No. 873. The armature of this electro-magnet, being attracted to its poles, loosens the shutter and permits it to fall, thus dis- playing the number. A similar device, or magigger, if my boy friends prefer the word, is very commonly employed by the magician. 310 THE WONDEE BOOK OP MAGNETISM who, on the touching of a push button, either by himself or by his confederate, permits a shutter to drop, disclosing to the great astonishment of the audience a ring or watch or a particular playing card hanging inside a glass frame. In order to increase the mystery of this trick the pushing of the button by his assistant, or by the magician himself, is coincident with the firing of a pistol. A magician's wand, though generally consisting simply of a suitably shaped stick of wood, is nevertheless often of no little assistance in the performance of tricks that require adroit sleight-of-hand ; for, it gives the magician an oppor- tunity for many movements that could not otherwise be made without awakening or attracting the attention of the audience to something the magician especially wishes should not be observed. Often the magician's wand is more complex than it appears to be; for, it is provided at one end with a minia- ture pistol that may either fire a percussion cap only, or may even have means for igniting a small charge of pow- der. In some cases it also contains a small voltaic bat- tery together with an induction coil arranged so that it is possible to give a fairly severe shock when incautiously handled by someone in the audience, and yet which can be safely handled by the magician, who knows its con- struction. Here is .another curious effect produced by electro-mag- netism, an effect by the way that is astonishing even to the intelligent and learned. It consists of a closed box inside of which is suspended a very heavy wheel which has been set in rapid rotation by a concealed electric motor and battery, or in any other way. When a body, possessing MAGNETISM AND MAGIC 311 eonsiderabjle weight, is rapidly rotating, a considerable force will be required to alter or change the plane in which it is rotating. When the box is handed to a person he finds no difficulty in carrying it in the direction of rotation, which, let us say, is along a due north and south line. If, however, he attempts to move it to the right or to the left, that is, to the east or to the west, a strange influence at once begins to oppose the carrying of the box in this direc- tion. In this manner, intelligent people have been consid- erably surprised; but, as soon as the trick was explained to them, their astonishment of course ceased. The great power with which electro-magnets attract their armatures or masses of soft iron to their poles is frequently utilised by magicians. A piece of soft iron is seen resting on a soft iron plate. A person called from the audience is asked to lift it, which he finds it easy to do. As soon, however, as the magician closes the circuit of the coils of an electro-magnet, the poles of which lie immediately under the plate of iron, the attraction is so great that it is impossible to raise it. This, as you will see, is a modern trick that we may regard as having been borrowed from the curious thing that happened to Magnes in Asia Minor, when he unexpectedly brought his crook in contact with a large piece of lodestone. It will interest you if I quote from a book called " Mod- ern Magic," by Professor Hoffmann, the role this trick played in Africa in the hands of the great Eobert Houdin : " The above trick is cited by Eobert Houdin in illustra- tion of the great difference which there may be, in point of effect, between the modes of presenting the same illu- sion. The reader may possibly be aware that Eobert 313 THE WONDEE BOOK OF MAGNETISM Houdin was employed by the French Govemmeiit, at one period of his career, in a mission to Algeria, with the object of destroying, if possible, the popular belief in the pretended miracles of the Marabouts, whereby these latter had obtained an extraordinary ascendency over the minds of the ignorant Arabs. The plan adopted was to show, first, that a European could perform still greater marvels, and then to explain that these seeming mysteries were mere matters of science and dexterity, and wholly inde- pendent of supernatural assistance. The 'Light and Heavy Chest' was one of the prominent features of the programme, but if presented by that name it would have produced but very little effect. The fact that the chest be- came immovable at command would only have been at- tributed by the Arabs to some ingenious mechanical ar- rangement beyond their comprehension, but exciting only a momentary wonder. With great tact Eobert Houdin contrived to turn the attention of his audience from the object to the subject of the trick, professing, not to make the chest light or heavy, but to make the person who volun- teered weak or strong at his pleasure. Thus presented, the trick had the appearance no longer of a mere achievement of mechanical or scientific skill, but of a manifestation of supernatural power. We will tell the rest of the story as nearly as possible in Eobert Houdin's own words, as