/VC J^ 
 
 $* 
 
 REESE LIBRARY 
 
 UK THK 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Received 
 <sions No. 
 
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TELEGRAPHY 
 
PRINTED BY 
 
 SPOTTISWOODE AND CO., NEW-STREET SQUARE 
 LONDON 
 
MANUAL 
 
 OF 
 
 TEL EGRAPH Y 
 
 BY 
 
 W. WILLIAMS 
 
 I 
 
 SUPERINTENDENT INDIAN GOVERNMENT TELEGRAPHS 
 MEMBER OF THE SOCIETY OF TELEGRAPH ENGINEERS AND ELECTRICIANS 
 
 ILLUSTRATED BY 93 WOOD ENGRAVINGS 
 
 LONDON 
 LONGMANS, GREEN, AND CO. 
 
 1885 
 
 All rights reserved, 
 

 PREFACE. 
 
 THE FOLLOWING MANUAL has been written by order of the 
 Director-General of Telegraphs in India, and is based on the 
 substance of the papers on technical subjects, set by the 
 Author, at the general examinations of the Indian Telegraph 
 Department. These subjects embrace a general description 
 of the various instruments, batteries, and circuits which the 
 telegraph official may be called upon to deal with, faults 
 which may be met with, and their remedy ; the conservancy 
 of, and modes of testing lines, batteries, instruments, 
 lightning conductors, and earths ; the general principle of 
 telegraph working in its various forms ; the electrical phe- 
 nomena which interfere with communication and the 
 various methods by which their obstructive effect is over- 
 come. 
 
 The objects of the manual are as follows : 
 
 First. To afford the staff a means of self-education in 
 practical telegraphy. 
 
 As the conduct of telegraph circuits and apparatus is ruled 
 by electrical phenomena, these are explained in detail, it being 
 
vi PREFACE. 
 
 essential that they should be clearly understood in order that 
 the instruments specially designed to utilise or overcome the 
 effects of such phenomena be also understood. 
 
 Second. To serve as a primer and companion to the 
 departmental ' Testing Instructions.' 
 
 With this view, unnecessary details are herein avoided ; but 
 principles, by which alone electrical facts can be accounted for, 
 are explained fully and in simple language which, it is hoped, 
 will be found intelligible to all readers. 
 
 It is believed that the manual will fall into the hands of 
 many whose opportunities of education in telegraphy have 
 been limited, and who, perhaps, late in life aspire to become 
 something more than manipulators ; and while the main 
 object of the book is to open the way for such to an intelli- 
 gent understanding of those details of telegraph work which 
 may have been looked upon merely in the light of rule-of- 
 thumb processes, it is further hoped that it may encourage a 
 systematic study of the science, for its own sake, outside the 
 limited scope of a technical manual. 
 
 Third. For the instruction of probationers. 
 
 The contents of the book are made progressive so far as 
 is possible in a work of the kind ; any allusion to a subject 
 or phenomenon not noticed in an earlier part of the treatise 
 being connected therewith by a marginal reference. The 
 marginal titles to paragraphs are inserted with the object of 
 affording the Instructor material for questions in the exami- 
 nation of training-classes. 
 
 Fourth. To form a text-book of ready reference. 
 
PREFACE. vil 
 
 The alphabetical index is framed with this object ; and, 
 further, the subjects are arranged under their several heads, 
 as specified in the table of contents. Section A is devoted to 
 the definition of all the most important technical terms in 
 general use, a clear notion of such terms being an important 
 step towards the comprehension of electrical reasonings gene- 
 rally. 
 
 Appendix A comprises those laws which determine the 
 strength of electric currents, in whatever circuits they may 
 exist ; the principles on which duplex and other systems of 
 working are based ; the strength of electro-magnets ; the 
 influence of derived circuits, extra currents, and all those pro- 
 perties on which the speed and efficacy of signalling depend. 
 
 Appendix B contains the mathematical solutions, by the 
 
 simplest step-by-step processes, of the formulae used in the 
 book. It is believed that this appendix, in conjunction with 
 the preceding one, will be found of value in showing how a 
 knowledge of electrical laws, combined with an elementary 
 knowledge of algebra, makes the solution of most electrical 
 formulae easy, explaining the meaning of those used in testing, 
 and enabling the student to originate formulae for himself in 
 the course of his own tests or other electrical work. 
 
 N.B. References herein to ' Testing Instructions ' relate 
 to the second edition of that work, published in 1878. 
 

CONTENTS. 
 
 A. DEFINITIONS OF TECHNICAL TERMS . 
 
 B. BATTERIES . 
 
 C. INSTRUMENTS . 
 
 D. CIRCUITS (TELEGRAPHIC) . . . 117 
 
 E. FAULTS IN INSTRUMENTS AND CONNECTIONS, THEIR DISCOVERY 
 
 AND REMOVAL ..... 
 
 F. TESTING OF BATTERIES 
 
 LINES .... 
 EARTHS AND LIGHTNING CONDUCTORS 
 INSTRUMENTS AND CONNECTIONS . 236 
 
 APPENDIX 
 
 A. LAWS AND PRINCIPLES BY WHICH THE CONDUCT OF ELECTRICAL 
 
 CURRENTS MAY BE UNDERSTOOD, AND ON WHICH THE SOLU- 
 TIONS OF FORMULA ARE BASED 
 
 B. ELECTRICAL FORMULAE AND THEIR SOLUTIONS 247 
 TABLE OF NATURAL SINES AND TANGENTS 3 02 
 
 ALPHABETICAL INDEX .... 33 
 
 SECTION A. 
 DEFINITIONS. 
 
 INTRODUCTORY REMARKS ON ELECTRICITY AND THE 
 ELECTRIC CURRENT. 
 
 DEFINITION 
 I 
 
 2 
 
 [ELECTRICAL.] 
 
 Electric quantity 
 
 Unit of quantity or current 
 
 Potentials, tension or intensity 
 
 Current .... 
 
 Difference of potential 
 
 Electromotive force 
 
 UnitofE.M.F. ...... -7 
 
X CONTENTS. 
 
 DEFINITION 
 
 Conductor . . . . . . . . . 8 
 
 Conductivity . . . . . . .9 
 
 Resistance . . . . . . . . . 10 
 
 Insulators . . . . . . . . .11 
 
 Unit of resistance . . . . . . . . 12 
 
 Circuit ......... 13 
 
 Electrification . . . . . . . . . 14 
 
 Capacity . . . . . . . . 15 
 
 Unit of capacity . . . . . . . . 16 
 
 Dielectric . . . . . . . . . 1 7 
 
 Specific inductive capacity . . . . . . . 18 
 
 Galvanic polarisation . . . . . . 19 
 
 Constant batteries . . . . . . . 20 
 
 Multiple arc ........ 21 
 
 Derived or divided circuit . . . . . . . 22 
 
 Extra current ........ 23 
 
 Return current . . . . . . . 24 
 
 Solenoid ......... 25 
 
 Electrolytic action Ions Anode Kathode Electrolyte . 26 
 
 Constant of galvanometer . . . . . . .27 
 
 Osmose . . . . . . . . . . 28 
 
 Earth currents or natural currents . . . . . .29 
 
 ' Differential ' . . . . . . . . . 30 
 
 Null method . . . . . . . 31 
 
 Astatic . . . . . . . . 32 
 
 Amalgamation ........ 33 
 
 Units . . . . . . . . . 34 
 
 Absolute or metrical units . . . . . . -35 
 
 Resultant . . . . . . . 36 
 
 Conjugate conductors . . . . . . -37 
 
 Reduced length . . . . . . . . . 38 
 
 [MAGNETIC.] 
 
 Magnetic polarity Equator Neutral line . . . . -39 
 
 Magnetic field . . . . . . . . . 40 
 
 induction ........ 41 
 
 meridian . . . . . . . . 42 
 
 Coercive force . . . . . . . -43 
 
 Residual magnetism . . . . . . 44 
 
 Portative force . . . . . . . -45 
 
 Magnetic moment . . . . . 46 
 
 Saturation point ........ 47 
 
 [ELECTRO-MAGNETIC.] 
 
 Magnetic inertia . . . . . . . 48 
 
 Amperian currents ........ 49 
 
 Vibration or oscillation . . . . . . 50 
 
CONTENTS. xi 
 
 SECTION B. 
 BATTERIES. 
 
 PARAGRAPH 
 
 Current mainly due to chemical action ..... I 
 
 E.M.F. dependent on chemical combination . . . . . 2 
 
 Current strength directly proportional to E.M.F. and inversely to resistance 3 
 Considerations in the choice of battery material . . . 4 
 
 DanielPs battery ........ 5 
 
 Minotto's ,, . . . . . . . . 
 
 Leclanche's,, ........ 7 
 
 Grove's ,, . . . . . . . . 8 
 
 Bunsen's ,,........ 9 
 
 Bichromate batteries . . . . . . . . 10 
 
 Fuller's bichromate of potash battery . . . . .11 
 
 Chemical action of the above-named batteries . . . ..12 
 
 Conditions to be fulfilled by telegraph batteries . . . 13 
 
 E.M.F. how obtained . . . . . . . . 14 
 
 Constancy, how obtained . . . . . . 15 
 
 Description of the Minotti cell . . . . . . . 16 
 
 The diaphragm, its use . . . . . . 17 
 
 Respective merits of sand and sawdust diaphragms . . 18 
 
 New cells, 'joining up' . . . . . . 19 
 
 Salt, its use and danger . . . . . . 20 
 
 To bring new cells into working order immediately . . . .21 
 
 Reserve cells to meet emergencies . . . . 22 
 
 Defective diaphragm . . . . . . 23 
 
 Connecting wires of battery cells . . . . 24 
 
 Junction of connecting wires . . . . . 25 
 
 Difference between ' line ' and ' local ' cells . . . . . 26 
 
 Causes affecting the internal resistance of cells . . . .27 
 
 Signs of exhaustion of battery cells . . . . 28 
 
 Mixing of liquids, objection to, and remedy . . . .29 
 
 Processes by which a cell comes into working order . . 30 
 
 E.M.F., constant, for similar cells . . . . . 31 
 
 C varies with R by Ohm's Law . . . . . . . 32 
 
 Local action ...... -33 
 
 Use of battery sponge or syringe . . . . 34 
 
 Formation of zinc sulphate in cells, its harm and prevention . . 35 
 
 To clean battery coppers and zincs . . . . 36 
 
 Leakage from cell to cell, cause, effect, and remedy . . -37 
 
 Insulation of batteries . . . . . . 38 
 
 Treatment of exhausted batteries . . . . . -39 
 
 The state of dismantled batteries forms a criterion of their careful preparation 
 
 and preservation . . . . . . 40 
 
xii CONTENTS. 
 
 PARAGRAPH 
 
 Adaptation of battery power to length and gauge of line and climatic changes 41 
 Arrangement of cells for local circuits . . . . .42 
 
 The standard cell . . . . . . 43 
 
 Summary of ru'es for the preparation and conservancy of battery cells 44, 45 
 
 Rough standards of efficiency for 'line,' 'local,' 'testing,' and 'standard' 
 
 cells . . . ... . . 46 
 
 Portable batteries, their construction and action . . . -47 
 
 SECTION C. 
 INSTRUMENTS. 
 
 Part I. Signalling Instruments. 
 
 * Signalling ' and ' testing ' instruments . . . . . . 48 
 
 Electro-magnetism ........ 49 
 
 Action of the current on a soft iron core Right- and left-handed helices 
 
 Polarised instruments . . . . . 50 
 
 Effect of a current on a magnet . . . . 51 
 
 Oerstedt's discovery . . . . . 52 
 
 Ampere's law .... -53 
 
 Effect of a current on a magnet, proportional to current strength and number 
 
 of convolutions . . . . . . 54 
 
 Principles on which galvanometers are made . . . -55 
 
 Application of electro -magnetism to the construction of signalling instruments 56 
 Attractive force of electro-magnets . . . . 57 
 
 Sounders and relays ........ 58 
 
 Siemens' relay, description . . 59 
 
 electrical action . . . . . .60 
 
 Bias Working force . . . . 61 
 
 rest force and force of restitution . . . .62 
 
 weak point of . . . . . 63 
 
 play . . . . . . . 64 
 
 force of restitution accelerated by a reverse current or E. M. 
 
 shunt . . . . . . . 65 
 
 adjustment . . . . . . .66 
 
 sensibility dependent on play of tongue. Use of micrometer 
 
 screw . . . . . 67 
 
 limit to smallness of play . . . . .68 
 
 sticking. . . . . . 69 
 
 remedy for sticking . . . . . -7 
 
 efficiency range . . . . 71 
 
 Sounders inserted in the local circuit . 72 
 
 Siemens' Morse sounder, description . . . . 73 
 
 electrical action . . . . -74 
 
 translation springs for prolonging contacts . . 75 
 
CONTENTS. 
 
