m&i BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henrg M. Sage 1891 A..lAf!.A.6. {Y^/9~l Cornell University Library arV18600 Telegraphists' guide to the new examinat 3 1924 031 273 877 olin.anx Cornell University Library The original of tliis book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 924031 273877 A MANUAL OP TELEPHONY, By "W. H. PEBBCE, C.B., F.E..S., President of the Institution of Electrical Engineers, Engineer-in-Chief and Electrician at the General Post Office, and A. J. STUBBS, Technical Officer, General Post Office, A.I.E.E. "With upwards of 300 Illustrations, mostly from original drawings. Appendix, Tables, and a full Index. 520 pages. Crown 8vo., 15s. Contents : — I. Transmitters and Eeceivers — II. Apparatus and Circuits — III. Simple Telephone Exchange Systems— IV. Multiple Switches — V. Miscellaneous Switching and other Systems — "VI. Construction, "Wires and Cables. "The most complete epitome of present-day telephonic practice." — Electrical Engineer, " Must find its way into the library of every scientist and into the hands of every one who takes the slightest interest in telephony."— i)oi!j/ Chronicle. "The work is exhaustive of its subject, without bemg overbiurdened with minute technical details."— Times. THE PRACTICAL TELEPHONE HANDBOOK. By JOSEPH POOLE, A.I.E.E. ("Wh. Sc. 1875), Chief Electrician to the late Lancashire and Cheshire Telephone Exchange Co., Manchester. With 227 Illustrations. Price 39. 6d. **It contains readable accounts of all the best known and most widely-used instruments, together with a considerable amount of information not hitherto published in book form." — Mlectnetan. "Will be found both useful and interesting to persons who use the tele- phone, as Mr. Poole's exposition of telephonic apparatus is both clear and comprehensive."— Satonia]/ Heview. London: WHITTAKER & CO., Paternoster- Square. . . READ . . "ELECTEICITY." BRIGHT, PRACTICAL, AND ALWAYS UP-TO-DATL Largest Sale of any Electrical Paper in the United Kingdom. WEEKLY - - ONE PENNY. Of all Newsagents and Booksellees. SAMPLE COPY FREE FROM " EI. H C mil CJ X T7 "X-," 29, liudgate Hill, Liondon, E.G. THE TELEGEAPHISTS' GUIDE TO THE NEW EXAMINATIONS IN TECHNICAL TELEGRAPHY. TOGETHER WITH AN APPENDIX DEALING WITH D»T AND SECONDAET CELLS, TJNIVEESAL BATTERY SYSTEM, DIBECT READING BATTERY INSTRUMENT, DUPLEX (Bbidge Method), NEW SYSTEM OE MORNING TESTING, EAST SPEED REPEATERS, &c. BY JAMES BELL, A.I.E.E. CEETIPICATED TEACHER CITY AKD GUILDS OP LONDON INSTITUTE. LONDON : "ELECTRICITY," 29, LUDGATE HILL, E.G. PREFACE. The following pages are a reprint of a series of articles wliioli appeared in the columns of Electeicity, under the title of " Postal Telegraphs. The New Technical Examina- tion." Evidence was abundantly furnished of the genuine and widespread appreciation with which the articles were received by readers of that journal, and this circumstance ■accounts for their re-appearance (with some additions and corrections) in the present form. The papers, it is believed, will meet the whole of the requirements of the new technical examination imposed on telegraphists aspiring to superior appointments in the 'Government telegraph service. The writer, however, has not by any means confined himself to the bare require- ments of that examination, and students preparing for i;he Examinations in Telegraphy conducted by the City ■and Guilds of London Institute will, it is hoped, find substantial aid in carefully perusing this little book. The questions, which are typical of those set at the Examinations of that Institute, and of the Science and Art Department, will be found useful by the student in testing his knowledge. Answers have been given to all the arithmetical questions. The writer has to express his indebtedness to Mr. S. Wilson, Belfast, for the ready help afforded him by that gentleman in preparing the articles on "Repeaters," ^' High Speed Wheatstone System," and " New System of Morning Testing." Sydney Cottage, Dundee, February, 1895. J. B. CONTENTS. PAOE. CEOSSING AND LOOPING WIEES -VVITII PACILITT AJTD CEETAINTY : 3 TEACING AND LOCALISING FAULTS IN INSTEUMENTS 7 TEACING AND LOCALISING PEEMANENT AND INTEEMITIENT EAETH, CONTACT, AND DISCONNECTION, EAIILTS ON AVir.ES 9 ilETHODS OP TESTING THE ELECTEO-MOTIVE EOECE AND EESISTANCE OF BATTERIES, AND A GENEEAE KNOW- LEDGE OF THE YAEIOirS DESCEIPTIONS OF BATTEEIES... 13 SYSTEM OF MOENING TESTING, BOTH AS EEGAEDS SENDING AXD EECEIVING CUEEENTS, WITH THE NECESSAET CALCULATIONS IN CONNECTION 'WITH THE SAME 19 MAKING UP SPECIAL CIECUITS IN CASES OF EMEEGENCY ... 20 JOINING UP AND ADJUSTING SINGLE NEEDLE, SINGLE CUEEENT, AND DOUBLE CUEEENT MOESE, BOTH SIMPLEX AND DUPLEX, AND WHEATSTONE APPARATUS 22 PITTING A "WHEATSTONE TRANSMITTER TO AN OEDINAEY KEY-WOEKED CIRCUIT 31 A GENERAL KNOWLEDGE OF THE PRINCIPLES OF QUADEUPLEX AND MULTIPLES WORKING 31 3IEASUEING EESISTANCES BY THE WHEATSTONE BRIDGE 41 The above arc the Questions on which the new Technical Examination is based. POSTAL TELEGRAPHS. The New Technical Examination. — I. By Jas. Bell, A.I.E.E., Certificated Teacher City acd Guilds of London Institute. A list of subiects in wticli, by tlie Postmaster-General's ■direction, telegraphists who are recommended for pro- motion, will, in future, be examined, was printed in the columns of Electeicitt, (Vol viii., 'So. 3) last July. The present is the first of a short series of papers written with the view of assisting those affected, in acquiring the necessary knowledge to enable them to qualify in these subjects. As the papers will be of an ■essentially practical character, theoretical points will not be touched upon — these being fully discussed in the many text books published on the Science of Electricity. At the same time it is hoped that much of what appears may prove of interest to many of our readers, who, tilthough not directly connected with the telegraph service desire to extend their knowledge of telegraph matters. The subjects will be considered in the order in which they are laid down by the Postmaster- General. 1. Crossing and looping wires with facility and certainty. In all telegraph offices of any importance the line wires are not led direct to the instruments, but are connected to brass terminals or binding screws, which are fitted on to •a wooden framework, technically called the test-box. The object of this test-box is to facilitate the opera- tions of crossing, looping, extending wires, &c. The arrangement shown is the one generally adopted, and the young telegraphist usually regards it with something like a feeling of veneration, there being a supposed certain mysteriousness connected with "the test-box." To ci'oss vrires is simply to interchange them. Thus, wh n a fault appears on a busy circuit—such as a Wheatstone •or Duplex — a good wire on a less important circuit 4 running between the two stations can be substituted: for the faulty one. No. 2, Fig 1, will show how this is done. Wires are looped, when it is desired t& dispense with the earth return, a metallic circuit behig thus formed. This of course can only be done when there is more than one wire connecting the two stations. Looping is also resorted to when earth currents are prevalent, and are of such strength as to interfere with the working of the wires. These earth currents sometimes attain considerable power ; the writer has registered currents of over 25 milliamperes, which is strong enough to actuate most telegraph instruments. A faulty wire is also sometimes looped to a perfect one, when it is required to ascertain the distance of the fault from the testing station. Examples 3 and 4, Pig. 1, show how two wires are looped. Although not expressly stated in the Postmaster- General's order, the general management of the test-box will no doubt form part of the telegraphist's future examination, while a knowledge of the arrange- ment will be of present service to him. Accordingly,, the following particulars will be useful : — The test-box is usually divided into two parts by a wooden ledge — the upper portion being reserved for the line wires, while, to the lower half, the sending battery wires are con- nected before they are led to the instruments. The dark terminals on the line-box are joined together at the back by a brass strip which is connected with the earth. The " down," or north line wires, occupy the upper part of the line-box, while the south, or " up " line wires are led to the lower row of terminals. But this order may be reversed at some stations. In the centre of the box is fixed a four-plug switch, the brass plates of which are connected as shown. A detector, or ordinary galvanometer, rests on the top of the box. The dotted lines show the internal connections. No. 1 gives the normal position of the wires. The line wire is soldered to the back of tha upper terminal, which is connected in front by a brasa strip to the " to instrument " terminal, from which a wire goes to the instruments, returning to the " from instru- ment" terminal, and thence to "earth." No. 2 shows o o o tow to cross two wires, No. 3 how to loop two wires (for testing purposes), and No. 4 how to loop two wires, •allowing the signalling apparatus to remain in circuit. This also shows how a wire may be extended to a more ■distant station — the office which makes the extension TO 4EN0 A z;«c C u R R E NT,. TO TEST roKi CONTUCT, To KCCCIVE A' CURRENT.' Pig. 2. ramaining in circuit, and thus becoming an intermediate instead of a terminal station. Example 5 shows how to disconnect, and Example 6 how to earth a wire. No. 7 is a, " through " wire, and No. 8 shows how to come