related in the story of his life: " 'An Arab of middle stature, but well-knit, wiry, and muscular, the very type of an Arab Hercules, came for- ward, with plenty of self-confidence, and stood by my side. " ' Are you very strong,' I inquired, eyeing him from head to foot. MAGNETISM AND MAGIC 313 " * Yes,' he replied carelessly. " ' Are you sure you will always remain so ? ' " ' Perfectly.' "'You are mistaken; for in one moment I shall take away all your strength, and leave you as weak as a little child.' " The Arab smiled scornfully, in token of disbelief. " ' Here,' I said, ' lift up this chest.' " The Arab stooped, lifted- the chest, and said disdain- fully, 'Is that all?' " ' Wait a bit,' I replied. Then, with the solemnity appropriate to my assumed character, I made a gesture of command, and gravely said: " ' You are weaker than a woman. Try now to lift that box.' " The strong man, perfectly indifferent about my magic spell, again catches hold of the box by the handle, and gives a vigorous pull to lift it; this time, however, the chest resists, and in spite of the most determined efforts, remains absolutely immovable. " The Arab wastes in vain over the unlucky chest an amount of force which would have lifted an enormous weight; till at last, exhausted, panting, and burning with shame, he ceases, looks dumbfounded, and begins to ap- preciate, the power of the magic art. He has half a mind to give up the attempt; but to give up would be to ac- knowledge himself conquered, and to admit his weakness; and after having been famed for' his muscular strength, to sink to the level of a child. The bare idea makes him furious. Gathering new strength from the encourage- ment which his friends offer him by word and look, he 314 THE WOKDEE BOOK OP MAGNETISM casts towards ihem a glance which seems to say, ' You shall see what the son of the desert can do.' Once more he bends over the chest, his nervous hands grip the handle, and his legs, planted one on each side of the chest like two columns of bronze, serve as a fulcrum for the mighty effort which he is about to make. It' seems almost impos- sible but that under such a strain the box- must fly to pieces. Strange ! this Hercules, a moment ago so strong and self-confident, now bends his head; his arms, riveted to the boz, are drawn by a violent muscular contraction against his chest; his legs quiver, and he falls on his knees with a cry of agony. " An electric shock, produced by an induction coil, had ]ust been communicated, at a signal from me, from behind the scenes to the handles of the chest. Thence the contor- tions of the unlucky Arab. To prolong his agony would have been inhuman. I gave a second signal, and the electric current was cut off. My athletic friend, released from his terrible bondage, raised his hands above his head. ' Allah ! Allah ! ' he cried, shaking with fright, then wrap- ping himself hastily in the folds of his burnous, as though to hide his disgrace, he rushed through the spectators, and made his way to the door of the hall." I might add many additional magical feats that can be performed by the assistance of electro-magnets, such, for example, as spirit-rappings, in which the rappings are due to the movements of the armatures of an electro-magnet. I might also describe the construction and operation of a device known as the magic bell, as well as the crystal cash box. But the, above must suffice. It appears that magnetism was employed over half a cen- MAGNETISM AFD MAGIC 315 tury ago as an aid in performing the so-called magical tricks. This will be seen from the following quotations taken from Hutton's " Eesearches in Science," published in 1851, and before referred to : " For some years past, the properties of the magnet have been employed to perform several tricks, which excited a considerable degree of astonishment in those who first beheld them. No means indeed more secret, and at the same time more proper for action, could be employed than magnetism, since its influence is stopped by no body with which we are acquainted, except iron. This idea was first conceived by the celebrated Comus, who varied, in a singu- lar manner, the different tricks performed by this agent, so that all Paris flocked, with the utmost eagerness, to the places where they were exhibited. He was admired by the ignorant, who considered him as a sorcerer, while the learned endeavoured to discover the artifice, which, how- ever, was a profound secret as long as no one suspected magnetism to be the principal cause of it. " We shall here endeavour to give an idea of some of these tricks, as they will form a fund of rational amuse- ment to those who know how to perform them. " Those who exhibit these tricks often employ a pre- tended magic telescope, by means of which one can see, it is said, through opaque bodies. It is nothing else than an instrument in the form of a telescope, at the bottom of which, that is towards the object glass, there is a magnetic needle, which assumes its proper direction when the tele- scope is placed upon the side which that object glass forms. " To construct this telescope, provide a turned tube of ivory, wider towards the end where the