 ^^^^ 
 
 PARAGRAPH 
 
 Siemens' Morse sounder, adjustment . . . . -76 
 
 efficiency . . . . 77 
 
 Siemens' sounder and relay combined electrical circuit of . . -78 
 
 Platinum points and their use . . . . . 79 
 
 Douglas, or State railway sounder, description . . . .80 
 
 adjustment and efficiency . 81 
 
 Dubern sounder, description . 4 . . . .82 
 
 adjustment . . . . . . . 83 
 
 efficiency . . . . . . .84 
 
 Portable sounder, description . . . . . . . 85 
 
 adjustment . . . . . .86 
 
 efficiency . . . . . 87 
 
 Morse characters . . . . . . . .88 
 
 The Morse alphabet . . . . . . 89 
 
 Rules for accurate signalling . . . . . .90 
 
 Inkwriters . . . . . . . 91 
 
 Siemens' inkwriter (relay &c. combined) . . . . .92 
 
 Inkwriter terminals and connections . . . . 93 
 
 Embossing recorders . . . . . . -94 
 
 Needle instruments . . . . . . 95 
 
 Single needle instrument . . . . . . 96 
 
 Double ,, ,, . . . . . . . 97 
 
 Electric bells, the Alarum or Trembler, description . . . .98 
 
 adjustment . . 99 
 
 efficiency . . . .100 
 
 Single-stroke bell, description . . . . . . 101 
 
 adjustment . . . . .102 
 
 efficiency . . . . . . 103 
 
 Transmitting apparatus . . . . . .10 
 
 Keys or handles . . . , . . 105 
 
 Double current keys . . . . . . .106 
 
 The tapper or pedal key . . . . . . . 107 
 
 Constant resistance key . . . . . . .108 
 
 Charge and discharge .... 109 
 
 ' Charge,' variable and permanent state . . . . .no 
 
 ' Discharge ' . . . . . ..ill 
 
 Return current of discharge . . . . . .112 
 
 Induction . . . . 113 
 
 Electro-static induction . .114 
 
 The Leyden jar ... . 115 
 
 The condenser . . ... . . .116 
 
 Resemblance of a telegraph line to a Leyden jar or condenser . . .117 
 
 Voltaic or electrodynamic induction . . . . .118 
 
 Extra current . . . 119 
 
 Extra currents in a pair of coils neutralised by joining them parallel . 120 
 
xiv CONTENTS. 
 
 PARAGRAPH 
 
 Summary of causes affecting the speed of signalling . . . .121 
 
 (a) Retardation by electro-static induction, charge, and discharge. 
 
 (b) Magnetic inertia. 
 
 (c ) Extra currents. 
 
 (d) Mechanical inertia. 
 
 (e) Residual magnetism. 
 
 The electro-magnetic shunt, its action . . . . . 122 
 
 Its results . . . . . . . . .123 
 
 Its efficiency ...... . . 124 
 
 Discharging arrangements . . . . . . .125 
 
 The discharging relay . . . . . . . 126 
 
 Its efficiency ........ 127 
 
 The discharging key . . . . . . . . 128 
 
 Its use as a zinc sender . . . . . . .129 
 
 Discharge, how affected by D'Arlincourt relay . . . 130 
 
 The receiver . . . . . . . .131 
 
 The discharger . . . . . . . . 132 
 
 Siemens' lightning discharger . . . . . 133 
 
 Its efficiency . . . . . . . . 134 
 
 Switches or commutators . . . . . . 135 
 
 S.T.D. switch . . . . . . . . . 136 
 
 P. switch ......... 137 
 
 Current reversers . . . . . . . . 138 
 
 Lever switch ........ 139 
 
 The commutator . . . . . . . . 140 
 
 The battery commutator . . . . . . .141 
 
 Part II. Testing Instruments. 
 
 The galvanoscope . . . . . . . . 142 
 
 Galvanometers . . . . . . . 143 
 
 Aids and obstacles to sensitiveness . . . . 144 
 
 Simple form of galvanometer . . . . . 145 
 
 Calibration . . . . . . . . . 146 
 
 The calibrated galvanoscope . . . . . 147 
 
 Its efficiency . . . . . . . . 148 
 
 Sources of inaccuracy in galvanometers . . . . -149 
 
 The sine galvanometer . . . . . . . . 150 
 
 The tangent galvanometer . . . . . . 151 
 
 The departmental tangent galvanometer . . . 152 
 
 Its efficiency . . . . . . . . 153 
 
 The astatic galvanometer . . . . . . . 154 
 
 Its efficiency . . . . . . . 155 
 
 Thomson's reflecting galvanometer . . . . 156 
 
CONTENTS. XV 
 
 PARAGRAPH 
 
 Shunts . . . . . . . . 157 
 
 Resistance of shunts . . . . . . . . 158 
 
 Multiplying power of shunts . . . . . 1 59 
 
 Resistance coils . . , . . . . . . 160 
 
 Heating effect of current on coils ...... 161 
 
 Bifilar winding of coils and its object . . . . . . 162 
 
 Principle of Wheatstone's bridge . . . . . .163 
 
 The Wheatstone bridge . . . . . . . . 164 
 
 The differential galvanometer . . . . .165 
 
 Part III. Magneto-electric Instruments. 
 
 Magneto-electric induction . . . . . . . 166 
 
 Principle of magneto-electric machines ..... 167 
 
 The magneto-electric machine . . . . . . . 168 
 
 The commutator . . . . . . .169 
 
 Magneto-electric currents of high potential . . . . 170 
 
 Insulator and joint detector . ...... 171 
 
 Wheatstone's A B C instrument . . . . . 172 
 
 The telephone . . . . . . . 172 (a) 
 
 The microphone . . . . . . 172 (b] 
 
 SECTION D. 
 CIRCUITS. 
 
 Battery circuit . . . . . . . 173 
 
 Internal and external circuit and resistance . . . . . 174 
 
 Conventional symbols for the various portions of telegraphic circuits . 175 
 
 Current strength the same at all points of a direct circuit . . 176 
 
 Earth circuit . . . . . ... 177 
 
 Earth plates . . . . . . . . . 178 
 
 Open and closed circuit . . . . . . -179 
 
 Open circuit or S working . . . . . . . 180 
 
 Local circuit ........ 181 
 
 Office circuit between line and signalling instruments . . . . 182 
 
 Translation circuit . . . . . . . .183 
 
 Circuit of T working with discharging arrangements . . 184 
 
 D and G working circuits . . . . . . .185 
 
 P working circuit . . . . . . . . 186 
 
 Circuit of A working . . . . . . .187 
 
 Looped open circuit (Morse) . . . . . . . 188 
 
 Closed circuit (Morse) . . . . . . .189 
 
 Looped open circuit (needle instrument) . . . . . 190 
 
 Reverse current working . . . . . . .191 
 
 Advantages of reverse current working . . . . . . 192 
 
 Disadvantages of reverse current working . . . . 193 
 
xvi CONTENTS. 
 
 PARAGRAPH 
 
 Circuits worked with positive currents . . . . . . 194 
 
 Circuits worked with negative currents . . . . .195 
 
 Circuit of zinc sender . . . . ^ ' . . 196 
 
 Circuit of A B C instruments . . . . . 197 
 
 Duplex telegraphy . . . . . . 198 
 
 Duplex circuit, bridge method . . . . . .199 
 
 Adjustable resistance W . . . . . . 200 
 
 To obtain permanent balance ...... 201 
 
 Duplex condensers . . . . . . . 202 
 
 To obtain transient balance . . . . . . . 203 
 
 Respective advantages and disadvantages of the ' Bridge ' and ' Differential ' 
 
 methods of duplex . . . . . . . . 204 
 
 Differential duplex working ...... 205 
 
 Constant resistance, keys for differential duplex . . . 206 
 
 Permanent and transient balance ...... 207 
 
 Double current differential duplex . . . . 208 
 
 Split battery duplex ....... 209 
 
 Permanent and transient balance 210 
 
 Circuits containing covered wires . . . . . .211 
 
 Joints in covered wires . . . . . . . 212 
 
 Connections between wires and terminal screws . . . .213 
 
 SECTION E. 
 FAULTS IN APPARATUS. 
 
 First causes of faults . . . . . . ..214 
 
 Injurious effects of dust and dirt . . . . . 215 
 
 Faults in working batteries and their indications . . 216 
 
 Mixing of liquids ........ 217 
 
 Local action ...... . . 218 
 
 Leakage ......... 219 
 
 Cross leakage . . . . . . . . 220 
 
 Disconnection . . . . . . . .221 
 
 Rise in resistance . . . . . . . 222 
 
 Fall in resistance . . . . . . . -223 
 
 Battery faults indicated by regular tests . . . . . 224 
 
 Instrument faults ........ 225 
 
 (1) Mechanical. 
 
 (2) Electrical. 
 
 (3) Faults of magnetisation. 
 
 (4) Faults of adjustment. 
 
 Faults in the relay . . . . , . . . 226 
 
 (a) Loose carriage. 
 (/>) Unequal coils. 
 
CONTENTS. xvii 
 
 PARAGRAPH 
 
 (c] Imperfect insulation of working contact. 
 
 (d] Demagnetisation of permanent magnet. 
 
 (e] Residual magnetism of cores or shoes. 
 (/) Oxidation of platinum contacts. 
 
 (g) Mechanical inertia of the tongue. 
 
 (h) Imperfect adjustment. 
 
 (i) Broken coil. 
 Faults in the sounder ....... 227 
 
 (a) Defects in common with the relay. 
 
 (6) Imperfect insulation of the pillars. 
 
 (c) Failure of antagonistic spring. 
 Faults in the inkwriter . . . . . . . 228 
 
 (a) Defects in common with the sounder. 
 
 (I)) Broken spring. 
 
 (c) Clogged wheels and bearings, the result of dust and rust. 
 
 (d) Friction of the paper wheel. 
 
 (e ) Failure of the inking wheel to mark. 
 
 Faults in the needle instrument ...... 229 
 
 (a) General faults. 
 
 (b) Imperfect continuity. 
 
 (c) Permanent deflection of the needle. 
 
 Faults in electric bells, electro-magnetic shunts, and discharging relays . 230 
 Faults in the condenser . . . . . . 231 
 
 (1) Deterioration of the dielectric. 
 
 (2) Surface leakage between the terminals. 
 
 Faults in keys ........ 232 
 
 (a) Dirty contacts. 
 
 (b} Weak antagonistic spring." 
 
 (c ) Excessive play. 
 
 (d] Electrolytic action. 
 
 (e} Weak contact springs of constant resistance keys. 
 Faults in the lightning discharger . . . . . 233 
 
 (a) Contacts. 
 
 (b) Imperfect connections. 
 
 Faults in switches and commutators ..... 234 
 
 (a) Defects in common with the lightning discharger. 
 (b} Corrosion of plugs. 
 
 (c) Imperfect conductivity of bars. 
 
 Faults in galvanoscopes and galvanometers . . . 235 
 
 (a) Defects in common with electro-magnets generally. 
 
 (b} Demagnetisation of the needle. 
 
 (c) Defective suspension. 
 Faults in resistance coils ....... 236 
 
 Effects of temperature. 
 Faults in the ABC instrument . . . . . . 237 
 
 General remedies for instrument faults ..... 238 
 
 (a) To remedy dirty or fused contacts. 
 
 (l>) To preserve steel springs from rust. 
 
xviii CONTENTS. 
 
 PARAGRAPH 
 
 (c) To secure pivots from unnecessary friction. 
 
 (d) To magnetise needles. 
 
 (e) To magnetise an astatic pair. 
 (_/") To render iron soft. 
 
 (} To protect instruments from lightning. 
 
 (h} To secure firm connections. 
 Faults in office connections . . . . . . . 239 
 
 (a) Perished wires. 
 
 (b} Leakage. 
 
 (c) Electrolytic action. 
 Faults in earth connections ....... 240 
 
 (a) Defects common to wire connections. 
 
 (b) Corrosion of plate. 
 
 (c) Polarisation. 
 
 Faults in the line circuit . . . . . . 241 
 
 (a) Total disconnection. 
 (b} Partial disconnection. 
 (c} Total or ' dead ' earth. 
 
 (d) Partial earth. 
 
 (e) Contacts, perfect and partial. 
 
 Natural causes obstructing communication ..... 242 
 (a) Atmospheric electricity. 
 (l>) Earth currents. 
 (c} Thermo-electric currents. 
 (d] Moisture. 
 
 SECTION F. 
 TESTING. 
 