in 6 circuit on such a wire. Wlien signals are received reversed, the test-box connections should be altered as shown in Examples 9 and 10 for down and up lines respectively. When it is desired to con- nect a "down" line to an "up" set of apparatus, the wires are joined as shown in No. 11, while JNos. 1^ and 13 illustrate how an "up" line should be connected with a " down " set. No. 14 is an Example of how a wire may be forked— that is, instead of extending a wire to another station as in example 4 the earth con- nection may be allowed to remain, and the two line wires connected as shown. By this plan the total resistance is much less, as compared with the other method. Example 15 will serve to show how a line wire may be discon- nected from its ordinary apparatus, and joined, say, to a Wheatstone or Duplex set. Nos. 16 and 19 illustrate how a current on the line may be neutralised. When a " switch " is accidentally turned to " send," a cun-ent To mc'j A from the similar pole of the battery, and of about equal strength to the switch current, should be sent to line. Communication can thus be preserved between the other stations in circuit. The diagram shows the connections for neutralising a switch current coming from a " down " station. No. 17 shows the normal position of the sending battery and instrument connections. To increase battery power, connect as shown in No. 18, while to change the battery entirely join up as shown in No. 20. The accompanying sketch (Fig. 2) shows a neat com- bination of test-box galvanometer and plug switch in use at certain large offices. The small black circles indicate the holes in which the plugs should be inserted when sending or receiving a current, or when testing for contact. To send or receive a current the line wire is connected to the right hand front terminal ; when testing for contact one line should be joined to the right hand and one to the left hand terminal. The position of the plugs in the form of switch, sketched in Pig. 1 (test-box), is shown in Fig. .3. 2. Teacing and Localising ITaults in Insteuments. A common fault in all telegraph instruments is discon- nection or loss of continuity. The needle of the galvano- meter remains unaffected when the single current Morse key is depressed, or when the switch of a double current key is put to " Send." The various terminals should be tightly screwed up, as it frequently happens that the loss of continuity is caused by a loose or imperfect connection The batteries should next be proved by inserting a detector between its poles, or between the terminals Z and C of the key. If, after being satisfied that the battery is in good order, the galvanometer needle is still unresponsive, a current should be sent through the instrument from the test box by inserting pegs in the plug-switch, as shown in Fig. 3, and touching the "To Inst." terminal with the test wire. Should the test-box galvanometer needle not move, this would point either to the fault being in the signalling apparatus or in the wires between it and the test-box. After removing the test wire the apparatus should be "short-circuited" by connecting, with a piece of good wire, the line terminal of the galvanometer and the middle back terminal of the key in the case of a D.C. circuit, and the line terminal of the galvanometer and the back terminal of the key in the case of a S.C. circuit (to short-circuit Single Needle and A.B.C. apparatus connect the line and earth terminals). If the disconnection is not in the instruments the galvanometer needle should now be deflected. But if the needle remains stationary, then, with the S.C. key still depressed, or the D.C. switch to "send," and with a good galvanometer in circuit, touch, in suc- cession, the several terminals until the testing galvano- meter needle is deflected, which will be the case when the faulty portion is bridged over. Thus, suppose there is a broken wire inside the relay (R), touch the binding screw U with the testing wire (T) — the needle will be unaffected, Now touch D and it will instantly respond, because the broken part has been cut out of circuit. Dirt on the contact points of the key is a frequent cause of disconnection. This is particularly the case with the front and back stops of the single current key. The contact points of all instruments should be kept scrupulously clean — a film of dust having been known tc temporarily break down a circuit. When the galvanometer needle is deflected more strongly than usual it points to an " earth " fault. To prove whether it is in the office or on the line the " to instrument " wire should be disconnected at the test-box. If on depressing the key the galvanometer still moves the fault is between the apparatus and test-box. It can be localised by breaking contact at the several points until the "earth" is passed, which will be indicated by the 9 Eeedle ceasing to move. The fault will sometimes be found in tlie lightning protector, which can be proved by removing the wire from, the upper disc, thus cutting the protector out. 3. Tracing and localising permanent and intermittent earth, contact, and discormection faults on wires. Eaeth. — First make certain that the fault is not in the office by testing the apparatus in the manner described in the last paper. If the galvanometer moves only a little stronger than usual, the fault will be at or near the dis- tant station. If, on the other hand, the needle is power- fally deflected, the fault will be near the home office. Supposing ourselves at Station A, and that the fault is between B and C, ask the nearest station on the line (B), to disconnectthe wire. If no deflection is obtained on the testing galvanometer when a current is sent (see diagram Fic S 3), the fault is beyond B. After B joins up, request C to disconnect. If now the needle is deflected, the fault will be between B and 0. If a good wire between B and C is available it should be crossed with the faulty length. (See first paper.) Contact. — When two wires leading into the same office are in contact one of the instrument wires should be disconnected from its line wire at the test-box, and the key of either instrument worked up and down. If the instrument connected with the other wire is afl'ected, the contact will be in the office, and can be traced in the same way as for earth fault described in last paper. 10 Suppose, however, the contact is on the line, say between C and D, Tig 6. Insert plug in test-box switch for receiving a current (diagram 3), request station B to disconnect the_ wires. Now send a current on the other line by depressing the instrument key — the galvanometer at A will not be deflected, thus showing the fault is beyond B. Aftei B joins up instruct to disconnect. The resiilt will be the same, but when D disconnects, the current will be received at A, showing the fault to be between C and D. When the contact is with an unknown wire, treat it as an earth fault, and test accordingly. To test for the locality of a contact with an ordinary lineman's detector and battery (a dry one will suit very well, and is very convenient) connect as shown in Fig 7. Dumur tti»t OisronNccrcD DiscoxxECTiON. — In this case the wire is put to earth at the various testing points, and a current is sent through the testing galvanometer, which will be deflected until the fault is passed. Instruct B to " earth " the wire ; the galvanometer needle will be deflected. The same result 11 will follow when C puts the wire to earth ; but when D " earths " the wire the needle will remain at zero, thus proving that the disconnection is between C and D. p— the second case, the differential Galvanometer, although well adapted for use in connection with the balancing of a duplex circuit, may not be sufficiently sensitive for testing purposes. £i.£.cr/r/c/rr Fic.54. The early form of the Wheatstone Bridge is the well- known four-sided figure shown below. A B C D are- called the Anns, Sides, or Branches of the Bridge. A and C are also spoken of as the Ratios and B as the Rheostat. "When the resistance of each arm is the same- no current will flow through the cross wire, and the needle- vsrill be undeflected. Suppose a current starts from the- positive pole of the Battery and all the arms are of •equal resistance, then when it reaches point 1 it wUJi 44 •divide, one half going by A 3 B to 2, and the other half l)y way of C 4 D to 2 where they both meet and pass on "to the Z or negative pole of the Battery. The needle will Tiot be affected because the potential at points 3 and 4 is the same, and without a difference of potential there can be no current. If, now say 10 ohms be inserted in A and the same in 0, and if 40 ohms each be placed in B and D the current will still equally divide at point 1 because the total resistance of A and B is equal to the total resistance ■of C and D, and consequently there will be no difference- ■of potential between 3 and 4, and the needle will not be moved. When A bears the same proportion to B that C •does to D, or when A is to C as B is to D, or when A^ multiplied by D is equal to B multiplied by C no current "will flow between 3 and 4. If we know three of the resist- £i,£crjr/c/rr A C\ lO OO _ £00 fOO ^Xf Jg ' ^5oo ■»^ ^ooo 2000 ^coo "-^coo O^ nc.36. unces, the resistance of the fourth is easily found. For example, let the unknown resistance be placed in D and 'unplug 10 ohms in A and the same in C, then, if when say 50 ohms are unplugged in B the needle comes to zero, the resistance in D is also 50 ohms, thus — 10 : 10 :: 50 : x. We may now proceed to the modem form of the Bridge and show how it is used in actual practice. The