object glass is 316 THE WONDEE BOOK OP MAGNETISM placed; but the ivory must be of sufficient thinness to ad- mit the light through to the inside. The narrow end is furnished with an eye-glass which serves to show more distinctly the inside of it. The other end also is furnished with a glass which has the appearance only of an object glass, the posterior surface of it being opaque, so as to serve for the base or bottom of a sort of compass or magnetic needle which turns on a pivot fixed in its centre. When the telescope rests on the end. containing the object glass, this needle assumes a horizontal position, and points towards the north, or towards a magnetic needle in the neighbourhood. It is necessary also to have a real tele- scope, similar in appearance to the other, in order that it may be shown instead of it, which may be done by dexter- ously substituting the one for the other. " When you wish to employ the pretended magic tele- scope, place it with the object glass downwards upon any- thing you intend to examine, and if there be a magnet, or piece of magnetised iron below it, the needle will turn to that side. " If you are desirous of employing the ten ciphers, take ten small squares, of an inch and a half on each side, and on the upper face of each make a groove; but let these grooves be in different directions; that is to say, the first intended for the number 1 must proceed directly from the top to the bottom; the second must deviate to the right, so as to form an angle equal to a tenth part of the cireum- ference; the third an angle of two-tenths; and so of the rest, which will give ten different positions. Then intro- duce into these grooves small bars of steel, well magnet- ised, taking care to turn their north poles to the proper MAGNETISM AND MAGIC 317 direction; cover these grooves and the face of the squares with strong paper, in order to conceal the bars. You must also provide a narrow box, capable of containing in its breadth one of these squares, and of such length that they can all be arranged in it. " Then desire a person in your absence to take several of these squares and arrange them in the box in any man- ner, at pleasure, so as to form any number whatever, and to shut the box; after which you are to tell the number which has been formed. Deposit your pretended telescope on the place of the first square, that is on the left, if the figure below it be unity; the needle will turn in such a manner that the north point or pole will be before you. If the figure be 4, it will turn to the fourth division of the circle, which is equally divided into ten parts; and so of the rest. It will thence be easy to discover the figure in each place, and consequently to tell it. " A word written in secret, with given characters, may be discovered in the same manner ; also an anagram, formed of a proposed word, as Roma, which gives amor, mora, orma, mora, etc.; or a question which has been selected from several persons, and put into the box. In short, this trick may be varied in a great many ways, exceedingly agreeable, but all depending on the same principle. " The box of metals, for example, is only a similar varia- tion of the same trick. You put six plates of different metals in a box, and bid a person take any one of them, and put it into another box, and shut it. You may then easily tell which one he has taken. These plates are of such a form that they can occupy in the small box only one position. Bach of them, that of iron excepted, contains 318 THE WONDEK BOOK OF MAGNETISM in its thickness a magnetic bar, arranged in situations which are known, and these situations are discovered by means of the pretended magic telescope; consequently the nature of the metal must be known. No magnetic bar is placed in the plate of iron because this would be useless; but one side of the plate may be magnetised, or if it be not magnetised, the indeterminate direction of the needle will announce that it is iron." CHAPTER XXIX GILBERT'S " DE MAGNETB " When its early date of piablication is considered, perhaps one of the most remarkable books that have ever been written on the subject of magnetism is the " De Magnete, or the Lodestone." Although its author, William Gilbert, vras an Englishman, yet the book is written in the Latin language. Gilbert was born in Colchester, in 1540. Without dis- cussing his personal history, I will merely say that he was a practising physician, and had received the honourable appointment as physician to Queen Elizabeth of England. He was elected Eellow of the Eoyal College of Physicians in 1573, and continued a Fellow of the same college until the year 1600, when he was elected its President. During his early life, Gilbert devoted himself to chem- istry. After having obtained considerable knowledge of this subject he spent the remainder of his life in the study of magnetism. It was the great interest Gilbert took in magnetism that led him to write " De Magnete." This is a famous work, and has properly been regarded as the most valuable book on magnetism that was ever published, not only before the year 1600, but also for many decades afterwards. Con- sidering the early date of its