 Object of testing . . . . . . . 243 
 
 To measure ordinary resistances . . . . . . 244 
 
 Resistances measured by the Wheatstone bridge . . . . 245 
 
 To test the balance of the bridge ...... 246 
 
 To test the insulation of the bridge . . . . . 247 
 
 Resistances measured by the differential galvanometer. . . . 248 
 
 Rules for testing the accuracy of the differential galvanometer . . . 249 
 
 Resistances measured by the deflection of a galvanometer . . . 250 
 
 Effect of altering the E.M.F. of the testing battery . . . . 251 
 
 Battery resistance when taken into account . . . . -252 
 
 Rule for calculating the strength of current with reference to arrangement of 
 
 battery power . . . . . . . 253 
 
 Testing batteries joined for ' tension ' or * quantity ' . -254 
 
 Measurement of low resistances . . . . . . . 255 
 
 Adjustment of battery power according to work to be done . . . 256 
 
 Deflections controlled by adjustment of E.M.F. and by galvanometer shunts 257 
 
 Resistances measured with the Thomson galvanometer . . . 258 
 
 Adjustment of circuit . . . . . . . 259 
 
-> 
 
 D NIVJEJ ;>SITV 
 Bup'om^ '' X1X 
 
 PARAGRAPH 
 
 To measure resistances with the tangent galvanometer . . . . 260 
 
 Use of thick and thin coils . . . . . . .261 
 
 Reduction coefficient of coils . . . . . . 262 
 
 To measure a resistance using both coils of the tangent galvanometer . 263 
 
 Useful rules to be remembered in testing with the tangent galvanometer . 264 
 
 Resistances measured by the sine galvanometer .... 265 
 
 To measure high resistances with the bridge and any sensitive galvanometer 266 
 
 To measure resistances containing electromotive force . . . . 267 
 
 Measurement of battery resistance ...... 268 
 
 First method, with the departmental tangent galvanometer . . . 269 
 
 Second method (Poggendorffs), with the Wheatstone bridge . . 270 
 
 Third method (Sir William Thomson's) . . . . . 271 
 
 Fourth method, with the sine galvanometer .... 272 
 
 Fifth method, the same with the tangent galvanometer . . . 273 
 
 Sixth method, with a differential galvanometer .... 274 
 
 vSeventh method, with a reflecting galvanometer . . . 275 
 
 Eighth method, Mance's bridge method ..... 276 
 
 Ninth method, Mance's method of opposing E.M.F. . . . . 277 
 
 Tenth method, with the calibrated galvanoscope .... 278 
 
 Measurement of electromotive force . . . . . 279 
 
 First method, with the tangent galvanometer .... 280 
 
 Second method, with the Wheatstone bridge . . . . 281 
 
 Third method, with any galvanometer . . . . * . 282 
 
 Fourth method, the same with a shunt . . . . . 283 
 
 Fifth method (Wheatstone's) ...... 284 
 
 Sixth method, by opposing equal electromotive forces . . . 285 
 
 Seventh method (Poggendorffs), by opposing batteries . . . 286 
 
 Eighth method, with a reflecting galvanometer . . . . 287 
 
 Regular tests of batteries . . ... . . . 288 
 
 Strength of current arriving at distant station . . ... 289 
 
 Measurement of received currents ...... 290 
 
 Line testing by strength of received currents . . . 291 
 
 Regular testing of lines ....... 292 
 
 Bridge used for testing . . . . . . . 293 
 
 Adjustment of testing battery ...... 294 
 
 Mode of connecting lines to be tested with the bridge . . . 295 
 
 The conduction test ....... 296 
 
 The circuit test ......... 297 
 
 The insulation test ........ 298 
 
 Objects of the circuit test . . . . . . . 299 
 
 Measured relay resistance, an indication of state of insulation of the line . 300 
 
 Correction of measured values rendered necessary by defective insulation . 301 
 
 Order of observations made in regular testing .... 302 
 
XX CONTENTS. 
 
 PARAGRAPH 
 
 Adjustment of branch resistances of bridge . .- . . 303 
 
 Routine of testing ........ 304 
 
 Relay resistance . . . . . . . . 305 
 
 Reduction of resistances to corresponding units .... 306 
 
 Reduction of resistances to corresponding temperatures . . . 307 
 
 Positive and negative readings differ owing to natural currents . . 308 
 
 Calculation of measured resistances, allowing for effect of natural currents . 309 
 Electromotive force of natural current . . . . -310 
 
 Correction of. measured values . . . . . 311 
 
 Corrected relay resistance, a test of uniformity of line . . 3 1 2 
 Correction of measured values of ' Conduction ' and ' Insulation ' in the case 
 
 of a uniform line . . . . . . 3 1 3 
 
 Resistance per mile expressed in ' reduced length ' . . . 314 
 Measured values, how corrected in the case of a line not uniform in insula- 
 tion and conduction . . . . . 315 
 
 Displacement of resultant fault . . . . . .316 
 
 Localisation of resultant fault . . . . . . . 317 
 
 Section tests ........ 318 
 
 Fault testing . . . . . . . 319 
 
 To discover whether fault is in office . . . . . 320 
 
 Nature of line faults and their distinguishing features . . . . 321 
 
 General principle on which faults are localised : 
 
 ( ist) in a line of uniform gauge . . . . . 322 
 
 (2nd) in a line composed of wires of different gauges . . . 323 
 
 Dead earth on a single wire ...... 324 
 
 Partial earth on a single wire . . . . . . . 325 
 
 Earth on a multiple line The loop test . . . . -326 
 
 Contact on a double line (ist method) . . . . . . 327 
 
 Correction for leakage . . . . . . .328 
 
 2nd method. Contact on a double line localised by the loop method . . 329 
 3rd method. Contact localised by the loop method when a third good wire 
 
 is available ........ 330 
 
 Disconnection Localisation by insulation test . . . . 331 
 
 Localisation by capacity tests . . . . . 332 
 
 Imperfectly insulated break . . . . . . . 333 
 
 The measurement of capacities . . . . . -334 
 
 Earth testing ... . . . 335 
 
 ist method. With tangent galvanometer and no battery . . . 336 
 
 2nd method. With tangent galvanometer and a testing battery . . . 337 
 
 3rd method. With the calibrated galvanoscope .... 338 
 
 4th method. With the Wheatstone bridge and no battery . . . 339 
 
 5th method. With the differential galvanometer and no battery . . 340 
 6th method. With the Wheatstone bridge and a testing battery . .341 
 
 7th method. With the differential galvanometer and a testing battery . 342 
 
 To test lightning conductors . . . . . . . 343 
 
 Tests of instruments and connections ..... 344 
 
CONTENTS. XXI 
 
 APPENDIX A. 
 LAWS AND PRINCIPLES. 
 
 LAW 
 
 Ohm's law . . . . . . I 
 
 Kirchhoffs laws ..... .2 
 
 Bosscha's laws or corollaries to KirchhofFs laws . . 3 
 Laws of mutual action of currents ...... 4 
 
 Maximum current of a battery . . . . . 5 
 
 Laws of current strength in derived circuit: s . . . .6 
 
 Law of electrolytic decomposition . . . . . . 7 
 
 Combined resistance of a derived circuit or multiple arc . . .8 
 
 Multiplying power of shunts . . . . . . . 9 
 
 Resistance of shunts . . . . . . .10 
 
 Resistance of telegraph wire . . . . . . . 1 1 
 
 Laws which govern the speed of signalling . . . . .12 
 
 Action of currents on magnets Oerstedt's discovery Ampere's rule . . 13 
 
 Polarity of electro-magnets ..... 14 
 
 Best resistance for electro-magnetic coils . . . . . 15 
 
 Law of the magnetic intensity of electro-magnets . .16 
 
 Principle of the tangent galvanometer . . . . . . 17 
 
 Deflection of galvanometer needles . . . . . .18 
 
 Principle of the sine galvanometer . . . . . . 19 
 
 Laws and principles of induced currents . . . . .20 
 
 Laws of extra currents . . . . . . 21 
 
 Laws of extra currents in coils of electro-magnets . . .22 
 
 Mutual action of magnets .... 23 
 
 Law of magnetic attractions and repulsions . . . .24 
 
 Law of the direction of the Amperian currents in magnets . 25 
 
 Law of portative force of a magnet . . . . . .26 
 
 APPENDIX B. 
 FORMUL/E AND THEIR PROOFS. 
 
 SOLUTION 
 
 Joint resistance of two conductors in derived circuit . I. 
 
 Joint resistance of any number of conductors in derived circuit . . II. 
 
 Ditto, on the principle of ' reciprocals ' III. 
 
 Multiplying power of shunts . . IV. 
 
 Resistance of shunts ... V. 
 
 Principle of the Wheatstone bridge . VI. 
 
 ,, differential galvanometer .... VII. 
 
XX11 
 
 CONTENTS. 
 
 SOLUTION 
 
 Principle of the tangent galvanometer . . . . VIII. 
 
 ,, sine galvanometer ..... IX. 
 
 Resistance measured by the deflections of a tangent galvanometer . X. 
 
 Ditto, using both coils of the galvanometer .... XI. 
 
 Battery resistance measured by^the departmental tangent galvanometer XII. 
 
 Ditto, with the Wheatstone bridge . . . . XIII. 
 
 Ditto, Mance's method of opposing E.M.F. . . . XIV. 
 
 E.M.F. of batteries measured by the tangent galvanometer . . XV. 
 
 Ditto, with the Wheatstone bridge (Poggendojff) . . . XVI. 
 
 Resistance of line corrected for natural currents . . . . XVII. 
 
 E.M.F. of natural currents ...... XVIII. 
 
 Corrected relay resistance . . . . . . XIX. 
 
 General formula for corrected insulation resistance of line (absolute) . XX. 
 
 General formula for corrected conduction resistance of line (absolute) XXI. 
 
 Position and resistance of resultant fault .... XXII. 
 
 Section tests, measured conduction and insulation . . . XXIII. 
 
 Section tests, corrected conduction and insulation . . . XXIV. 
 
 Distance of a fault in miles . . . . . . XXV. 
 
 Localisation of partial earth in a single line . . . . XXVI. 
 
 Localisation of earths by the loop test . . . . XXVII. 
 
 ,, partial contact in a two- wire line . . . XXVIII. 
 
 Individual resistance of earths measured in pairs . . . XXIX. 
 Earth resistance measured by the tangent galvanometer and a testing 
 
 battery ........ XXX. 
 
 To calculate resistance to be inserted, to measure the range of an 
 
 instrument . . . . . . . XXXI. 
 
 To calculate the battery power required for the same purpose . XXXII. 
 
 Reduction of resistances to the same temperature . . . XXXIII. 
 
 TABLE OF NATURAL SINES AND TANGENTS . . . 302 
 
 INDEX . 303 
 
SECTION A. 
 
 DEFINITIONS 
 
 INTRODUCTORY REMARKS 
 
 1-38. ELECTRICAL 
 39-47. MAGNETIC 
 48-50. ELECTRO-MAGNETIC 
 
 <P 
 
INTRODUCTORY REMARKS. 
 
 Electricity 
 and the 
 Electric 
 Current. 
 
 THE following are simple definitions of technical terms, 
 which, though in everyday use in telegraphy, are far from 
 being generally understood. 
 
 It is of great importance that the telegraphist should 
 begin by making himself perfectly familiar with these terms, 
 for a clear comprehension of them removes much of the 
 difficulty often experienced in understanding electrical phe- 
 nomena both in practice as well as in theory. 
 
 In regard to the latter, as treated of in books, and more 
 especially in * testing ' and other departmental instructions, 
 many points will become clear and easy of comprehension, 
 which in the absence of such elementary knowledge are 
 found difficult and obscure, because they contain terms, 
 unfamiliarity with which attaches to them a degree of sup- 
 posed difficulty which they do not really possess. 
 
 It is not within the design of this manual to discuss the 
 various hypotheses and theories propounded to explain the 
 existence of electricity. Suffice it to say that, like light, 
 it is a natural agent, the effects of which are developed by 
 force. 
 
 This force may be in the shape of chemical action, heat, 
 or other agency, the result being that, by its exercise on a 
 certain body, that body exhibits properties such as those of 
 attraction or repulsion, the production of a spark, the de- 
 flection of magnetic needles, the magnetisation of iron, &c. ; 
 by virtue of which it is said to be electrified, or to be in a 
 state of electricity, or, in more definite language, manifests 
 an electric potential. 1 
 
 Practice shows that whenever electricity is developed in 
 a body, by any means whatsoever, that body assumes two 
 different electrical states or 'potentials,' equal in amount but 
 
 B 2 
 
 SECTION* 
 A. 
 
 Def. 
 
MANUAL OF TELEGRAPHY. 
 
 opposite in kind, termed for the sake of convenience posi- 
 tive ( + ) and negative ( ) respectively, these potentials of 
 different sign being analogous to the north and south polarity 
 of an iron bar which has undergone the process of mag- 
 netisation ; l the tendency of the equal + and potentials J Def. 39, 
 always being to unite and restore equilibrium when afforded 
 the means of doing so by a conducting medium. 
 
 As explained above, electricity is generated by different 
 means ; the form of electricity with which we have princi- 
 pally to deal in the following pages, viz. that mainly used 
 for telegraphic purposes, is known as galvanic,- or voltaic, or -' so called 
 
 dynamic, or current electricity. fr ? m Galvani, 
 
 '.__, , , whose dis- 
 
 Ihe two last terms imply motion, and convey the idea coveryiedto 
 that electricity is a current, which flows like a river ; hence fe^efet 
 in telegraphy * the current ' is universally referred to as form of chemi- 
 flowing from one point to another, as this term is found to ^^ ttery 
 express most simply the phenomena observed when a tele- 
 graphic signal formed at one end of a line is reproduced at 
 the other. 
 
 The student will, however, discover that this is a mis- 
 nomer when, for example, he goes on to observe the elec- 
 trical phenomena of duplex telegraphy, in which signals are 
 transmitted from both ends of the same wire at once ; and 
 in order to prevent subsequent confusion, arising from a 
 wrong conception of what a ' current' 1 really is, it should be 
 clearly understood from the outset that electricity does not 
 actually travel, but that what is described as the ' passage of 
 the current ' is simply an electrical change of state such as 
 that described above, resulting from the tendency of + and 
 potentials to combine through a conductor and restore 
 equilibrium. 
 
 Every change does not necessarily signify the restoration 
 of equilibrium by the combination of + and potentials, 
 for if two bodies exhibiting different degrees of potential of 
 the same sign be connected together by a conductor, the 
 potentials will become equalised in precisely the same way 
 as two bodies of water would find the same level when con- 
 nected by a canal. 
 