P.O. ^pattern is sketched in Fig. 35. The figures indicate the value in ohms of the various f esistance coUs. " If the resistance in A C is equal to that -in A A the resistance in B D measures that of the Kae. 45 If that in A A is ten times or one-tenth that in A C the- resistance in B D -will correspondingly be ten times or one- tenth that of the line." Let it he desired to measure the resistance of a Morse Sounder, Galvanometer, or other- instrument. Attach one of the terminals to L (Fig. 35), and the other to E. Make the ratios A A and A C equal — say 10 ohms each. "Now vary resistance in B D until the needle stands at zero. If 20 ohms have been unplugged in B D the resistance of the Sounder is the same. But suppose that -when 20 ohms are unplugged the needle is- deflected a degree or two to the right of zero, and that 19 ohms causes a deflection the other -way, then the- resistance of the Sounder -will be between 19 and 20 ohms, and the ratios A A and A C must be adjusted for fractions. Withdraw the lO-ohm plug in C and 100 ohms in A. If /"/C 56 a balance is no-w obtained -when 192 ohms aie unpluggeJ in B D then the resistance of the Sounder -will be 19'2, f or- 100 : 10:: 192 : x. This is the plan adopted -when very small resistances have to be measured. If the resistance- of a short piece of wire is to be measured, and with ratio* as in the last example, a balance is obtained when 5 ohms are unplugged in B D, then the resistance of the wire will be -5 or half an ohm; thus 100 : 10::5 : x. With l.OCO' ohms in A A and 10 ohms in A C the resistance would be. ■05 of an ohm. CONDTJCTIVITT OE "WiEE EeSISTAKCE TeST. This test, which is for the purpose of detecting incipient faults in the continuity of the line -wire, is made at least ■once a month. The distant station is requested to pat the -wire to he tested to "earth" for a few minutes. As the wire resistance is usually well within the range of the Rheostat B D the ratios A and C may be -equal — say 100 ohms. The connections for this test are shown in Fig. 36. The test is invariahly made with hoth positive and negative currents, and the mean of hoth tests is then ^^aken thus — ^—— where R, is the measurement ■obtained when a current from the zinc pole of the battery is sent to line ; and Bg the positive current measurement. The resistance per mile is found by dividing the total resistance by the length in miles of the wire tested. Thus SI telegraph line, twenty -one miles long, offered a total resistance of 230 ohms, when a negative current was sent to line, and with a positive currrent 240 ohms. The mean— 235 (^§30 + 240 ^ 335) divided by 21 gives the resistance per mile as 11'19 ohms. A reversing ■switch is placed at the end of the bridge, and the posi- tion of the plugs for making the necessary connections ^th the battery is indicated in Fig. 37. C TOUf/£ ZrOUNC nc.37. ^s the earth nsually offers resistance, and earth currents also being sometimes present, it is better in making the ■conductivity test to dispense with the earth entirely, and where a second wire is available a metallic circuit may be formed by requesting the distant station to loop the wires. The second wire is then joined to D. Assuming that the 47 ttickness and route of tlie two wires are identical tlie resistance of each wire will be equal to the total resistance •of the loop, divided by two — e.g., if the mean total resistance be equal to 600 ohms, the resistance of each wire will be 250 ohms. If, however, the wires do not follow the same route and are of different gauges, then, if a third wire can be obtained, proceed as follows in order to ascertain their individual resistance. Ask the distant station to loop Nos. 1 and 2. Note the resistance unplugged in B D vo obtain a balance — say, 490 ohms (a). Next request Nos. 1 and 3 to be looped — say the resistance is 500 (b). Finally get Nos. 2 and 3 looped, and let the resistai^ce be 510 (c). This gives a total resistance for the three tests of 1,500 ohms. Dividing this total by two, we get 750 ohms, and subtracting from this the last measure- ment of the three tests — 510, the result (240) gives the resistance of No. 1 wire. The resistance of wire No. 2 is found by deducting 500 (b) (the second of the three tests) from 750, thus giving :250 ohms as the resistance of No. 2. Similarly by subtracting 490 (a) (the first of the three tests) from 750 the resistance of No. 3 will be 260 ohms. The algebraic formula may be expressed thus : — Ri = 2+1+^ _ c=^240 ohms Rj == a+l+o _ j^250 ohms R3 = 1±t±£, - a=260 ohms or, El = a + ^-o ^ 240 ohms E^ = «-+<'-^ = 250 ohms E,, — _ ^+c- .«' =3 260 ohms 48 The IjtJsuLATioN Resistance Test. THs test is also made once a month. By its means' the amount of leakage from a wire to earth at the various points of support may he determined. In wet weather there is, of course, more loss of current at the poles than in dry weather. The connections are the same as in the test for conductivity (Fig. 36), but instead of putting the wire to earth the distant station should, disconnect the wire. The zinc pole of the hattery- ehould always be joined to K, in making this test, because this tends to increase the leakage, whereas a current sent from the copper pole would tend to diminish it, and so a- slight fault might be passed over. The resistances in A and will require to be in the ratio of 10 to 1,L00 as the- insulation resistance of a wire is generally much greater than the total value (11 110 ohms) of the resistances in B D. Thus, suppose a balance is obtained when the- resistance unplugged in B D is 10,000 ohms then the insulation resistance would be one megohm, or one million ohms, for 10 : 1,000 : : 10,000 : 1,000,000. To obtain the insulation resistance per mile, this total is multiplied by the length in miles of the wire tested. It should be kept in mind that the higher the resistance \o be measured the greater should be the resistances in the ratios A and C. If the insulation resistance of a wire is expected to be under 11,000 ohms the ratios may be made equal — 1,000 ohms in each. If it be over 11,000 ohms, then 100 ohms in A and 1,000 ohms in C should be inserted. With very high resistance to be measured, 10 ohms in A and 1,000 in should be used. In making any test with the bridge, the battery key (K,) should be first depressed and short contacts made with the galvano- meter key (K) until, by adjusting the coilS in the Rheostat (B D), the needle is brought to zero. To' prevent the galvanometer being short circuited it is- necessary to have some resistance always in A and 0. A test may be unreliable if the current from the battery is applied too long, because the resistance of the coils wiU be increased through the heat thus generated. Testing for the Distance of Faults. Disconnection. — It is not possible to measure the dis- tance of a disconnection from the testing station. It can 49 only be estimated by sending a powerful current to line by depressing the Key (Fig. 38), and then allowing the wire to discharge itself through the galvanometer. Note the deflection, and compare it with the reading obtained when the wire is in its normal condition. This method is not of much practical account, and can only be r(i> 1 9 / ^/c. js. made in very fine weather, when the leakage from the wire is at a minimum. Marth. — If it is a " dead " or perfect earth, having no resistance, join up the bridge as for a conductivity test (Fig. 36), and the distance of the fault will be at once r/c. S9. found by dividing the resistance unplugged in B D by the resistance per mile of the wire under test. Say the needle is balanced with 100 ohms in the Rheostat B D, 50 then taking 10 ohms as the resistance per mile, the distance of the fault will be 10 miles from the testing point. The Loop Test.— As. has already been stated, the earth always offers some resistance, and it is therefore better, in testing for the distance of an earth faidt, to get the faulty wire looped to a good one at the distant station. Two tests are made, the bridge being connected for the first test, as shown in Tig. 39. Let the resistance unplugged be called A. For the second test connect the bridge as in Pig 40. A C" ^, INb ~ LINE t 1 1 1 '. D 1 '^ ° « LINE ^ f / y .J 1 3 £i,eCTfrtc*T9 f/c. 40. Call the second measurement B, then A — B = R = the total resistance to the fault. Dividing this in the usual way by the resistance per mile, we get the distance in miles to the fault. Suppose, in the first test 420 ohms are unplugged, and in the second test 250 ohms, then, deducting 250 from 420, and dividing the result by 2, we get the resistance to the fault, and this, divided by the resistance per mile, gives the distance of the fault in miles from the testing-station. Contact. — Two wires in metallic contact really form a loop, the resistance of which can be measured by the bridge iu the ordinary way. The distance of the contact from the testing office will be haK of the total resistance divided by the resistance per mile of the wires. 