publication, this book is remarkable by reason of the extended knowledge it shows that Gilbert had gained on the subject of magnetism. I have purposely left a brief description of this book to this 319 320 THE WONDEE BOOK OP MAGNETISM last chapter of " The Wonder Book of Magnetism " because you are now in a position to appreciate many of the facts of magnetism it describes. In my study of this book, I have employed an ezcellent translation into English by P. Fleury Motteley. The translation is well done and, there- fore, makes an exceedingly interesting book, which I would advise you to read whenever the opportunity presents itself. Like all scientific men, I have the highest regard for Gilbert's work; for, I find it shows, for this early date, a wonderful grasp of the science of magnetism. I regret to say, however, I cannot agree with the following state- ment of Dr. Whewell: " Gilbert's work contains all the fundamental facts of the science, so fully examined, indeed, that even at this date we have little to add to them." ISTow, in point of fact, Gilbert's book contains no refer- ence whatever to such fundamental discoveries as the rela- tions of magnetism to electricity as first discovered by Oersted, nor of the production of electricity by magnetism as discovered by Faraday, although it is true that Gilbert does trace with fair clearness, considering the time at which he wrote, some of the resemblances as well as some of the differences between electricity and magnetism. I agree, however, with Dr. Thomas Thomson's statement that the book, " De Magnete," is " one of the finest examples of inductive philosophy that have ever been presented to the world." This is the more remarkable because the " De Magnete " preceded Bacon's " Novum Organum," in which the methods of the inductive philosophy were publicly ex- plained for the first time. Poggendorff, the German physicist, had so high an opin- GILBEET'S "DE MAGNETE " 331 ion of the ability of Gilbert that he called him " The Gali- leo of Magnetism." It is a fitting tribute to the work accomplished by Gil- bert that when scientific men adopted the plan of naming the practical electric and magnetic units after the -world's great scientific men who have finished their labours on the earth, they selected the name of Gilbert as the name for the practical unit of magneto-motive-force. This is a far greater honour than you may think; for, in order to have one's name given to an electric or magnetic unit, one must have occupied a very prominent rank in some par- ticular branch of science. You can see this when I mention some of the men whose names have been given to these units ; they are Faraday, Oersted, Ampere, Coulomb, Ohm, Volta, all of whom unquestionably occupied the highest rank in their particular branch of science. I believe I cannot close this Wonder Book on Magnet- ism in a better way than by. calling your attention briefly to some of the many important facts concerning magnetism that are to be found in the " De Magnete." In the first place Gilbert was the first to give the name " poles " to the ends of the magnetic needle. He called the pole that pointed to the* earth's north magnetic pole the south pole of the needle, and that which pointed, ap- proximately, to the earth's south pole, the north magnetic pole of the needle. This, of course, was before the time when men agreed to call the end of the needle that pointed approximately to the north pole of the earth the north pole of the needle, remembering of course that it is a south magnetic pole that exists in the earth's Northern Hemi- sphere. 323 THE WONDER BOOK OP MAGNETISM Gilbert does not seem to have known that there were any other bodies than iron or the lodestone that possessed magnetic properties; for he called particular attention to the fact that all other substances that possessed magnet- ism to some degree owe their power to the presence of iron in some condition or other. Perhaps the following quotation from John Robison will give you an excellent idea of how Gilbert's " De Mag- nate " was regarded by scientific men in 1822 : " This work of Dr. Gilbert's relates chiefly to the lode- stone, and what we call magnets; that is, pieces of steel which have acquired properties similar to those of the lode- stone. But he extends the term magnetism and the epithet magnetic to all bodies which are affected by lode- stones and magnets in a manner similar to that in which they affect each other. In the course of his investigations, indeed, he finds that these bodies are only such as contain iron in some state or other; and in proving this limitation he mentions a great variety of phenomena which have a considerable resemblance to those which he allows to be magnetical, namely, those which he called electrical, be- cause they were produced in the same way that amber is made to attract and repel light bodies. He marks, with care, the distinctions between these and the characteristic phenomena of magnets. He seems to have known that all bodies may be made electrical, while ferruginous substances can be made magnetical. It is not saying too much of this work of Dr. Gilbert's to affirm that it contains almost