 The term ' current ' may thus always be understood to 
 signify such change of potential, whether from + to , or 
 from a greater + to a less + ; its direction being determined 
 with reference to the fall of the greater + potential. 
 
INTRODUCTORY REMARKS. 
 
 Bearing these remarks in mind, allusion to the * electric 
 current* and its ''direction'' as described in the following 
 definitions, and to its action and the laws which govern it, 
 as explained in subsequent paragraphs, need create no mis- 
 understanding by reason of the use of the term. 
 
 ELECTRICAL. 
 
 1. Electric quantity is the amount of electricity 
 present in an electrified body. As applied to galvanic 
 electricity it represents the current available, and is that 
 property of it which manifests itself in signals, deflections, 
 or, in fact, work of any kind. 
 
 The force with which that work is done depends firstly 
 upon the difference of potentials which gives rise to its 
 E.M.F., 1 and secondly upon the resistance opposed to it ;' 2 
 E.M.F. tending to strengthen or increase the quantity of 
 current, and resistance tending to reduce it. 
 
 This is, in fact, the truth expressed by Ohm's law : 
 
 C=^-,* 
 
 where C represents the strength or quantity of the current, 
 E its E.M.F. 
 R the resistance it has to overcome. 
 
 2. The unit of quantity or current is called an 
 Ampere, Weber, or Oerstedt. 
 
 3. Potential, as the term implies, is the power which 
 any quantity of electricity has for doing work. As stated in 
 the foregoing introductory remarks, a body which shows 
 signs of electricity is said to manifest an electric potential ; 
 and, just as a magnet assumes a north and south pole, so 
 does the electrified body assume two different states of 
 potential, the one + and the other , the tendency of the 
 + being always to fall to the . Now in this fall work is 
 done, as it would be in the case of a volume of water the 
 fall of which through a pipe was used to raise a fountain ; 
 and just as, in this case, pressure causes the water to flow 
 through the pipe, so does potential, or tension, or intensity, 
 
 1 Defs.4-6, 
 and $ 14. 
 * De'f. 10. 
 
 * Law i, 
 'App. A. 
 
MANUAL OF TELEGRAPHY. 
 
 Def. 
 
 Current. 
 
 Difference of 
 Potential. 
 
 E.M.F. 
 
 Unit of 
 E.M.F. 
 
 Conductor. 
 
 Conductivity. 
 Resistance. 
 
 Insulators. 
 
 Unit of 
 Resistance. 
 
 as it is sometimes called, cause a quantity of electricity or a 
 current of electricity to flow along a conductor. 1 
 
 The standard by which potentials are compared is that 
 of the earth, which is taken as zero ; so that in speaking of 
 the potential of the current at any point, the difference of 
 potential between that point and the ground is understood, 
 being in kind + or , according as it is above or below 
 the potential of the earth. 
 
 4. Now it is obvious that in the case of the water pipe 
 the flow of the water is from the end where the pressure is 
 greatest towards that where it is least ; so with electricity, 
 the direction of its apparent flow, called a current, is from 
 that part of a conductor at which the potential is greatest to 
 that where it is least. 
 
 5. And upon this difference of potential depends the 
 force with which the current flows, and by which it over- 
 comes obstacles to its passage, this power of overcoming 
 resistance 2 being termed electromotive force (generally * Def. 10. 
 written E.M.F.)- 
 
 6. From what has been said above, it is clear that if the 
 potential at two points of a conductor be equal, there can 
 be no current, in the same way that if there were equal 
 pressures at two ends of a water pipe no water would flow 
 
 'out of it. 
 
 On the other hand, it follows that the strongest flow of 
 current (E.M.F.) is obtained when the difference of poten- 
 tials is greatest. 3 
 
 7. The standard or unit by which various E.M. forces 
 are compared is called a ' volt.' 
 
 8. The current has been described above as flowing 
 through conductors, a conductor being any substance offer- 
 ing a passage to the flow of electricity. 
 
 9. The property admitting of this passage of electricity 
 through a substance is called conductivity. 
 
 10. The reverse, that is, the property exhibited by some 
 substances which resists the passage of electricity, is called 
 1 resistance.' Thus, conductivity and resistance are the 
 converse or reciprocals of one another. 
 
 11. The above property of resistance is possessed by all 
 substances more or less, but those showing it to the greatest 
 degree are called insulators. 
 
 12. The unit by which resistances are most commonly 
 
 14. 
 
Def. 34. 
 
 114. 
 
 DEFINITIONS: ELECTRIC ' 
 
 measured is called an ohm, or British Association unit 
 (B.A.LL). There are other units of less common use. 1 
 
 13. An electric circuit consists of a battery (of whatever 
 form) and everything between its two poles which takes a 
 part in conducting the current. 
 
 14. Is the term applied to the condition of a body which 
 is charged with electricity or electrified ; e.g. supposing a 
 current to be applied to the conductor of an insulated tele- 
 graph cable ; the cable is then said to be in a state of 
 electrification. 
 
 15. The quantity of electricity which a cable or land line 
 or any other conductor thus holds is called its capacity. 
 The same is implied by the term electro-static capacity 
 or inductive capacity. 
 
 16. The unit by which capacity is measured is called a 
 farad, or more generally for convenience a microfarad. 2 ~ Def. 34 
 
 17. In an electrified cable induction 3 takes place be- 
 tween the conductor inside and the water or earth out- 
 side the cable ; in just the same way as ' it does between 
 the inner and outer plates of a Leyden jar, 4 the insulating 
 medium through which this action takes place being called 
 the dielectric. 
 
 Thus the dielectric in the case of the cable would be the 
 indiar ubber or guttapercha ; in that of the Leyden jar, the 
 glass ; in that of a land line, air. 
 
 18. On the dielectric depends this inductive effect, and 
 air being taken as the standard dielectric, the specific 
 inductive capacity of any insulating substance is its power 
 of effecting induction as compared with air. 
 
 19. A galvanic cell or battery is said to be polarised 
 when hydrogen in its free or uncombined state is deposited in 
 bubbles on the negative plate. 5 The first effect of these 
 bubbles is to enormously increase the internal resistance of 
 the battery, and, as a ' natural consequence, to reduce the 
 current thereof, or even to stop it altogether. 
 
 Again, they have the effect of altering the potential of 
 the negative plate, the surface of which is no longer that of 
 the metal e.g. copper in a Daniell's battery but a com- 
 plete surface of innumerable hydrogen bubbles which assume 
 a potential closely resembling that of zinc ; the original 
 difference of potential between the copper and zinc plates 
 being thus destroyed, no current flows at all. 
 
 4- 
 
MANUAL OF TELEGRAPHY 
 
 SECTION 
 A. 
 
 Constant 
 Batteries. 
 
 Multiple Arc. 
 
 Derived 
 Circuit. 
 
 Extra 
 Current. 
 Return 
 Current. 
 
 Solenoid. 
 
 Electrolytic 
 Action. 
 
 - J Sec. B. 
 
 s ee ng . 
 
 n 9 . 
 
 Batteries of low internal resistance, or joined through 
 low external resistance, polarise more readily than when the 
 resistance of the circuit is high ; l and, conversely, polarisa- ' 173, 174 
 tion can be reduced or prevented altogether by the insertion 
 of resistance ; so, when the poles of the battery are discon- 
 nected, the bubbles of hydrogen disappear. 
 
 The injurious effects of free hydrogen and the means 
 adopted to overcome them will be more fully discussed 
 under the head of ' Batteries' 2 
 
 20. Those batteries in which polarisation, or any other 
 cause which would render the E.M.F. variable, is prevented 
 are called constant batteries. 
 
 21. Suppose two points, a and b? to be joined by two or 
 more conductors ; these conductors are said to be joined in l6 3- 
 multiple arc, and form 
 
 22. a derived or divided circuit. 
 
 23. Is the name given to the instantaneous current pro- 
 duced by the inductive action of a current upon itself. 4 
 
 24. When observed in a telegraph line it is what is tech- 
 nically called a return current. 4 The current of discharge 
 from a line 5 is also termed a return current. 
 
 25. A solenoid is a helix or spiral of wire conveying a 
 current, the coils being equal in size, and wound at right 
 angles to a common axis, and so close to one another that 
 the currents circulating in them may be considered parallel. 
 The ends are brought back to the middle of the coil and 
 parallel to its axis with this object, that the current flowing 
 in from the ends towards the middle may counteract the 
 opposite and parallel current in each of the portions between 
 successive coils. 
 
 Such an arrangement behaves like a magnet, exhibiting 
 polarity, and following the laws of attraction and repulsion 
 of magnets. 
 
 26. Electrolysis is the name given to a property which 
 the electric current possesses of decomposing compound 
 liquids into their component elements or groups of elements 
 which are called Ions, the one appearing at the anode or 
 positive electrode (i.e. the continuation of the + pole of the 
 battery), 3 the other at the negative or kathode. The liquid 
 decomposed is called an electrolyte. 
 
 Electrolytic action takes place at every point of 
 leakage in a telegraph line or cable, causing at such point a 
 
DEFINITIONS : ELECTRICAL. 
 
 deposit, the nature of which depends on which pole of the 
 battery is joined to the line (the other pole being connected 
 with the earth) ; l if copper, then oxygen is deposited, form- 
 ing an oxide which tends to increase the resistance of the 
 fault ; if zinc, hydrogen is deposited, reducing the resistance 
 of the fault in other words, the insulation of the line or 
 cable at that point. 2 a 
 
 27. The constant of a galvanometer may be de- 
 scribed as that current which produces unit deflection on 
 that particular galvanometer e.g. a deflection of i on a 
 reflecting galvanometer, 3 45 on a tangent galvanometer o 
 (tan 45=i), or 90 on a sine galvanometer (sin 90= i). 
 
 28. In all two-fluid batteries there is a tendency for the 
 liquid surrounding the active plate to mix with that in which 
 the negative plate is immersed, through the diaphragm or 
 porous material which separates them. 4 This action is called 4 
 osmose, and tends to impair the efficiency of the battery, 
 as it causes chemical action to take place, and this again 
 causes useless consumption of material. The Daniell, 5 5 
 which is a two-fluid battery, is subject to this fault, which is 
 shown by the copper solution rising higher in the cell than 
 the zinc solution. 
 
 29. Earth currents, called also natural currents, 
 are observed in telegraph line circuits, 6 and are caused by e 
 difference of potential ^ either between the earth at two different 
 stations, or between different points of the line subject to 
 different temperatures. They may also be caused by induction 
 from passing clouds. They tend to strengthen or weaken 
 the signalling currents in the line, according as their direc- 
 tion is the same or contrary to that of the battery currents ; 
 and, being variable in their nature, they interfere with the 
 permanent adjustment of receiving instruments. 7 7 
 
 30. The term differential, as applied to any system of 
 working or to any instrument, implies that two circuits are 
 open to the passage of the current ; as, for example, in the 
 differential galvanometer, 8 the difference of current in either * 
 circuit working the needle or producing a signal. 
 
 31. In those differential circuits which are so arranged 
 that the currents are equalised and neutralise one another, 
 thus producing no movement of the needle, such a system 
 is called a Null method. 
 
 32. A pair of magnetic needles so suspended as to be 
 
 176-177. 
 
 194- 
 
 156. 
 
 5- 
 
 173- 
 
 58-87- 
 
 165- 
 
IO 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 A. 
 
 Amalgama- 
 tion. 
 
 Units. 
 
 parallel to one another, with their poles in opposite direc- 
 tions, is called an astatic system, because it stands apart 
 from the directing influence of the earth's magnetism. Gal- 
 vanometers in which such a system is used are called astatic 
 galvanometers. 1 
 
 33. Ordinary commercial zinc, when used as the active 
 plate of a galvanic battery, is liable to be uselessly wasted 
 by local action, 2 the result of foreign matter or irregularities 
 in the surface of the plates. To remedy this, they are, 
 when required for batteries in which the zinc plate has to be 
 immersed in acid, subjected to the process called amalga- 
 mation as follows : The zinc plate is first cleaned with 
 sulphuric or hydrochloric acid, and then rubbed with 
 mercury, which unites with a little pure zinc, and forms 
 an amalgam, which is spread over the surface of the plate 
 uniformly. The best way of rubbing the mercury on is 
 by means of rag bound at the end of a stick. 
 
 By amalgamating ordinary zinc, it is given, so far as its 
 surface is concerned, all the properties of pure zinc, which 
 is not used owing to its much greater cost. 
 
 34- A unit is a standard by which measurements are 
 compared ; as, for example, a foot for length, or a Ib. for 
 weight. For scientific measurements the metre (=39*37 
 inches) is usually chosen as the unit of length ; 
 
 [weight (=15*43 
 I grains Troy); 
 time ; 
 
 the gramme 
 
 the second 
 the ohm or | 
 B.A.U. J 
 the volt 
 the Ampere, 
 Weber, or 
 Oerstedt 
 the Farad 4 
 
 i may be considered the unit of deflection for a re- 
 flecting galvanometer ; 5 45 for a tangent j and 90 for a sine 
 galvanometer, tan 45 and sin 90 being = i, respectively 
 The following prefixes are commonly used : 
 micro meaning one millionth part ; 
 mega one million times ; 
 niilli one thousandth part ; 
 centi ,, one hundredth part. 
 
 resistance 
 E.M.F. ; 
 
 f quantity or 
 t current ; 
 
 capacity. 
 