51 Testing Earths. It is important that the connections with the earth at terminal offices should be perfect, and that the earth should offer as small a resistance as possible. The following, known as Pomeroy's method, is usually adopted for testing the resistance of an " earth." Join up the Bridge, according to Fig. 41. Line 1 is a wire (any line- wire will do) which is put to earth at the distant station, and E is the " earth," whose resistance is to be measured. Make the ratios A and equal. Call R the resistance unplugged in the Rheostat to obtain equi- librium. Now reverse the Battery and connect the Bridge, as shown in Eig. 42 — the zinc pole of the Battery f/c. -f/. being joined to a second line wire running at about right angles to Line 1. The second wire is also put to earth at the distant end. A second balance is now obtained. Call this R^. The resistance of the earth at E will then be half the difference between the two measurements, or R — Rt _ 90 — 70 _ ,Q 2 ~" 2 ~ The resistance of a good earth should never be higher than 10 ohms. Battery Testing. To find the internal resistance of a Battery by means of the Wheatstone Bridge, join up as in Fig. 43. The Half Deflection method is the simplest, and has already been explained in Article IV., where an ordinary Rheostat is substituted for the Bridge. If the astatic galvanometer 52 which is generally used with the Bridge, is employed in making the test, its resistance (1,000 ohms) should he deducted from the extra resistance inserted in B D to f/G. ^2. halve the deflection. The wires attached to the astotic galvanometer may, however, be detached and connected with the short coil of the Tangent Galvanometer, in which case the resistance unplugged in B D in order to halve the deflection will he equal to the internal resistance of the Battery under test thus — R = Ri where R is the C'l.e.CTmictT f/c. 43. internal resistance of the Battery and R^ the added resistance. The E.M.F. of a Battery is tested as described in Article IV. (equal deflection method), the connections for the Bridge being the same as in last test, Fig. 43. 53 All the principal tests in which the Wheatstone Bridge is employed have now been enumerated. Those readers who are interested in the theoretical and mathematical parts of the subject are referred to Kempe's "Electrical Testing," which may be obtained from the publisher of this book. A convenient and inexpensive form of Bridge is The Metre Bridge, which may be constructed by any one willing to take the trouble. The instrument takes its name from the French f/c. 44, metre = 39'37 inches. Procure a piece of seasoned pine or other suitable wood, 30iu. long, Sin. wide, and about half an inch in thickness. Cut from a sheet of thin copper or brass two pieces shaped like A A and E E, each 2in. wide. Fasten them to the base with small brass pins, in the position shown in Fig. 44. Nail a similar strip of the same width between the short arms of A A and E E, leaving a space on either side of about 2|in. Now stretch a length of German silver wire (copper wire may be used if the Grerman silver wire cannot con- veniently be obtained) between the long arms of A A and E E. Gum below the wire a strip of paper of the same length and an inch wide, marked off into suitable divisions to the right and left of zero, which should be exactly in the centre. A movable binding screw, or small 54 piece of "wood (G), with a contact spring and terminal attached to it, slides along the wire, and is termed the slider. Small binding screws are next fixed in the positions shown hj the small circles, and the Ijistrmnent is complete. The connections with the Buttery and Galvanometer are as indicated in the diagram, the known resistance being inserted at B, and the unknown at D, the ratios being to the right and left of the slider G. Sometimes the Battery and the Galvanometer (which need not be graduated) are transposed. The original form of the Bridge may be reproduced by screwing into a piece of wood about 18in. square a number of terminals (say 8), as shown in Fig. 45. This is a useful form for measuring small resistances. The letters and figures correspond to those of Fig. 34. The known resistance is inserted at B, and the wire or apparatus whose resistance is required is placed in the arm, D. The ratios A and C are of equal resistance, and, as in the original Bridge, cannot be varied, hence only comparatively small resistances can be measured. APPENDIX. 56 Appendix to Batteries. Dry cells, wliich were brought into prominence by Dr. Gassner, possess many practical advantages over the ordinary wet varieties. They are now pretty extensively used in the Postal Telegraph Service, both for testing and signalling purposes. They are employed in the new system of morning testing, and for temporary use in connection with specially made-up circuits at race, political, or other meetings, they are invaluable on account of their compactness, portability, unbreakable- ness, and cleanliness. They are always ready for use, and may be placed in any position, as their contents will not spill, even when they are placed upside down. The term " dry cell " is, of course, a misnomer, as without M-- moisture of some kind the chemical action necessaiy for the generation of a current could not take place. A great many varieties of these cells are now in the market. Some give every satisfaction, others are not so good, and are quickly exhausted. Speaking generally, however, their behaviour has been found to be about equal to the ordinary form of Leclanche, of which they are but a modification. The above sectional drawing will explain their construction. 67 The containing vessel (Z) is a box or canister about 7in. high and 2fin. in diameter. It is made of zinc and forms the positive element. The interior of this zinc box is coated to the thickness of about ^in. with a mixture of stucco or plaster of Paris, 25 parts ; sal ammoniac 10 parts, and water 55 parts (L). The zinc canister is then packed with a mixture of crushed carbon, 75 parts ; manganese dioxide, 10 parts ; zinc chloride, 5 parts ; sal ammoniac, 10 parts, and glycerine, 2 parts — the whole being made into a stiff paste by the addition ,of water. This mixture is denoted by M. In the centre of the packed mass is inserted a carbon plate (C), the top being sur- mounted by a binding screw and connecting-wire (P). A copper wire (N) is then soldered to the outside of the zinc canister, the top of which is sealed with pitch (S). If desired, the zinc chloride and glycerine may be omitted. The current obtained from a cell of this size is sufficient to operate a Morse Sounder or ring an alarm. A cell something similar to this has been in the writer's possesion for nearly two years, and is quite as efficient now as when it was made. The great disadvantage of the ordinary sealed-up dry cell is that ■ it cannot be renovated. When the contents become dry the cell is useless. It is true it may be recharged by connecting the terminals with a dynamo or primary Battery, as will be explained further on, but the result cannot be said to be wholly satisfactory. To meet this objection the Leclanche-Barbier patent dry cells have been invented. They are most efficient in action, and when exhausted can be renewed by withdrawing a plug and pouring in a solution of sal ammoniac, when, after allowing the cell time to absorb the liquid and pour- ing off all excess, it will be again in good working order. In the Post Office the dry cells (of which various makes have been tried), are placed in boxes in sets of ten, and, owing to their low internal resistance, the current is discharged through a small 10 ohm. resistance block placed inside the boxes. The E.M.F. of the cells is about I'5 volts and the internal resistance varies from 0'5 to 0'6 ohm. They should be employed for the same kind of work as ' '' the Leclanche, and not where continuous currents are wanted. 58 Secondary oe Storage Batteries are now being made use of in telegraphy to a large extent, and in those offices which are favoured with the electric light, the adoption of accumulators is only a question of time, as they can be conveniently and economi- cally charged from the electric lighting mains. They Lave been employed in the Berlin Central telegraph office for more than two years with the most gratifying results, both from a financial and telegraphic point of view. Their internal resistance being very low, and the current derived from them being of uniform strength, they are specially suited for working circuits on the Universal Battery system, which will be described in a subsequent paper. In London they are used on the Continental lines to great advantage. In America, at all the chief offices, the circuits are supplied with current direct from the electric light mains, or from dynamos on the premises so that batteries — whether primary or secondary — ^will soon be things of the past. It is stated that in New York from thirty to forty thousand cells have been superseded by dynamos, and at Boston the current is derived from the electric light mains, at a cost of only £600 a year, as compared with £4,000 the former annual cost of maintain- ing 10,000 cells. The Western Union Company in New York employ 51 small dynamos, and in Chicago 46, for the supply of current to the various circuits. In this country, however, the telegfraph lines cannot be worked direct from dynamos, as the " open circuit " system is universally employed — that is, the circuit is only closed during the transmission of a message ; whereas in America and some other parts of the world, the " closed circuit " system is used — that is, the current is always flowing, and is only inteiTupted when a telegram is being transmitted. Figs. 47 and 48 will illustrate the difference between the two kinds of circuits. To English minds the American plan appears to be wasteful, but apparently it is the method best adapted for America, just as the open circuit has been proved to be the most advantageous system for this country. Another objection to the use of the Dynamo is that it is unsuitable for Wheatstone circuits on account of the variation of the current which, of course, 59 is detrimental to high speed working. On the whole