everything that we know about magnetism. His unwearied diligence in searching every writing on the subject, and in getting information from navigators, and his incessant oc- GILBBET'S "DB MAGNETE " 323 cupation in experiments, have left very few facts unknown to him. We meet with many things in the writings of posterior inquirers, some of them of high reputation and of the present day, which are published and received as notable discoveries, but are contained in the rich collection of Dr. Gilbert. We by no means ascribe all this to mean plagiarism, although we know traders in experimental knowledge who are not free from this charge. We ascribe it to the general indolence of mankind, who do not take the trouble of consulting originals, where things are mixed with others that they do not want, or treated in a way and with a painful minuteness which are no longer in fashion. We earnestly recommend it to the perusal of the curious reader. He will (besides the philosophy) find more facts in it than in the two large folios of Scarella." I need hardly say that I must make the same objection to the above statement by Eobison that I made to that of Whewell, although this statement is less objectionable since it only states that Gilbert's book contains almost everything we know about magnetism. But even this I am sure is quite inaccurate. In his book, Gilbert refers very frequently to what he calls a terella or little earth. This consisted of a piece of lodestone cut or shaped into the form of a sphere and pro- vided with two opposite poles situated approximately at the ends of its axis. He also frequently refers to a mag- netised versorium, or a permanently magnetised bar or needle supported in a horizontal position on a vertical point, so as to be able to point or be moved under the influence of the earth's magnetism, or of the terella. His versorium was practically a compass needle. 324 THE WONDEE BOOK OF MAGNETISM In the first chapter of Book VI, on the globe of the earth as a lodestone, Gilbert calls attention to the fact that all the experiments that have been tried with the terella can be repeated on the earth itself ; that a lodestone, placed so as to be free to move, is attracted by the opposite mag- netic poles of the earth just as it is by the poles of a ter- ella; that iron is endowed with polarity by being placed in contact with the earth in a direction extending between its two poles; that magnetism can be destroyed by heat; that a bar having, thus lost its magnetism can be again endowed with this property by leaving it in contact with the earth. He also discovered that, if a bar of steel is heated to a red heat while in such a position that the earth's magnetism passed through it in the direction of its length, it will become permanently magnetised when cooled. Eeference is also made to the dip of the magnetic needle. Indeed, since Gilbert had so clearly recognised in his book the fact that the entire earth acted as one huge magnet, it is evident that he thoroughly understood the cause of the dip or inclination of the compass needle. He calls at- tention to the fact that when a bar of unmagnetised iron is accurately balanced on a vertical support so that it shall come to rest in a true horizontal position with each pole at the same distance from the earth's surface, that, as soon as it is magnetised, if free to move in a vertical plane, it will lose its horizontal position and will dip or incline towards the earth, and that the amount of this dip in- creases the nearer to the magnetic poles the needle has been carried. He probably obtained this knowledge of the dip of the needle from Forman. Gilbert was too good a chemist not to know that the GILBERT'S "DE MAGNETE" 335 lodestone is merely a peculiar form of iron ore, and that it acquired its magnetic properties by simply coming in con- tact with the magnetised earth. He also appears to have recognised the advantages that we have already pointed out that are possessed by masses of soft iron called pole pieces, when closely attached to the surface of the lodestone. He calls attention to some experiments he made of a mass of lodestone that could only hold or support a piece of iron weighing four ounces, but was able to sustain a weight of twelve ounces when suitably provided with masses of soft iron. As you will remember, Jamin at a much later date employed the same device for strengthening the attractive power of his com- pound steel horseshoe magnet. I might readily greatly extend the number of facts referred to in Gilbert's book, but I think the above will suffice to show the remarkable grasp its author had on magnetic science at this very early date. Before closing I wish to express the hope that I have presented some of the simple effects of magnetism, and, indeed, I may say most of the effects of magnetism, in such a manner as will inspire you, after having read " The Won- der Book of Magnetism," to wish to know more about this curious force, and that you may, therefore, be induced to read more advanced books and thus extend the range of your knowledge. If I am successful in doing this, I shall not regret the labour of writing this book. THE END