 154- 
 
 33- 
 
 3 Siemens' 
 unit = '9564 of 
 an ohm [1-045$ 
 s.u. =i ohm"!. 
 Varley's nnit = 
 25 ohms. 
 
 4 Or micro- 
 farad. 
 
 156 
 
DEFINITIONS: ELECTRICAL. rl 
 
 Thus 
 
 a microfarad means i millionth of a farad 
 a megohm i million ohms ; 
 a millioerstedt ToVo tn f an oerstedt 
 a centimetre tri f a 
 
 Absolute 35- For practical purposes, it is usually sufficient to 
 
 or Metrical know the value of an electromotive force as expressed in 
 
 Units. ,- , , ... - 
 
 volts, or of other electrical quantities in terms of one or 
 other of the electrical units mentioned above ; for theo- 
 retical purposes, however, it is obviously of great advantage 
 to be able to express such quantities in terms of the general 
 units of mass, length, and time, without being obliged to 
 employ arbitrary co-efficients, in themselves sources of error. 
 In the system of absolute units, a gramme is chosen 
 as the unit of mass, a metre of length, and a second of time, 
 the fundamental unit of force being * that force which, acting 
 upon a gramme of matter for a second, generates a velocity of 
 a metre per second.' According to this system, 
 
 i ohm = io 7 or 10,000,000 absolute units approximately, 
 i volt = io 5 or 100,000 ., 
 
 Thus 
 
 r Ampere, *\ 
 
 i<( Weber, or > = =io~ 2 or o-oi. 
 
 I Oerstedt J 
 
 In the more recent system, known as the centimetre- 
 gramme-second or C.G.S. system of units, the 
 
 fundamental unit of force is ' that force which, acting upon a 
 gramme of matter for a second, generates a velocity of a centi- 
 metre per second. ' 
 
 The advantage of employing the centimetre instead of 
 the metre as the fundamental unit of length is that the unit 
 of mass is (in the C.G.S. system) identical with the mass 
 of the unit of volume of water (the standard of specific 
 gravity), the mass of i cubic centimetre of water being 
 approximately i gramme. According to the C.G.S. system, 
 i ohm = io 9 C.G.S. units approximately. 
 
 i volt = io 8 
 
 Thus 
 
 c Ampere, 
 i< Weber, or ^ T/ ^ -1 
 
 I Oerstedt 
 
 l = io- ] 
 
12 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 A. 
 
 Conjugate 
 Conductors. 
 
 Reduced 
 Length. 
 
 36. However many forces may act upon an object, it can 
 only take up one fixed position. A single force applied in 
 a certain direction can produce the same result. 
 
 This single force is termed the resultant of all the forces. 
 The same term is applicable to electrical forces ; for ex- 
 ample, a telegraph line containing many points of leakage 
 would be subject to a certain loss of current ; the same re- 
 sult precisely might be produced by one single leakage, 
 which would represent the resultant fault or loss of current 
 due to the leakage all along the line. Thus, ' The resultant 
 fault of any line or cable is that fault which, if applied alone 
 to the line at the proper point, would produce the same effect 
 with respect to the sent and received currents as all the actually 
 existing faults do? * 
 
 37. ^ In any system of linear conductors, any pair of con- 
 ductors are said to be conjugate to one another when a varia- 
 tion of the resistance of, or the E.M.F. in the one causes no 
 variation of the current in the other? ' 2 
 
 38. Every resistance which may be measured must ob- 
 viously correspond with the resistance of a certain length of 
 a standard conductor, so that resistance may thus convey 
 the idea of length ; for example, in the case of a telegraph 
 line made up of wires of different gauges, and consequently 
 each offering a different resistance per mile, the whole re- 
 sistance may be expressed in terms of that of a standard 
 wire ; 3 that is to say, the actual resistance of any line must 
 represent the resistance of a certain length of wire of the 
 standard gauge. This length is called the reduced length 
 of the line. 4 
 
 1 Schwendler 
 (t. i, App. xi. 
 a). 
 
 2 Brough (t. i, 
 p. xii. vol. i). 
 
 3 In practice 
 No. i wire 
 I.W.G. is 
 taken as the 
 standard, the 
 resistance of 
 which =252 
 ohms, or 
 264-24 s.u. at 
 80 Fahr. 
 
 4 The conve- 
 nience of 
 reducing 
 resistance to 
 length for 
 testing pur- 
 poses will be 
 seen hereafter 
 (Sec. F). 
 
 MAGNETIC. 
 
 Polarity 
 (Magnetic). 
 
 39. A magnetic substance is said to be polarised when 
 it assumes equal and opposite poles, called north and south, 
 the former being that which would turn towards the north 
 pole of the earth, and the latter towards the south, if the 
 substance were free to move. Between the poles (generally 
 midway) there is a point at which no magnetic force is 
 exhibited ; this is called the equator or neutral line. A 
 piece of steel exhibiting these properties is called a magnet, 
 and an imaginary line drawn from pole to pole would repre- 
 sent what is called the axis of the magnet. 
 
DEFINITIONS: MAGNETIC. 
 
 Magnetic 
 Field. 
 
 Magnetic 
 Induction. 
 
 Magnetic 
 Meridian. 
 
 Coercive 
 Force. 
 
 Residual 
 Magnetism. 
 
 Portative 
 Force. 
 
 Magnetic 
 Moment. 
 
 Saturation 
 point. 
 
 40. The space in the neighbourhood of a magnet through SECTION 
 which its influence is felt is called a magnetic field. -_ ^' _- 
 
 41. Certain magnetic substances, such as soft iron, 
 become magnetised when placed in a magnetic field ; the 
 property by which this phenomenon occurs is called mag- 
 netic induction. 
 
 42. The magnetic meridian of any place is indicated 
 by the position a magnetic needle, free to move in a hori- 
 zontal plane, would take up at that place." 
 
 43. There is a force which is exhibited in magnetic sub- 
 stances, notably in hard steel, which resists the process of 
 magnetisation : this is called coercive force, which has 
 the further property of causing magnetism once acquired to 
 be retained. In perfectly soft or pure iron this property is 
 scarcely perceptible. 
 
 44. A bar of perfectly soft iron, exhibiting no coercive 
 force, would be capable of instantaneous magnetisation and 
 demagnetisation on the application and withdrawal of any 
 magnetising influence, such as a current or inducing mag- 
 net ; * but, according to the degree of coercive force in the * 49-54- 
 bar, so would it retain, after the removal of the magnetising 
 current or magnet a trace of its magnetism, taking an appre- 
 ciable time to fade away, this remanent trace being called 
 residual magnetism. It is obviously a source of retard- 
 ation in all electro-magnetic instruments ; 2 the property is, 
 however, utilised in the D'Arlincourt relay. 3 
 
 45. The portative force of a magnet is represented 
 by the weight which it can carry. 4 
 
 46. The magnetic moment of a magnet is deter- 
 mined by multiplying the strength of the poles into the 
 length between them. 
 
 47. By the process of magnetisation 5 it is possible to 
 impart to a bar of steel a greater amount of magnetism than 
 it will permanently retain ; the result being that a portion of 
 the newly acquired magnetism will disappear, leaving the 
 bar permanently magnetised to an extent less than that 
 exhibited immediately after it was magnetised : the point at 
 which the magnetism remains constant is called the point 
 of saturation. 
 
 5*. 
 
 4 Law 26, 
 App. A. 
 
 238 (d). 
 
MANUAL OF TELEGRAPHY. 
 
 ELECTRO-MAGNETIC. 
 
 Magnetic 
 Inertia 
 
 Amperian 
 Currents. 
 
 Vibration or 
 Oscillation. 
 
 48. It is found in practice that the soft iron core of an 
 electro-magnet takes time both to receive and to part with 
 its magnetism, by virtue of what is called magnetic inertia. 1 
 Its retarding effects are felt more in the process of magnetisa- 
 tion than in demagnetisation, as soft iron parts with its mag- 
 netism more quickly than it acquires it ; so, when received 
 currents follow one another in rapid succession, the core of 
 the electro-magnet may not have time to be saturated, 2 and 
 so not be able to exert its full attractive force. 
 
 The stronger the current, 3 and the shorter the iron core, 
 the less the magnetic inertia, and, consequently, the more 
 rapid the magnetic action. 
 
 49. Ampere attributed all magnetic phenomena to elec- 
 tricity, assuming each individual molecule of a magnetic 
 substance to be traversed by a closed electric current, the 
 direction of these amperian currents being in conformity 
 with Ampere's law. 4 
 
 50. By a single vibration or oscillation of a magnetic 
 needle is understood its motion from its maximum deflection 
 on one side of zero to the same limit on the other. 
 
 121 (b). 
 
 Def. 47- 
 
 Defs. 1-4. 
 
 4 Law 13, 
 App. A. 
 
SECTION B. 
 
 TELEGRAPH BATTERIES 
 
 i- 4. GENERAL PRINCIPLES ON WHICH BATTERIES 
 ARE MADE 
 
 5-12. VARIOUS FORMS OF BATTERIES AND THEIR 
 CHIEF FEATURES 
 
 13-18. PREPARATION OF BATTERIES 
 19-45. CONSERVANCY OF BATTERIES 
 
 46. ROUGH STANDARDS OF EFFICIENCY OF VARIOUS 
 BATTERIES 
 
 47- PORTABLE BATTERIES 
 
TELEGRAPH BATTERIES. 
 
 Current mainly 
 due to Chemi- 
 cal Action. 
 
 E.M.F. De- 
 pendent on 
 Chemical 
 Combination. 
 
 I. A battery may be composed of one or more cells, 
 each of which consists, generally, of two conducting plates 
 (usually two dissimilar metals) immersed in liquid. Now 
 any two dissimilar metals placed in an acid which can 
 chemically affect either or both of them, manifest different 
 electric potentials ; and when the poles, i.e. the extremities 
 of the metal plates outside the liquid, are joined by a con- 
 ductor, the tendency of these potentials is to unite and 
 produce equilibrium by the fall of the higher to the lower, 
 which necessarily results in a 'flout of current? as described 
 in Section A (Introductory Remarks) ; the chemical action 
 which goes on in the cell, however, maintains the initial 
 difference of potential between the poles, the degree of this 
 difference depending mainly upon the readiness with which 
 one metal (the active or electro-positive, as it is called) is 
 attacked by the acid, as compared with the other or electro- 
 negative plate. 1 
 
 Investigations have proved that the mere contact of two 
 dissimilar metals also causes them to assume different po- 
 tentials, but the effects manifested from contact alone bear 
 no proportion to the results of chemical action. 
 
 2. Now the force of this chemical action consti- 
 tutes the electromotive force of the cell, a property 
 of the highest importance in the construction of any form 
 of battery, for it is that on which the effective strength of 
 the current directly depends. 2 It is desirable, therefore, 
 to select materials which combine to develop this property 
 to the greatest degree, which, as has been shown above, is 
 done by employing a combination in which the active plate 
 is highly electro-positive as compared with the negative 
 plate. 
 
 SECTION 
 B 
 
 1 It is custom- 
 ary to call the 
 electro-positive 
 or active plate 
 the + plate, 
 and the electro- 
 negative the 
 plate, and to 
 consider the 
 current as 
 generated by 
 and starting 
 from the 
 former in the 
 direction of the 
 arrow shown 
 in fig. 43, 
 173, sec. D. 
 The direction 
 of the current 
 outside the 
 liquid would, 
 therefore, be 
 from the 
 extremity of 
 the - plate, 
 which is called 
 the + pole, to 
 the extremity 
 of the + plate, 
 which is called 
 the - pole. 
 
 2 Law i, 
 App. A. 
 
i8 
 
 Current 
 Strength 
 Directly Pro- 
 portional to 
 E.M.F., and 
 Inversely to 
 Resistance. 
 
 MANUAL OF TELEGRAPHY. 
 
 The following list represents an electro-chemical series 
 of metals, arranged, according to a determination of Fara- 
 day, in the order of the relative potentials they exhibit when 
 immersed in dilute sulphuric acid, each being electro -positive 
 to that which follows it on the list, so that the greatest 
 difference of potential obtainable would be exhibited if the 
 first and the last were taken to form a pair : 
 
 >-, 
 - - * 
 
 Considerations 
 in the Choice 
 of Battery 
 Material. 
 
 g c 'S 
 .5 <L> 
 
 N H HH 
 
 g .3 
 
 C/3 r^ 
 
 CQ <I 
 
 I I 
 
 The above order slightly varies with different acids, but 
 always holds good for the same combination, so that the 
 E.M.F., so long as nothing occurs to affect its constancy, 
 is always the same for any particular combination. 
 
 3. Electromotive force, however, is not the only property 
 which has to be considered in the composition of a battery 
 cell, for although, according to Ohm's law, the strength of 
 current elicited is directly proportional to E.M.F., it 
 is at the same time inversely proportional to resistance, 
 and this latter property, as relating to the internal resistance 
 of the cell, is dependent ' not only upon the nature of the 
 materials used for the plates, but also upon their size, their 
 form, the mechanical arrangement of the cell and the nature 
 of the liquid in which the plates are immersed. Pure water 
 offers very high resistance ; the addition of acids reduces 
 this, but the resistance even of acid solutions is very high as 
 compared with that of metals. 
 