everything points to the Aocuranlator or Secondary- Battery as being the Battery of the future — at least, at all offices where a connection with the electric light mains can be made. OPEN f/c ^7 The term "Accumulator" is largely responsible for making many people suppose that electricity can be stored up in much the same way as gas is stored up in a gas- holder and drawn upon when required. But it is really chemical energy — and not electricity- — that is stored up when a secondary cell (a better term than accumulator) is charged. A simple form of cell may be made by sus- CL OS ED r/c. 48 pending two large strips or plates of lead into a vessel containing dilute sulphuric acid — 10 parts of water to one part of acid. By connecting the strips with a battery of two or three Bichromate, or 4 Daniell cells, and allowing the current to pass through the arrangement for 10 or 15 60 minutes, and then detacliing it from the Battery, it will give out a current immediately the two strips are con- nected. This can be readi?y proved by inserting a galva- nometer in circuit. The cell is improved by being repeatedly charged first in one direction and then in the other, the surface of the plates being gradually rendered more spongy or porous, a condition which is spoken of as forming. The oxygen in the liquid attacks the positive plate, forming on its surface lead-dioxide (P6O2). The negative plate remains almost unaltered — the hydrogen which is given off there not combining with it. The action is symbolically represented thus : — P6 + 2 H2O + 2 H2SO4 + Fb = P6O2 + 2H2 SO4 + 2Hs + P6. During the discharge of the cell, some of the oxygen in the lead dioxide (P6O2) goes to the negative plate, and forms lead monoxide, or litharge (P60). This action goes on until both plates are coated with lead monoxide, when the cell ceases to give out current, or is dis- charged as it is called. On again charging the cell, the oxygen leaves the negative plate, and the positive plate is again coated with lead dioxide. The negative plate is rendered slightly porous, or convei'ted into spongy lead. The Faure Accumulator, from which all other varieties have been derived, in constructed as follows : — Two sheets of lead, each about 3ft. long. Gin. wide, and x^in. thick, are smeared with a paste of red lead (PfesO^) and sul- phuric acid. The sheets are insulated from each other by two or three strips of gutta-percha a little longer than the sheets, but only ^in. wide. The sheets are then rolled into a tight spiral, and immersed in a jar containing dilute sulphuric acid. A terminal is attached to one end of each sheet. Treating the plates in the way described enables the accumulator to be used after it has been charged two or three times in opposite ways. Otherwise, when " forming " is produced electrically instead of artificially, the cell would require to be charged and discharged for 14 days or so before it could be utilised. A useful accumulator may be made with the lead which covers the interior of tea-boxes. Cut the lead into four 6x8 inch plates, roughen their surface, and smear them 61 over witli a paste of red lead mixed with water. When dry insulate the plates with strips of muslin or other suitable material ; place the whole in a water-tight box containing dilute sulphuric acid, and charge the cell from three or four Daniells for some hours ; then discharge it through an exterior resistance of about 10 ohms. Repeat this for a few days until it is capable of acquiring a full charge. Thn E.M.F. of a secondary cell averages about two volts, while the internal resistance is less than an ohm. By multiplying together the current given by a cell, and the number of hours during which it flows, the capacity of a secondary cell is obtained — this capacity being measured in ampere-hours. Thus a cell having a capacity of 60 ampere-hours would furnish a current of 1 ampere for 60 hours, 2 amperes for 30 hours, 3 amperes for 20 hours, 4 amperes for 15 hours and so on. The cells may be charged either in " Series," or in " Quantity," and the charging current should never be less than i\ volts per cell in order to produce the necessary chemical changes which must take place during the process of charging. When exhausted, dry cells, like accumulators, may be recharged by connect- ing the positive pole of the cells to the positive pole of a dynamo or a primary battery, the negative poles being also joined together. GrEOUPING OF CeLLS. In order that the best results may be obtained from a battery, a knowledge of the different methods of grouping and arranging the cells is necessary. Cells may be con- nected together to form a battery in various ways, the plan adopted in any case being dependent upon the exterior resistance of the circuit ; where the external resistance is low as compared with the internal resistance of the cells, they should be joined in parallel, abreast, or quantity — that is, the copper or carbon plate of one cell should be connected with the copper or carbon of the next cell, and so on throughout the series. The zinc plates are likewise joined one to another. This virtually forms one large cell. Where the external resistance is high sm compared with the internal resistance, it is best to join 62 up the cells in " series " — that is, the positive pole of one cell is connected with the negative pole of the neighbour- ing cell, and so on. Then, both the " series " and " quan- tity " arrangements may be combined, some cells being joined in series, and some in quantity, to make up one battery. By Ohm's law we can calculate the strength of current obtainable from cells joined up according to one or other of these groupings. This law teaches that " the strength of current is directly proportional to the E.M.F., and inversely proportional to the resistance of the circuit," and is expressed thus C = — where C is the current in amperes, E the electromotive force in volts and R the resistance in ohms. It may also be written C R = E — "the electromotive force is proportionate to the product of the current strength and the resistance " — for example, a Daniell cell has an E.M.P. of 1'07 volts and a total resistance of 10 ohms, what will be the strength of current ? V 1"07 — :^ C or — — = 'lO? ampere, or 107 milliamperes. E 1"5 A Leclanche cell = = C'5 ampere, or 500 ma. R 3 E 2 Similarly a Bichromate cell := ,^-— = 1 ampere or 1,000 R A ma. The E.M.F., or difference of potential, is only affected by the number of cells connected in series, and not by the size of the plates. On the other hand the internal resist- ance of a cell is dependent upon the size of the plates. Suppose a Battery consisting of 8 cells is joined up in quan- tity the result would be equal to having one cell 8 times as large as anyone of the single cells. The E.M.F. of the Battery would be the same as that of one cell, but the internal resist- ance of the Battery would be one-eighth of the resistance of a single cell. Example. — ^A Daniell cell has an E.M.F. of 1 volt, and an internal resistance of 2'8 ohms. When six of these are joined up in quantity through an external resistance of half an ohm, what will be the strength of current ? C =^ = 2^ , . 1 , ^^ R -g- + o = -Q^ ^ 1'04 ampere. 63 In the third grouping of cells,' where some are joined in quantity and some in series to form a Battery, the E.M.F. of such a Battery is found by multiplying the E.M.F. of one cell by the number of cells joined in series. The internal resistance of the Battery is obtained by multiplying the Fic.49. resistance of one cell by the number of cells joined in series, and dividing the result by the number of cells Q w "D joined in quantity — or R' ^ — ^r — where R'' is the resistance of the Battery, S the number of cells in series, R the resistance per cell, and Q the number of cells in Quantity. Example. — " What would be the total E.M.F. and the total resistance of six Daniell cells joined 3 in series and 2 in quantity — (see Fig. 49) — the E.M.F. per cell = 1 volt and resistance per cell = 5 ohms " (CandGr Exam). Here total E.M.F. 3x1^3 volts and total resistance 3x5 ^15 ^- 2 = 7"5 ohms. The current strength in amperes is obtained in the ordinary way by dividing the E.M.F. by the resistance — external and internal — of the circuit. Thus, " A battery of 30 Daniell cells arranged 10 in series, and 3 in quantity, with an external resist- ance of 20 ohms — the internal resistance per cell being 6 ohms, and the E.M.F. per cell 1 volt, what will be the strength of the current ? " p,^ E _ 1 X 10 = 10 E ~E ~ 6"x~10"^30"R 10 E .