 4. Again, the cost of materials requires consideration. 
 Looking at the foregoing list (para. 2), though the best 
 effects as regards E.M.F. would be obtained by the use of 
 a zinc and silver pair when immersed in dilute sulphuric 
 acid, it might obviously, for purposes of economy, be ad- 
 vantageous to select some other metal for the negative plate. 
 Next above silver on the list we find copper, which is con- 
 siderably less costly than silver, and which, when combined 
 with zinc as the positive plate and immersed in dilute sul- 
 phuric acid, is productive of good electromotive force, and 
 forms one of the simplest and earliest galvanic elements 
 devised. Such a combination, however, in common with all 
 single-fluid arrangements, is productive of polarisation ; * 1 Def * 
 
TELEGRAPH BATTERIES. t 
 
 and here another and highly important point for considera- SECTION 
 
 tion, in regard to the construction of telegraph batteries, is ^ B> . 
 
 introduced. The chemical action which takes place in the 
 
 form of cell above described, when the poles are joined, is 
 
 as follows : The solution is decomposed, hydrogen being 
 
 liberated at the copper or negative plate, to which it adheres 
 
 in the form of bubbles, and oxygen combines with the zinc 
 
 or active plate to form zinc oxide, which is again dissolved 
 
 by the acid of the solution forming sulphate of zinc, this 
 
 action being thus attended by consumption of the zinc plate. 
 
 The acid, however, is also consumed, and this, as will be 
 
 understood from what has been said above (para. 3), must 
 
 result in a corresponding increase of the resistance of the 
 
 cell. But this is not all ; for the E.M.F. is impaired and 
 
 rendered inconstant by the deposition of free hydrogen on 
 
 the negative plate, giving rise to polarisation, as described 
 
 in def. 19 (Sect. A). 
 
 Again, the free hydrogen thus deposited has the further 
 property of reducing metals from their salts, and tends to 
 cause the deposition of zinc (from the sulphate of zinc in 
 solution) on the copper plate, which, being thus faced with 
 zinc, assumes the potential of the zinc plate, and this results 
 in the destruction of the E.M.F. of the cell, which depends 
 on the difference of potential manifested by the respective 
 metals. 
 
 For telegraph batteries, therefore, which are required for 
 continual work, it is obvious that these injurious effects, 
 resulting from the deposit of hydrogen, should be pre- 
 vented. 
 
 Various forms of battery aim at effecting this in various 
 ways. In Daniell's battery it is done by the introduction of 
 a second liquid containing a copper salt, by which the 
 liberated hydrogen, instead of collecting in bubbles on the 
 negative plate, is employed in depositing metallic copper 
 thereon, thus preventing the inconstancy and loss of E.M.F. 
 which would be caused by the deposit of free hydrogen. 
 
 Minotti's form of this battery is that which has been 
 hitherto adopted, almost universally, for telegraphic pur- 
 poses in India ; but before describing this in detail it may 
 be well to draw attention to some of the most important or 
 typical forms of battery, as described briefly in the follow- 
 ing paragraphs, with the object of illustrating how, by each 
 
 C 2 
 
2O 
 
 MANUAL OF TELEGRAPHY. 
 
 Daniell's 
 Batter}-. 
 
 combination, the properties essential to the acquisition of a 
 strong and constant current are obtained. 
 
 It will be observed that the high E.M.F. and low re- 
 sistance necessary for the production of a strong current 
 may be obtained at the expense of constancy ; and that the 
 most constant current is not the strongest which can be 
 produced : further, that the acquisition of electrical qualifi- 
 cations may be attended with cost, inconvenience, or me- 
 chanical difficulties disproportionate to the purposes for 
 which the battery may be required. 
 
 Taking all these points into consideration, the respective 
 merits and demerits of the various batteries described, as 
 relating to the special nature of the work for which they 
 are designed, can be compared. Their resistances, being 
 affected by variety of size, shape, mechanical arrangement, 
 nature of porous vessel or diaphragm, strength of acid 
 solutions, &c., cannot be compared in the same definite 
 terms as their E.M.F., which, for constant batteries, is the 
 same for every combination of the same kind independently 
 of its size or form. 1 
 
 5. Daniell's battery consists of a zinc and copper 
 pair ; the zinc, which is the active plate, is immersed in 
 dilute sulphuric acid contained in a porous vessel ; the 
 copper or negative plate is placed in an outer jar containing 
 a solution of sulphate of copper, which is kept saturated by 
 the insertion of crystals of the copper sulphate. 
 
 In this, as in every form of cell, the outer jar should be 
 made of some insulating and watertight material, such as 
 glazed stoneware, porcelain, guttapercha, &c. 2 
 
 E.M.F. = 1-079 volts. 3 
 
 Chief feature -.Constancy. 
 
 Polarisation is prevented by surrounding the negative 
 plate with the copper solution. 4 
 
 Advantages: (i) Constancy. 
 
 (2) Inexpensiveness. 
 
 Disadvantage : Waste of material due to internal 
 action when the battery is not at work, an unpreventable 
 cause of which is the property called ' osmose' 5 
 
 Use : Adapted by its constancy for continual work, 
 and consequently well suited for purposes of telegraphy. 
 
 2 Sometimes 
 the outer vessel 
 is made of 
 copper, and so 
 forms the 
 negative plate 
 as well ; but 
 this would not 
 do for a tele- 
 graph battery, 
 whose cells 
 should always 
 be as well 
 insulated as 
 possible ( 38). 
 
 5 Defs. 7 and 
 34- 
 
 4 Def. 19, and 
 
 4- 
 
 5 Def. 28. 
 
TELEGRAPH JJ^-LJ. .*^yvy.c,07>^>--J ^ '-- i^<y 
 
 The zinc solution, which by the action of the battery SECTION 
 becomes sulphate of zinc, 1 requires daily attention and re- . ^ _^ 
 plenishment with water, to prevent its becoming saturated i i 2 . 
 and forming crystals. 2 * 35. 
 
 On the other hand, the copper solution should be kept 
 saturated by replenishment, as required, with crystals of 
 sulphate of copper (Hindustani tutia), and any excess of that 
 solution owing to osmose action should be removed by 
 means of a sponge, or, if necessary, a syringe. 3 5 34. 
 
 It may here be mentioned that all the materials used for 
 batteries should be of the purest description obtainable. 
 The use of inferior chemicals is likely to result in setting up 
 action other than that designed, which must result in im- 
 pairing the electrical and even the mechanical efficiency of 
 the battery. 
 
 The water used for replenishing battery cells should 
 always be the softest procurable. Cells should not be too 
 full. 
 
 Minotti 6. In the Minotti battery the zinc plate is immersed 
 
 Battery. j n waten j ts chemical action produces the same results as 
 that of the Daniell; their E.M.F. is, therefore, the same. 
 By the use of water, however, without acid, the original re- 
 sistance of a Minotti is greater than that of a Daniell cell, 
 so it takes longer to come into working order. 4 Its normal 4 2 i. 
 resistance, moreover, is much higher than that of an ordinary 
 Daniell of corresponding size, owing to the nature of its 
 diaphragm, which is of sand or sawdust instead of a porous 
 vessel. 
 
 With this exception, the Minotti differs only from the 
 Daniell in the mechanical form and arrangement of its 
 materials, by which simplicity, economy, and portability are 
 gained, and the action of gravity is utilised and assisted by 
 the horizontal diaphragm, by which the tendency of the 
 liquids to mix is reduced. 5 * 29. 
 
 The construction of this form of battery, which has been 
 
 found the most suitable for general use in India, will be 
 
 described in detail hereafter. 6 G 16 et seq. 
 
 Leclanche J. The battery of Leclanchd consists of a zinc plate, 
 
 Battery. usua lly in the form of a cylindrical rod, immersed in a strong 
 
 solution of salammoniac, which occupies the outer jar ; into 
 
 this is placed a porous vessel containing a plate of carbon 
 
 surrounded by a mixture of binoxide of manganese and 
 
22 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION carbon in coarse grains ; these form a compact mass in the 
 
 B. 
 
 porous vessel 
 
 Grove's 
 Battery. 
 
 E.M.F. = 1-48 volts. 
 
 Chief feature : No consumption of material when 
 not in work ; but when the poles of the battery are joined 
 continuously the action is attended by the deposit of free 
 hydrogen on the carbon plate, and polarisation is the 
 result. 1 
 
 Advantages: (i) Freedom from loss of current by 
 local action. 2 
 
 (2) Of very long duration from the same cause. 
 
 Disadvantages : Polarisation and consequent incon- 
 stancy when used continuously. 
 
 Use : As the bubbles of free hydrogen soon disappear 
 when the poles of the battery are disconnected, the Leclanche 
 is peculiarly adapted for the ringing of bells and all kinds of 
 occasional work ; the great length of time it will last when 
 thus employed, without requiring attention, renders this 
 form of battery particularly suitable for dwelling-houses, 
 private telegraph or telephone offices, where incessant com- 
 munication is not kept up. 
 
 The solution of salammoniac (hindustani nausada) re- 
 quires renewal after a time ; likewise the zinc plate, which 
 must be kept clean. 3 
 
 8. In Grove's, as in all the other forms of battery de- 
 scribed herein, the active or positive plate is zinc, which is 
 immersed in dilute sulphuric acid contained in the outer 
 jar ; into this is placed a porous vessel containing the nega- 
 tive plate platinum immersed in concentrated nitric acid. 
 
 E.M.F. = from 1*63 to 1*956 volts, 
 
 being dependent on the strength of the acid solutions. 
 
 Chief feature : Great strength of current without 
 polarisation, as the hydrogen given off combines with the 
 oxygen of the nitric acid to form water, 4 and is thus pre- 
 vented from being deposited on the negative plate. 
 
 Advantages: High E.M.F. with very low re- 
 sistance, and consequently a strong current. 5 
 
 Disadvantages: (i) The materials, especially the 
 platinum, are very expensive. 
 
 (2) The fumes given off (N 2 O 4 ) are injurious. 
 
 1 Def. 19. 
 
 2 33- 
 
 3 A recent and 
 convenient 
 form of this 
 battery is that 
 known as the 
 
 ' agglomerate 
 block, "in which 
 the porous cell 
 is dispensed 
 with and a 
 conglomerate 
 block of the 
 binoxide of 
 manganese is 
 attached to 
 each of the two 
 faces of the 
 carbon plate ; a 
 porcelain or 
 earthenware 
 separator is 
 placed between 
 this block and 
 the zinc rod, 
 the whole being 
 bound together 
 by indiarubber 
 bands and im- 
 mersed in the 
 solution 
 opalammoniac, 
 
 4 12. 
 
 3- 
 
TELEGRAPH BATTERIES. 
 
 Bichromate 
 Batteries. 
 
 (3) The strength of current falls off as the nitric acid is 
 consumed. 
 
 (4) Frequent attention is necessary from the above 
 cause. 
 
 Use : Frojn the features of this battery enumerated 
 above, it will be evident that it is greatly superior in point 
 of current strength to any of the forms of battery previously 
 described ; but it is wanting in the lasting power of either 
 the Daniell or Leclanche. It is thus adapted for occasional 
 use where great strength of current is required, being espe- 
 cially valuable for the lecture-room, for laboratory work, the 
 production of the electric light, or for any unprotracted ex- 
 periments where current strength is the chief desideratum. 
 
 It is advisable to dismantle the cells of Grove's battery 
 when not in use, to prevent waste of material. They can be 
 quickly set up again ready for use as required. 
 
 As a preventive against local action, the zincs are always 
 amalgamated. 1 i Def. 33. 
 
 p. Bunsen's battery is a modification of Grove's, and 
 in its construction differs from the latter only in the nature 
 of the negative plate, which is of carbon instead of plati- 
 num. 
 
 E.M.F. = 2 volts (about). 
 
 The current elicited from a Bunsen is somewhat less 
 than that of a Grove, owing to the latter having the lower 
 resistance of the two. 
 
 The chemical action is the same in both, and conse- 
 quently the principal disadvantages of the Grove are common 
 to the Bunsen ; the chief advantage of the latter, as com- 
 pared with the Grove, being its lower prime cost, owing to 
 the substitution of carbon for platinum as the negative plate. 
 
 The precautions taken to prevent waste of material in 
 the Grove battery are necessary also in the case of the 
 Bunsen. The solutions removed from these batteries can 
 be used again for recharging them. 2 
 
 10. Various forms of bichromate batteries are made in course be kept 
 which bichromate of potash, combined with a powerful separ 
 acid or agent, is used as the exciting liquid or compound. 
 The most common form is that in which zinc and carbon 
 plates are immersed in a solution composed of bichromate 
 of potash and sulphuric acid, this combination being pro- 
 
 2 They must of 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 B 
 
 Fuller's 
 Battery. 
 
 ductive of high E.M.F. and low resistance, thus forming a 
 powerful and comparatively inexpensive battery, and one 
 which is much used for surgical purposes, either for work- 
 ing electro-motors or even for electric lighting on a small 
 scale. 
 
 The inconstancy of the ordinary bichromate battery, 
 however, renders it unsuitable for telegraphic purposes. The 
 liquid requires to be often stirred or otherwise kept in mo- 
 tion ; the plates have to be kept out of the liquid when the 
 battery is not in use, to prevent useless waste of the zincs, 
 which further require to be amalgamated to prevent local 
 action, 1 which would otherwise take place to a very preju- x 33. 
 dicial extent. 
 
 The crystals of bichromate of potash are first dissolved 
 in hot water, and the sulphuric acid is not added until the 
 solution is cold. 
 
 II. Fuller has, in his mercury-bichromate of potash 
 battery, devised an important modification of the fore- 
 going : the plates are zinc and carbon, the former being 
 always kept amalgamated by the use of mercury in the porous 
 cell, thus remedying one of the evils to which the ordinary 
 bichromate battery is exposed. He further introduces a 
 cylindrical porous vessel in which stands the zinc, which is 
 cast into a shape peculiar to Fuller's form of battery, and 
 consists of a cylindrical rod widening at its base which just 
 fits into the porous vessel. Sufficient mercury is placed in 
 the porous vessel to cover the flattened base of the zinc ; a 
 small quantity of sulphuric acid is added, the porous vessel 
 being then filled up with water. The outer jar contains the 
 carbon plate, immersed in a solution of bichromate of 
 potash and sulphuric acid, to which water is added. 
 
 This form of battery, also, is liable to polarise when joined 
 through a very low resistance, but is practically constant 
 when the external resistance is considerable, as in the case 
 of an ordinary telegraph line worked on * open circuit.' 2 - 179. 
 
 The Fuller has been used with advantage in one or two 
 of the chief offices in India for line circuits ; and one of 
 these cells when thus used is found to do the work of at 
 least two Minotti. It is, however, neither so portable nor 
 more than one quarter as lasting as the Minotti ; neither is 
 it (owing to its greater tendency to polarise) as suitable for 
 low- resistance circuits, such as in the case of earth testing, 
 
TELEGRAPH BATTERIES 
 
 Chemical 
 Action. 
 
 &C. 1 Its cost of maintenance, too, exceeds that of the 
 Minotti. 
 
 Both the outer and inner jars of the Fuller are kept re- 
 plenished with water, and any appearance of blue solution 
 in the porous vessel is at once removed. 
 
 When the outer solution becomes blue, more potash is 
 added ; or, if it retain its orange colour and the current 
 fails, more sulphuric acid is added, some of the inner solu- 
 tion being at the same time removed, and the porous vessel 
 replenished with water. 
 
 If the potash solution is over-saturated, crystals of 
 chrome alum form on the carbon plate ; some of the solu- 
 tion should in this case be removed, and a little sulphuric 
 acid and water be added. 
 
 12. The chemical action, which takes place in the 
 above-mentioned batteries when the poles are joined, may 
 be briefly described in chemical symbols as follows : 
 
 SECTION 
 B. 
 
 CIRCUIT OPEN (i.e. poles disconnected). 
 
 DANIELL- 
 
 Zn S0 4 H 2 
 
 [ Zinc plate Sulphuric acid 
 
 SO 4 Cu 
 
 Sulphate of copper 
 
 CIRCUIT CLOSED (i.e. poles joined). 
 
 Cu 
 
 Copper 
 plate 
 
 ZnSO 4 
 
 Sulphate of zinc 
 
 H 2 SO 4 
 
 Sulphuric 
 acid 
 
 CuCu 
 
 Copper deposited 
 on copper plate 
 
 Zn 
 
 Zinc plate 
 
 LECLANCHE"-! 
 
 2NH 4 C1 
 Salammoniac 
 
 OPEN. 
 
 2MnO 2 
 
 Binoxide of 
 manganese 
 
 CLOSED. 
 
 Zncl 2 2NHa H 2 O 
 
 Chloride of Ammonia Water 
 zinc 
 
 Mn 2 O 3 
 
 Sesquioxide of 
 manganese 
 
 c 
 
 Carbon plate 
 
 C 
 
 Carbon plate 
 
 GROVE 
 
 OPEN. 
 
 Zn H 2 SO 4 
 
 Zinc plate Sulphuric acid 
 
 DIAPH- 
 
 N 2 O 5 C 
 
 Nitric acid Carbon plat 
 
 CLOSED. 
 
 ZnSO 4 
 
 Sulphate of zinc 
 
 I 
 
 -RAGM. 
 
 H 2 N 2 4 C 
 
 Water Nitrous acid Carbon 
 (gas) plate 
 
26 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 B. 
 
 
 Zn H 2 S0 4 
 
 Zinc plate Sulphuric acid 
 
 PE 
 
 a 
 
 N. 
 
 N 2 O ; -, C 
 
 Nitric acid Carbon plate 
 
 BUNSEN- 
 
 CD 
 
 DS 
 
 iD. 
 
 
 ZnSO 4 
 
 Sulphate of zinc 
 
 -RAGM. 
 
 H 2 N 2 4 C 
 
 Water Nitrous acid Carbon 
 (gas) plate 
 
 Conditions to 
 
 Telegraph by 
 Batteries. 
 
 Def. 19. 
 
 3 8. 
 
 4 Def. 20, and 
 
 13. The design of any battery is to produce a 
 current with the object of doing certain work, and the 
 stronger the current the more work it will do in a given 
 time. Now, by Ohm's law, 1 we know that the greater the i Law 
 E.M.F. of a battery, and the less its resistance, the stronger A PP- A 
 the current it will give ; so, to get the strongest possible cur- 
 rent, all we have to do is to make up a battery which shall 
 produce the greatest E.M.F., and offer the lowest resistance 
 possible. We have seen, however, that where everything 
 else is sacrificed to these two attributes high E.M.F. and 
 low resistance the current produced, though strong, suffers 
 in other ways, the principal of which are polarisation 2 and 
 falling off in strength as the acid solutions are consumed by 
 the action of the battery. 3 
 
 Both of these causes produce inconstant currents* which 
 would never do for telegraph work, for which a steady and 
 continuous current is required ; and, as low internal resist- 
 ance facilitates polarisation, there is a limit to which we 
 can reduce the resistance of batteries without endangering 
 their constancy. Hence, the main points to be aimed at in 
 the construction of a telegraph battery are really maxi- 
 mum E.M.F. and constancy. Cost of materials, chemical 
 simplicity, and freedom from local action 5 also demand 
 consideration. 
 
 DanielPs battery combines these qualifications better 
 than any other class of battery yet devised. It stands 
 highest of all in order of constancy, and although its E.M.F. 
 is only half that of a Grove, yet there is no other electro- 
 chemical combination which can produce such a high 
 E.M.F. at so low a cost ; and E.M.F. can, in the case of 
 telegraph batteries, be multiplied by increasing the number 
 of cells in series, as will be more fully explained hereafter. 6 
 Further, copper, zinc, and sulphate of copper are all easily 
 procured, even in India. 
 
 33 . 
 
 252-254. 
 
TELEGRAPH BATTERIES. 
 
 E.M.F. ; how 
 obtained. 
 
 Constancy : 
 how obtained. 
 
 The Minotti form of this battery is the most portable 
 and convenient ; the zinc or active plate is easily inspected 
 and removed ; the diaphragm of sand or sawdust is inex- 
 pensive and unbreakable, and forms a convenient means of 
 regulating the resistance of the cell ; and the use of water 
 instead of an acid around the zinc, renders the Minotti 
 less liable to local action from surface impurities l than the l 33 
 ordinary Daniell, and dispenses with the necessity for amal- 
 gamation. 2 - Def - 33* 
 
 14. The property of electromotive force, being the result 
 of chemical action, causing a difference of potential to be 
 manifested by the respective plates of the battery when 
 immersed in liquid solutions, 3 it becomes necessary to select 3 2. 
 such a combination as shall produce the greatest possible 
 difference, compatible with constancy, between the poten- 
 tials of the plates. This is done by immersing them in a 
 solution, or solutions, which shall act vigorously on one (the 
 active) plate, and as little as possible, or not at all, on the 
 other. 
 
 The combination used in the case of the Minotti 
 battery 4 is productive of fair E.M.F., the exciting liquid 4 6andi6. 
 not being a strong acid, as in the case of the Grove and 
 Bunsen, but merely water which is converted by the action 
 of the battery into sulphate of zinc. This zinc sulphate 
 solution is kept replenished with water, and should never 
 be allowed to become more than semi-saturated, for the 
 following reasons : 
 
 1. The semi-saturated condition is that of least resist- 
 ance to the current. 
 
 2. When saturated it becomes more dense than sulphate 
 
 of copper, so the principle of the gravity battery 5 is no 5 6 and 29. 
 longer retained. 
 
 3. The crystals formed by saturation are productive of 
 various evils as explained in paragraph 35. 
 
 15. The great advantage of the Minotti is the constancy 
 of its E.M.R, which remains the same from the time the 
 battery comes into action to the time it is exhausted. It 
 does not depend upon the strength of acid solutions, and 
 the hydrogen generated by the action of the battery, which 
 is the great enemy to constancy when in its free or uncom- 
 bined state, by virtue of its polarising effects, is prevented 
 from remaining free by its immediately combining, by 
 
28 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 B. 
 
 Description of 
 the Minotti 
 Cell. 
 
 chemical affinity? with the sulphate of copper solution, and 
 taking the place of a certain amount of copper which it 
 reduces from the solution, depositing it on the copper plate 
 immersed therein, the plate being thus continually supplied 
 with a fresh bright copper coating, so long as the action of 
 the battery continues. 
 
 16. The Minotti cell is constructed as follows : 
 Into a cylindrical insulated jar (generally of highly glazed 
 stoneware), standing on three projections, which are made 
 with the object of reducing surface-leakage to a minimum, 2 
 is first placed a circular disc of copper to which a covered 
 leading wire has been previously attached in the following 
 way : The wire (which is usually Hooper's core) is cut 
 into a length of 15 inches for each cell ; the end which is 
 to be attached to the copper disc is bared for 2 \ inches, and 
 well cleaned. Into the disc, which is also well cleaned, are 
 punched three holes one near the circumference, one at 
 the centre, and one between the two ; each just large enough 
 to receive the bare copper wire, which is then threaded 
 through them until the end of the insulating covering is 
 drawn close down to the disc. 
 
 The bare end of the wire and the disc are then ham- 
 mered together so as to secure a good contact between 
 them. 
 
 The disc, with its leading wire attached, is then laid flat 
 at the bottom of the jar into which it just fits ; the leading 
 wire (the upper end of which is bared for i inch, and subse- 
 quently forms the copper or + pole 3 of the cell) is led up 
 the side of the jar. Upon the disc is then placed sulphate 
 of copper in powdered crystals, the quantity being deter- 
 mined according to the work required of the cell. 4 
 
 Over this layer of sulphate of copper is placed a disc of 
 linen, or porous paper, or felt ; then the diaphragm, which 
 consists of a layer of sawdust or sand, on which is placed 
 another disc similar to that below the diaphragm, which is 
 thus kept together by means of these discs. 
 
 On this is laid the zinc plate, which consists of a thick 
 circular disc, the upper surface of which has a vertical pro- 
 jection or neck, into which a brass terminal is introduced at 
 the time of casting the disc, and which forms the pole of 
 the cell. Water is then carefully poured in until it rises 
 just above the level of the flat surface of the zinc, so that 
 
 1 i.e. the 
 natural ten- 
 dency which 
 certain chemi- 
 cal bodies have 
 to unite with 
 one another. 
 
 38. 
 
 i (note). 
 
 26. 
 
TELEGRAPH BATTERIES. 
 
 Use of the 
 Diaphragm. 
 
 Respective 
 Merits of Sand 
 and Sawdust 
 diaphragms. 
 
 the connection between the zinc neck and the brass ter- 
 minal is above water. Hard water should not be used ; 
 but rain water, if possible, as being most free from impurities. 
 The circular zinc and diaphragm discs are each made with 
 a groove or nick in its circumference, through which the 
 leading wire passes. 
 
 17. The diaphragm is the partition used in two-fluid 
 cells to keep the liquids apart as much as possible, 1 and, at 
 the same time, to allow of the passage of the current through 
 it without offering too great resistance thereto. In most 
 forms of battery the diaphragm consists of a porous, unglazed 
 earthenware vessel : the sand or sawdust diaphragm of the 
 Minotti, however, has special advantages, already referred 
 to. 2 
 
 Sand, if used, should be river, not sea sand, and should 
 not, of course, be so fine as to be rendered imporous. If 
 sawdust is used, it should be of teak or some hard wood. 
 Either should be well washed before use, the water being 
 squeezed out of it before it is placed in the cell. It should 
 be packed in little by little, each layer being flattened down 
 by the hand or with a rammer of wood of similar shape to 
 the zinc disc, the pressure being increased as more is added, 
 especially around the leading wire, which should be kept as 
 rigid as possible. 
 
 18. Sawdust has this advantage over sand for a dia- 
 phragm, that, being compressible, it holds the leading wire in 
 a fixed position, whereas with a sand diaphragm the slightest 
 movement of the wire outside the cell is communicated to 
 the inner part too, the result being just the same as if the 
 cell had been shaken, 3 for it causes the copper solution to 
 work up to the zinc, creating local action 4 and possibly de- 
 stroying the E.M.F. of the cell. The sand diaphragm, 
 on the other hand, has this advantage, that it sinks as the 
 sulphate of copper beneath it is consumed, whereas sawdust 
 has been known to form a cake which clings to the sides of 
 the jar, thus creating a space between the diaphragm and 
 the receding sulphate of copper, whereby the resistance of 
 the cell is increased. Further, cells with sand diaphragms 
 last longer than those in which sawdust is used. If, how- 
 ever, the cells require to be moved, there is less danger from 
 shaking when sawdust is used, owing to its forming a more 
 rigid mass than sand. 
 