„K 3 =20+20=40^= 25 ampere, or 250 ma. The maximum current from a battery is obtained when the internal and external resistances are as nearly as possible equal. Thus " 8 cells having a resistance per 64 cell of 4 ohms, and each cell an B.M.F. of 1 volt, the external resistance being 2 ohms, from -which of the following methods of joining up would the strongest cuiTcnt be obtained — (1) When the 8 cells are joined in series ; (2) 4 in series and 2 in quantity ; (3) 2 in series and 4 in quantity ; (4) when they are all joined in quantity ? " The No. 3 arrangement would give the maximum current because the internal resistance and external 2 resistance are the same — = '50 ampere. The 2 + 2 ^ Q other examples are worked thus (1) -^^ ="235 amp. o2 -\- 2 Ohm's law enables us to determime the number of cells necessary to work a telegraph circuit when it is known what current strength is needed to operate the instru- ments, and what the total resistance of the circuit amounts to. The current required by various intruments has already been given in these papers, and in some cases also their resistances. The following question will illustrate this point — "Having given the length of a telegraph line, the resistance per mile of the wire, the resistance of the apparatus in circuit, and the number of milliamperes of current required to work the instru- ment, how would you calculate the number of Daniell cells required to work the line allowing for a 60 per cent, loss of current through insulation leakage ? The resistance of the Battery cells may be assumed to be negligible " (C. and G-. Exam. 1893). Let the length of the line be 40 miles, the resistance per mile be 20 ohms, and the resistance of the apparatus 200 ohms = 1,000 ohms. Let the number of milliamperes required be 15 (which is the current allowed for Single Needles and Relays). A Daniell cell on a working circuit has an average E.M.P. of 1 volt — therefore, lj[S^ = 1,000 ohms 1 milliampere, but 15 ma. are required, therefore, 15 65 Daniell cells would give tte necessary current, but allow-' ing for a 60 per cent, loss of current 22 or 23 cells would be provided. The TJniveesal Battery System is extensively adopted in the telegraph, service — both postal and railway — of this country. As one Battery serves several circuits, a considerable saving in money and ElECrfK-'rr plant is effected. At the Central Telegraph Office, London, as many as 120 Single Needle circuits are worked from one double set of eighteen cells. In the case of long and busy circuits, however, this number is very con- siderably reduced. Circuits worked from one Battery ^i^Mcnnotrf Fig. si. should be as nearly as possible of the same resistance. If the resistances vary more than 25 per cent., equalising resistances are inserted in the Battery leads, as shown in Fig. 50, where R is the artificial resistance. The E 66 Batteries employed should be large Daniells or Bicliro- mates, or preferably accumtdators, owing to their low resistance and constancy of electromotive force. The rule for finding the Battery power to use, or which form of Battery is best suited for single current groups, is — " Divide the resistance of the circuit which has the greatest resistance in the group by the number of circuits ; then the resistance of the Battery to work the group must always be less than the result of such division." RELAtr FiC. 52. Thus, suppose there are five circuits worked from one Battery, the circuit of highest resistance being 1,400 ohms, then the resistance of the Battery should be under 280 ohms — 1,400-5-5=280. A large Daniell cell averages 5 ohms per cell ; therefore 50 of these cells, giving a total resistance of 260 ohms, should be used. Pig. 51 illus- trates the grouping of single current circuits. For " Down " Station instruments the Relay and Battery connections should be reversed. In the case of double current circuits, the Battery is "earthed" in the centre, or rather a Doable Battery is used. Both "Up" and "Down" Stations may be grouped together, but for a " down " station the Relay 67 and Battery connections are reversed. Fig. 52 shows the connections for an Up Station instrument. At Intermediate Stations, where the Down line is longer than the Up line, a resistance coil is introduced between U of the Key and U of the Relay. This resist- ance should be equal to the difference between the resistances.of the Up and Down sections. When the Up Down Fic. 53. line is longer than the Down line, the Up line is connected to D of the Key ; the Battery connections are reversed ; the middle back terminal of the Key is connected to U of the Relay ; the equalising resistance coil is placed between U of Key and D of the Relay ; and the " Down" line con- nected to D of the relay. Single Needle instruments are connected as shown in Fig. 53, but the Coil and Battery connections should be 68 reversed for a " Down " Station. The Batteries employed in universal working require to be carefally attended to, and should be kept in a high state of efficiency, because a failure in the Battery means the stoppage of all the circuits grouped together. Direct Reading Battery Testing Instrument. It has been mentioned that at certain large offices there had been introduced an instrument by means of which the fall of E.M.P. and resistance per cell could be obtained without calculation or reference to tables. The latest form of instrument, known as "Battery Testing Instrument B," is illustrated in Pig. 54. It is the inven- tion of Mr. Eden, of the Electrician's department, and is believed to be theoretically as well as practically accurate. The galvanometer used is the new double-wound Tangent Galvanometer, which will be described hereafter. The Battery Instrument consists of two Rheostats, B and R, the figures surrounding the former indicating ohms, and those on R representing the number of cells under test. P is a plunger Switch which is de- pressed momentarily while the resistance of a Battery is being measured. The Galvanometer Switch S is turned to DAN, LEG, or BIG, according as Daniell, Leclanche, or Bichromate cells are being tested. Before proceeding to use the instrument the " constant " of the galvanometer is first taken by connecting the standard cell to the Tangent Galvanometer, both plugs being out and terminals 2 and 3 connected by the brass strap as shown in Fig. 56. The deflection should be 80 divisions on the outer scale, but if the resistance of the standard- cell is over 3 ohms, the needle should point to about 79^ divisions. To ascertain the E.M.F. of a Battery place the arm of Rheostat R at the number of cells to be tested and then join up as shown in Pig. 54. The 750 ohm resistance- coil in the galvanometer should be cut out by inserting the plug. The Galvanometer Switch S is placed at DAN, LEG, or BIO. If a deflection of 100 divisions on the outer scale of the Galvanometer be obtained, the Battery is in perfect order. Any deflection under 100 indicates the fall of E.M.F., the amount per cent. 69 being shown on the inner scale. 25 per cent, of a fall is allowed for Leclanches, 15 for Bichromates, and 10 for Daniells. The actual E.M.F. in volts is ascertained by multiplying the theoretically perfect E.M.F. by the outer scale reading, and dividing the result by 100. Thus, the E.M.F. of a Bichromate which shows a fall of 5 per cent. ^i,m^rm'trr-r Fig. 54. would be = 2-14 X "95 = 2-033 volts. Similarly, a Leclanche also giving 95 divisions on the outer scale would be = 1-60 X -95 = 1-520 volts, and a Daniell = 1-07 X -95 = 1-017 volts. To estimate the resistance per cell, the connections are the same as before, and Rheostat B should be placed at 70 ■tlie maximtim resistance per cell allowed. The plunger P is depressed for a second (which period should never be exceeded), and the Galvanometer deflection noted at the sa me time. The Rheostat B should be adjusted between each depression of the plunger, until the needle is undeflected when the plunger is depressed. The resistance per cell is then read off the Rheostat B. If the resistance of a cell is required without any reference to its E.M.r., any sufficiently sensitive Galvanometer with a resistance not exceeding 320 ohms may be used. The resistance is ascertained in the way already mentioned. ffC. 55- The Tangent Galvanometers, both of the earlier and later forms, have their dials gradu- ated with a scale corresponding to the tangent divisions (outer scale), as well as a degree scale. A plan of the earlier pattern is shown in Fig. 55, and a diagram of the new form is given in Fig. 56. It will be seen that while the earlier pattern is only supplied with one shunt, in the new form there are no less than six shunts, the usefulness of the instrument being thereby greatly increased. The 71 ring is double-wound with two wires, each having a resistance of 160 ohms, so that when the two are joined in series the resistance (320 ohms) is the same as in the earlier pattern. The winding of the two coils being differential the instrument may be used as a differential galvanometer. The ends of the coils are connected as shown in the diagram, the shunts being connected with terminals 1 and 4. The values of the shunts are tV*^) aVth, Ath, ■s'ijth, T^sth, and -sitjth, the sensitiveness of the instrument being reduced to these values, and at the same time the resistance between terminals 1 and 4 F/C. 66. reduced from 320 ohms (the two 160 ohm coils being in series) to 32, 16, 8, 4, 2, and 1 ohms respectively. A resistance coil of 750 ohms as in the earlier pattern, is also inserted between terminal 4 and the coils. When the short circuit key is depressed terminals, 1 and 4 are directly connected. As shunts play such an important part in telegraphy an explanation of these may fitly be introduced here. Currents equal to 2'5 milliamperes can be measured on the new Tangent Galvanometer by con- necting the line to one terminal and Earth to the other, terminals 2 and 3 being strapped across as in Fig. 66. 72 But as the currents used in telegraphy are much stronger it is necessary to employ a shunt by joining a resistance coil across the Galvanometer terminals. When the Gral- vanometer is thus shunted the current divides inversely as the respective resistances and the strength of the current in each path, in terms of the resistance of the Galvanometer G, and Shunt S, may be found from the following formulae : — Let 0, denote the current in the Galvanometer, C, the current in the Shunt, and C = Ci + Ci := the total current. (1). ThenG.=^andC, = 1 Dividing ^=% whence C, = 0, |l (2) . Substituting this value of d in equation 1, we get That is to say, the current in the Galvanometer, multi- plied by (which is called the multiplying power of the Shunt, and is usually denoted by n), is equal to the total current. From equation 2 we have the current in the Galvanometer C, = C =_ (4). G + S n Calculating the value of Cj in the same manner, we have the current in the Shunt C^ = C ^ = C G ^ + ® The resistance of the Shunted Galvanometer is ^ ^ = G+S — which is always less than the Galvanometer alone; hence, when the external resistance is small, compensation 73 resistance imist be added to the main circuit. This resistance R can be found from the formula G- + S n n ^ ' It is, however, seldom necessarrj to add resistance in ordinary tests. The multiplying power of the Shunt has been stated to be ^ — = n from which we may obtain the value of S for any given multiplying power. n = ?-i^ »s - s = G- and S = -A_ (7). a n — 1 Example. — In the Post Office Tangent Galvanometer of 320 ohms if the -^"^ Shunt will measure the required current, then to = 10 and S = -^.^ = ^^ =85-5 ohms. The new tangent instrument, as already explained, is provided with xj? tjj to mj tto and shi shunts, the resist- ances of which can be calculated from formula (7). A current of 1 milliampere in the unshunted Galvanometer produces 80 divisions, from which the number of divisions for each shunt can be calculated. With the unshunted Galvanometer 2'5 ma. can be measured ; with the tV shunt inserted, 25 ma., and so on. At those offices where the new Tangent has not been supplied, a 32 ohm coil has been provided, with the ts shunt plug in, and the 32 ohm coil joined across the Tangent Terminals 50 ma. can be measured. But the current required for Direct Sounders is 60 ma. Instead, therefore, of connecting the 32 ohm coil, withdraw the i^s shunt plug, and join a resistance of 8 ohms across the Tangent Terminals, when any strength of current up to 100 ma. can be measured, two divisions representing 1 ma. With a coil of 4 ohms so connected, 200 ma. can be measured ; with 2 ohms, 400 ma., and with 1 ohm 800 ma., as in the new form of Tangent Gal- vanometer. Shunts are used across batteries in some of the tests for their internal resistance, but unless the shunt is very much higher than the resistance of the battery, polarisa- tion is soon set up in the Leclanche and Bichromate cells. 74 The E.M.F. in any circuit is equal to CE., the product of the current and resistances. In a shunted Galvanometer the E.M.F. in the Galvanometer circuit is Cj X G, and in the shunt circuit Cg X S, or, including the external resistance R, it is Cj (R + G) and Cg (R + S) respectively. fic.56. The New System of Moening Testing. There were several inaccuracies in the system of morning testing described at p. 19. A new system has therefore been devised, and is being gradually introduced. The new tangent Galvanometer, as was explained in the last article, is wound with two coils of 160 ohms each. ffc. S7. These are connected to four terminals, so that either coil or both coils in series can be introduced into the circuit by means of switches. The 50 Daniell cells are replaced by 40 dry cells, each of -75 resistance. The standard cell constant (the connections for which are shown in Pig. 56) is fixed in the usual way, both plugs 75 being out. The dry cell constant is then fixed, two 10,000 ohm coil being connected in circuit with the battery, and one coil of the Gralvanometer (as shown in Pig. 57), deflection 110=2-75 ma., 750 ohm coil to be short-circuited. One or two cells can be added if necessary. The total resistance in circuit is 20,190 ohms,- and current 2'75 ma.; the E.M.P. is therefore 55-5225 Tolts. Insulation Resistance Test. The necessary connections for this test are given in Fig. 58. The wires, instead of being earthed at the distant end and tested singly, are looped and tested in pairs, the current being sent by the testing office. Considering the - Fic.58. current as travelling from the Zinc pole of the battery, it passes through the 2-4 coil of the galvanometer, and returning by the second wire, flows through the 3-1 coil in the opposite direction to the out-going current and exactly neutralises its magnetic efBect on the needle if the line is perfect. No deflection is, therefore, produced. The greater the leakage on line the less will be the effect of the incoming current and the greater will be the deflection. Tables are provided showing the insulation for various deflections on different lengths of line. The standard insulation is still 200,000 ohms per mile and when the insulation is lower, localising tests are made by means of the Wheatstone Bridge, and the responsible officers are advised. At offices where a Bridge is not provided two other tests are taken (Fig. 59) the first shows the difference in the strength of the currents in the wires 76 on each side of the fault, call the deflection d. The second (Fig. 60) shows the current in one wire from the fault to the testing office, call the deflection D. Then the distance in ohms R of the fault is obtained from the formula : — ^_(L+G)D "- 2D-d " -160 where L is the resistance of the loop, and G the resistance of the galvanometer. Fic.59 TSST d Example — ^Let L=:2590, one wire being 1390, and the other 1200 ohms, G=320, d=42, D=124-5. R= (2590+320) xl24-5_^g0^^^30 124-5 X 2 — 42 LfNE Fic.GQ Distance of fault on long wire, 1590—1390 = 200 beyond the looping office, or 1200—200=1000 on the short wire, which, divided by the resistance per mile, give the distance in miles. 77 These three tests — insulation, d, and D — are now re- quired daily for all telephone trunk lines. Calculations can then be made at head-quarters showing the condition of the lines at any time. When there is a single wire only to be tested, the dis- tant office joins it to earth through 10,000 ohms. At the testing office it is joined through 10,000 ohms to one coil of the Galvanometer, the testing battery, and to earth (Fig. 61). The formula C = ^ will show the deflection which should be obtained if the line is perfect. The greater the leakage the greater the deflection in this case also. The deflec- tions showing the state of the insulation are found as follows : — Deduct the perfect current deflection frora the deflection obtained w^hen the wire is under test, and multiply the result by 2. Example — perfect current 100, deflection obtained 140, = (140 - 100) X 2 = 80. If the insulation resistance is below 200,000 ohms per mile, and the wire is made up of both cable and land wires, the cable is now taken into account in multiplying by the number of miles — five miles of cable being reckoned equal to one mile of land line. Example — total land length of a loop 44 miles, total cable length 54 miles. Then 54 44 -f- — = 44 -h 10'8 = 54-8 is the multiplier instead 5 of 44, the land length, as formerly. Two switches with nine possible combinations are pro- vided for making the various tests. Sufficient has been said to enable readers to understand the essential points of the new system of morning testing, but we add the following particulars which will be specially helpful to those officers whose duty it is to make up the weekly return for the information of the engineering department. Wcyrhing Standard (Column 9). — The galvanometer deflection noted in this column shows whether the wire is of sufficiently high resistance to allow of standard speed being attained. 78 The insulation resistance of the line mnst not be below the total conductor resistance. For example, if the line is 250 miles long and conductor resistance 4,100, the total insulation resistance must not fall below 4,100 ohms. The insulation resistance per mile is 250x4,100=1,100,000 ohms. If one office tests 100 miles, the standard deflection must correspond to a resistance of 11,000 ohms. If the other office tests 150 miles, the deflection must correspond to 7,300 ohms. The length of the section tested is found in column 5 of the weekly Return, the total length in column 7, and the conductor resistance in column 8. Then the figures in column 7 x column 8 -i- column 5 = column 9, the insulation resistance of the section tested, the equivalent deflection for which will be found in the tables. Maintenance Standard (Column 10). — The minimum insulation resistance of the open work is 200,000 ohms per mile, cable and underground 1,000,000 per mile. If the wire is all open work, 200,000 divided by the number of miles gives the minimum total insulation resistance. If cable or underground included, then — 200,000 = A, and 1,000,000 open work in miles -> - - cable and underground = B. The joint resistance of A and B is the minimum 79 insulation resistance of the line, the equivalent deflection for which will be found in the tables. Example : — 20M = 4545 and ^'Q""'""" = 18518. 44 54 . = 3649, equivalent deflection being 123. Insulation Test (Column 11). — The E.M.F. has been stated to be 55*5225 volts. Let the resistance of the line be 1000 ohms. The current sent is e ouiiexiu BBxiL 20000+1000 + 320+30= 2'6005 ma. As one milliampere produces 40 divisions through one coil of the tangent, 2"6005=104"02 divisions for a line of 1000 ohms. If the line is perfectly insulated the current sent and received will be equal, but if the line is faulty, for every division lost on the line, half a division extra leaves the battery and half a division less reaches the Galvanometer, but the sum of the sent and received currents is the same in each case. Example. — If the line is perfect and 100 ma. are sent 100 ma. will be received the sum of the two is 200. If the line is faulty and 10 divisions are lost on the line 105 ma. will leave the battery and 95 ma. will be received — 105 + 95 = 200. An increased current leaves the battery because the joint resistance of the fault, and the line beyond the fault is less than the resistance of the perfect line. The received current bears the same proportion to the current lost at the fault, as the resistance of the fault does to the line beyond the fault. In the last example and line 1000 ohms, if the apparent fault is due to general leakage, the fault will appear to be at the centre of the line. Then -^S — = the total resistance of the fault, the deflection for which will be found in the tables. Calling the original current C, the current lost at fault 2 c, then the current sent when the line is faulty will be .+ c, and current received C — c, total restance of line R and fault resistance F, we have— J, ^ C - X ^R ^ ^ C -c 2 c 4 c 80 The High Speed "Wheatstone System. — Duplex Bridge Method. The speed, which for many years had increased bnt slowly, has in later years advanced by " leaps and bounds." In 1870, the highest working speed attainable between London and Dublin was 75 words per minute. Now 500 words a minute can be signalled between the two places. In 1874, the working speed between London and Belfast was about 40 words a minute. Two years afterwards this was increased to 60 words a minute. In 1880, it had risen to 150, five years later to 250, and in 1887 to 400, at which it has since remained. In 1875, the highest working speed on land lines was 100 words a minute, in 1880 it was doubled, and now on the long lines between London and the North of Scotland 450 words a minute is possible. These results are due to the skill, energy, and perseverance of the Post Office elec- trical engineers guided by Mr. Preece. The sending and receiving apparatus and batteries have been improved, the wires better insulated, copper wires have been sub- stituted in many cases for iron wires, fast speed- Repeaters (which will be dealt with in a separate paper), have been inserted in long circuits, and the evil effects of electro- magnetic inertia have been almost entirely eliminated. An important change was made in 1889 on all the postal telegraph cable circuits by substituting the Bridge method of working duplex for the difBerential system, which has already been described. This change resulted in increasing the speed by 30 per cent., and the practical speed on duplex circuits between London and Dublin is now 300 words a minute — that is, a total of 600 words per minute may now be signalled on the London-Irish wires. The Bridge method of working duplex is illustrated in Fig. 62. Those readers who have perused the articles on the difBerential system, and the Wheatstone Bridge, will have no difficulty in understanding the principle ot the Bridge method. When the key K, at Station A, is depressed, a current flows from the battery, and di rides at D, one part passing (the arms a, b, being properly 81 proportioned) to line, and tlie other part to earth. The potentials at points c and d being the same, no current passes through the Relay R, which is consequently not afEeoted. When B. alone sends to A,., the same thing occurs there, his instrument also being unaffected. But when the current reaches the distant station it divides into two branches, one part proceeding to earth by way of c, d, through the Relay which it affects, and by means of the local battery L B, the Sounder S is worked. The other part passes to earth by c, a, D and K. When both stations send at the same time, the currents from b6th batteries being equal in strength and opposite in direction, their effects will be neutralised, and no current will flow to line, butthe potentials at the points between which the Relay is connected being altered, that instrument is affected and signals are recorded on the Sounders S and Sj. In order to obtain the best results, the resistance in the branch b d should be half that of the branch a c, — the resistance in the latter being about half that of the line, and the batteries employed should be of low resistance. One great advantage of the Bridge method of duplex telegraphy is, that ordinary apparatus may be used, it being unnecessary to wind the electro-magnet coils differentially. It is also possible to work a Sounder or Morse printer at one end of 82 the line, and the Hughes typewriter, or other instrument, at the other end. The Bridge method, however, requires more hattery power than the differential system. The connections for Wheatstone Automatic Duplex (Bridge method) apparatus are shown in Fig. 63. S C and S Ci are the signalling condensers. E C is the reading condenser. R, and E^ the arms of the Bridge. The connections for simplex working are shown in dotted lines. The had effects of electro-magnetic inertia have already been noticed. Inertia (resistance to motion or rest) was early recognised to be one of the most formidable foes to UP i.//VF- or £ rJis^ 'v^Mi^ F/C. 63. high speed, other improvements effecting but little good so long as the electro-magnet could not be magnetised and demagnetised more than a certain number of times per second. One of the first efforts was to join the coils in parallel or " quantity." The so-called extra currents in the two coils were then said to completely neutralise each other. This has since been found to have been an erroneous conclusion, the effect really being that it reduced the time- constant of the coils. When joined in parallel the self- induction is only one-fonrth of the quantity when joined in series, but only half the current is available. Every 83 electro-magnet has a certain co-efficient of self-induction, L, whicli determines the rate at which the current rises and falls, and its time-constant is expressed by the ratio, — -. The self-induction L is a varying quantity, and R requires the aid of advanced mathematics in its calcula- tion, but it always means the ratio of the self -produced magnetic induction to the current which has produced it. The time-constant must be determined for each electro- magnet, as it depends, among other things, upon the quality, &c., of the iron, the number of convolutions of wire, and the strength of current. The latter is the only variable quantity, and the self-induction increases as the square of the number of turns. As has already been indicated, joining the coils in series doubles the pull of the coils, but quadruples the electro-magnetic inertia. The time-constant of the present Receiver coils and B T ^ relays in series is ^=-57^7; = -015 second for a current of 20 milliamperes. A condenser of capacity K, with its plates connected by a resistance R, has also a time-constant K R., and its pction is exactly the opposite of that of the electro-magnet. When therefore an electro-magnet and a shunted condenser are joined in series as in Fig. 64, and suitable values given to the condenser and shunt, their effects are said to oppose and neutralise one another. Without a condenser, when the current ceases to flow, the self-induced current flows out and prolongs the signal, and the first part of the reverse current is lost in neutralising it, if the currents are not of the same strength. The analogy of starting and stopping a train may aid in obtaining a clearer idea of the effects of the coils and condenser. It requires a cerlain force to start the train in a given time, but if a much greater force is used the same work will be performed in a shorter time, and the same reasoning will apply to stopping the train. When the coils discharge, a certain quantity of current is required to neutralise it, and before that has been completed the current has been again reversed before a signal has been formed. But if, while 84 keeping the CBrrent, and, therefore, the self-induced current, low, we apply a high electro-motive force at the proper time, the self -induced current will be neutralised almost instantaneously, and there will he sufficient E.M.P. current, and time left to overcome the opposition to magnetisation afresh, and to form a signal before the current is reversed. These effects are attained by insert- I fl" WLmcr^/cir^ Fig. 6^. ing the shunted condenser between the coils and earth, the shunt reducing the resistance and the discharge of the condenser supplying a high E.M.F. The mathematical proof of the shunted condenser arrangement is beyond the scope of this article, but it may be said that the quantity in the electro-magnet, is E L =5 =- and the quantity discharged from it is E L j^ — , where B is the E.M.P. and L the self- (B -|- Bj)'' induction of the coils. The quantity in the condenser Bi is E K .g Jrg- and the quantity discharged from it E -|- Bj B ^ through the coils is E K -^g — ^ but when the shunt (B -h Bj)-' and condenser are properly adjusted EL ,„ ^ „.„ = EK 1l L = K Bj!- (B + B)2 (E -I- Ei)3 It has been found from experiment that a Eeceiver joined in short circuit with the coils in series requires a shunt of 2,000 ohms and a capacity of 7-5 mf . when the current is about 20 ma. If the current is reduced by increasing the shunt, less capacity will suffice. 85 It may be mentioned that a Transmitter was exhibited in June last, by the Postmaster-General, which dispenses with the 421b. weight, a pneumatic motor being applied directly to the eccentric axle, the whole of the train work and fly-wheel not being required. The speed is regulated by opening or contracting the nozzle of the regulating air supply. The power is so small that the Transmitter can be driven at a moderate speed by simply blowing into it with the mouth. rig. 65 shows the short circuit connections necessary to prove a Transmitter or Receiver. The wire joined to D is removed and is replaced by the wire on U.