 SECTION 
 B. 
 
 1 Def. 28, and 
 29. 
 
 13. 
 
 5 29. 
 4 33^ 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 B. 
 
 New Cells: 'On 
 Short Circuit." 
 ' In Series. ' 
 'Parallel.' 
 
 'Salt : its Use 
 ;and Danger. 
 
 Ip. A cell prepared as above offers a very high resist- 
 ance, and must continue to do so until, by some chemical 
 process, the water, which, as we have shown before, is the 
 worst of liquid conductors, is converted into some solution 
 of less resistance. This is attained by joining together the 
 poles of the cell, and thus allowing the current (however 
 weak at first) to pass between them. Chemical action at 
 once sets in, 1 the water above the diaphragm eventually be- i 30. 
 coming sulphate of zinc, and that below, sulphate of copper. 
 The diaphragm itself also becomes saturated, the upper part 
 with zinc sulphate and the lower with copper sulphate. As 
 this change goes on so does the internal resistance of the 
 cell decrease, until the zinc solution becomes semi-saturated 2 2 *4- 
 and the copper solution completely so, the strength of the 
 current increasing accordingly, until the cell comes into 
 working order. 3 3 3- 
 
 In order that the current may have no unnecessary ob- 
 stacle in the shape of external resistance, the poles of the 
 cell are joined directly one with the other, in which condition 
 the cell is said to be * on short circuit.'' 
 
 When cells are joined together to form a battery, with 
 the copper or + pole of one to the zinc or pole of the 
 next, and so on, they are said to be joined 'in series? If, 
 however, all the + poles are joined together to form one 
 electrode, 4 and all the poles together to form the other, 4 Def. 26 
 then the cells are said to be joined * parallel.' 1 
 
 To bring new cells into working order, each cell is joined 
 on short circuit, so that the current of each has only its own 
 cell's resistance to overcome, and the action of a whole battery 
 is not impeded by any undue resistance which may be ex- 
 hibited by any individual cell, as would be the case were 
 they all joined in series together. 
 
 20. As the action of a cell reduces its resistance, so in- 
 versely, if the resistance of a new cell be reduced by any 
 mechanical means, such as the addition of an acid or salt, 
 its action is accelerated thereby. On this principle, when it 
 is required to hasten the action of a newly prepared cell, it 
 may be done by surrounding the zinc disc with a solution of 
 common salt and by saturating the diaphragm with the same 
 before it is put into the cell. Time would be further saved 
 by adding water to the powdered sulphate of copper before 
 the diaphragm is placed over it. This is a course, however, 
 
TELEGRAPH BATTERIES. 
 
 To bring New 
 Cells into 
 Working 
 Order imme- 
 diately. 
 
 Extra Battery 
 Power for 
 Emergencies. 
 
 Diaphragm 
 and its Defects. 
 
 2 33- 
 
 3 Def. 20. 
 
 not to be recommended except under the most exceptional 
 circumstances, and when sulphate of zinc is not obtainable ; 1 
 as the use of salt favours local action, 2 prevents the uniform 
 deposit of copper on the copper disc, and interferes with the 
 constancy of the cell. 3 
 
 21. The best way to bring a new cell into immediate 
 working" order on an emergency is to pour in enough 
 water to nearly cover the powdered sulphate of copper on 
 the copper disc, then to soak the sand or sawdust to be used 
 for a diaphragm in a solution of sulphate of zinc, after in- 
 serting which the zinc disc is laid on, as usual, and sufficient 
 water added to cover its surface. 
 
 Sulphate of zinc in solution can be obtained by removing 
 the liquid round the zinc discs of working batteries. Any 
 copper which this may contain can be removed by heating 
 the liquid over a slow fire in an earthen vessel with pieces 
 of zinc in it. The liquid should then be evaporated off in 
 shallow earthen dishes, when the crystals of sulphate of zinc 
 which remain can be collected. 
 
 22. To prevent the necessity for resort to such expe- 
 dients, reserve cells are kept ready for use, in case extra 
 battery power may be required, whether it be for the pur- 
 pose of working through an increased length of line or to 
 compensate for leakage caused by imperfect insulation from 
 atmospheric or other causes. 4 
 
 The reserve cells should be prepared in the ordinary way, 5 
 brought into working order, 6 reduced to the proper resist- 6 19. 
 ance 7 and tested, after which the zinc discs should be re- t 4 6. 
 moved (to prevent waste through local action), 8 and, having 
 been cleaned and dried, be placed ready for immediate use. 
 
 23. Sometimes cells do not come into working order 
 owing to the presence of air which is prevented from escaping 
 by the tightness of the diaphragm, and which has the effect 
 of greatly increasing the resistance of the cell. 
 
 Bubbles of hydrogen would produce a similar effect, and 
 may be the result of joining the poles of the cell through 
 very low resistance. 9 Should the latter, only, be the case, 
 the bubbles will soon disappear on disconnecting the poles 
 of the cell ; if this treatment does not get rid of them, then 
 the diaphragm itself is probably the cause of the mischief, 
 and should be pierced with a sharp fine stick, the bubbles of 
 air or hydrogen gas, as the case may be, will then escape, 
 
 SECTION 
 B. 
 
 4 289. 
 
 5 16 and 17. 
 
 33- 
 
 Def. 19. 
 
MANUAL OF TELEGRAPHY. 
 
 Connecting 
 Wires of 
 Battery Cells. 
 
 Junction of 
 Connecting 
 Wires. 
 
 Difference 
 between Line 
 and Local 
 Cells. 
 
 Internal 
 Resistance 
 of Cells. 
 
 and the resistance being thus reduced, the cell will come 
 into working order. 
 
 The stronger the current and the lower the resistance in 
 its path, the more readily will hydrogen be liberated some- 
 times faster than it can be taken up so that it becomes 
 ' free,' l a fault which may thus occur more often in a local l Def. 19. 
 than in a line cell. 2 2 26. 
 
 24. Reference has been made 3 to the necessity for 3 16. 
 drawing the bare end of the leading wire which is connected 
 
 to the copper disc so far through it that the insulating 
 covering extends quite down to the disc. This is to prevent 
 the bare copper wire being eaten away by the corrosive 
 action within the cell. On the same principle, the insulating 
 covering of the wire should be perfectly free from cracks or 
 defects of any kind. 4 4 37> 
 
 25. The reason why the junction between the bare 
 wire and the disc is made by hammering, 5 and not by 5 16. 
 the help of solder, is that the latter would set up local action 
 
 in the cell (because it would manifest a potential different 
 from that of the copper), and waste of material would be 
 the result. 6 6 33 . 
 
 26. The consumption of sulphate of copper in pro- 
 perly prepared cells being in proportion to the strength 
 of current elicited, which we know by Ohm's law to be 
 inversely proportional to the resistance in circuit, 7 the E.M.F. 7 Law i,. 
 being constant, it follows that those cells which are joined pp ' ' 
 through low resistances, such as that of a ' local circuit] 8 8 180, 181. 
 which is generally about 50 ohms, would become exhausted 
 
 more rapidly than those cells which are joined through a 
 telegraph ' line circuit] 8 the resistance of which may be 
 several hundreds, or even thousands, of ohms. Twice as 
 much sulphate of copper is therefore used for local cells as 
 for line cells, the former being made to contain twenty-four 
 ounces and the latter twelve. Notwithstanding this, a line 
 cell should last much longer than a local cell, for it has 
 been proved 9 that the average current passing through a 
 line cell is about one-tenth of that required for working a 
 ' sounder,' 10 the usual resistance of which is about 30 ohms. 10 58. 
 
 27. Again, the internal resistance of a battery or 
 cell decreases with the consumption of material ; any 
 sudden increase of resistance, therefore, would show that 
 something was wrong. The defect may be in the diaphragm, 11 n 23. 
 
TELEGRAPH BATTERIES. 
 
 or may be due to the formation of some salt in the cell 
 introducing undue resistance. This is the case sometimes 
 when the initial chemical action of the cell deposits the in- 
 soluble oxide of zinc upon the zinc disc before the current 
 is sufficient to reduce it to sulphate of zinc. 1 When this is 
 the case, the zinc presents a white appearance, and should 
 be at once replaced by a clean disc. 
 
 28. The consumption of sulphate of copper is 
 indicated by the level of the zinc disc ; when this has 
 sunk to a depth corresponding to the original thickness of 
 the layer of sulphate of copper with which the cell was 
 charged, it will be found that the cell has become exhausted, 
 and its internal resistance reduced to the saturated dia- 
 phragm alone, about 5 ohms. 
 
 29. One of the advantages claimed for the Minotti is 
 that it is constructed on the principle of a 'gravity 
 battery,' the object of such being to prevent the natural 
 tendency always displayed by the liquids in a two-fluid cell 
 to mix with one another. In view to preventing this, the 
 gravity battery was devised, in which the heavier liquid (or 
 that of greater specific gravity) occupies the lower portion 
 of the jar, and the lighter liquid the upper ; porous vessels 
 or partitions are dispensed with, their office being per- 
 formed by the action of gravity alone. It will be readily 
 understood that the slightest movement of such a battery 
 would be liable to derange it, absolute stillness being neces- 
 sary to allow of the liquids maintaining their respective levels. 
 
 We have this principle in the Minotti in which the 
 saturated solution of sulphate of copper in the lower part of 
 the jar is heavier than the semi-saturated solution of sul- 
 phate of zinc in the upper part ; these solutions, however, 
 are kept apart, not by gravity alone, but by the porous dia- 
 phragm of sand or sawdust. Nevertheless, in this, as in 
 the case of the ordinary gravity battery, the greatest care 
 should be taken to avoid unnecessary movement or shaking 
 of the cells, the result of which would be that the copper 
 solution would be forced up to the zinc disc, on which a salt 
 of copper would be deposited and the E.M.F. of the cell 
 consequently endangered ; local action would be set up and 
 material uselessly consumed. 
 
 There is never perfect separation of the liquids even in 
 the Minotti ; so . there is always some waste, but this is 
 
 D 
 
 33 
 
 SECTION 
 B. 
 
 30- 
 
34 
 
 SECTION 
 B. 
 
 Processes by 
 which a Cell 
 comes into 
 Working 
 Order. 
 
 E.M.F. Con- 
 stant for 
 Similar Cells. 
 
 C varies with 
 R t by Ohm's 
 Law. 
 
 Local Action. 
 
 MANUAL OF TELEGRAPHY. 
 
 reduced to a minimum by care in the manipulation of the 
 cell. The utility of the diaphragm as a separator is greatly 
 dependent on the care with which it has been prepared. 1 
 
 To avoid shaking up the solutions, it is advisable to 
 place newly- prepared cells, before the water is added, in the 
 position which they are to occupy permanently. 
 
 30. When a cell is prepared in the ordinary way, 2 the 
 water has first to penetrate through the diaphragm down to 
 the sulphate of copper ; immediately it has done so, the 
 chemical action of the cell is started, and its E.M.F. esta- 
 blished, which, as explained before, remains very constant 
 up to the time the cell is exhausted. 
 
 Not so, however, the resistance, which at first is very 
 high, 3 but is reduced as chemical action goes on by the 
 formation of sulphate of copper in solution in the lower part 
 of the cell and diaphragm in place of water. As the resistance 
 is thus reduced, so does the force of the current increase, 
 according to Ohm's law, and as the current increases, so 
 does the action which it creates in the short-circuited cell 
 also increase, in virtue of which the solution occupying the 
 upper part of the cell and diaphragm is converted into sul- 
 phate of zinc. Thus, when the poles of a newly-prepared 
 cell are joined, the internal resistance continues to fall, and 
 the current strength to increase, until it is fit for use. 
 
 31. The E.M.F. of a Minotti being a constant quantity 
 so long as there is any action at all in the cell, and being 
 the same for all similar chemical combinations, 4 we can 
 multiply the E.M.F. of a battery by increasing the number 
 of cells joined in series, 5 remembering, however, that each 
 cell carries with it its own resistance, and that the current 
 emitted by a battery or cell, though directly proportional to 
 
 its E.M.F., is affected at the same time by its internal 
 
 p 
 resistance (C = -^).* 
 
 32. Hence it follows that when the internal resistance (jR) 
 of a battery cell varies (E being constant), the current (C) 
 also varies ; and as the current (C) is the property of the cell 
 which produces signals, deflections, &c., it is evident that the 
 larger 7? becomes, the less working power the cell displays. 
 
 33. Ohm's law thus teaches us that when we have high 
 E.M.F. and low resistance, we may expect a strong current. 
 We know also that the strength of current produced by a 
 
 1 17 and 44. 
 
 16. 
 
 19- 
 
 4 2. 
 
 5 19. 
 
 * Law : 
 App. A. 
 
Use of Battery 
 Sponge or 
 Syringe. 
 
 Formation of 
 Zinc Sulphate. 
 
 TELEGRAPH BATTERIES. 
 
 battery or cell is proportional to the amount of battery 
 material consumed. To avail ourselves, however, of the 
 full value of the current, it is necessary that it should accu- 
 mulate at the poles or electrodes of the battery or cell, l 
 otherwise it is lost to practical use, and battery material is 
 consumed to no purpose. 
 
 This waste is due to what is called local action, the 
 effect, often, of impurities (principally iron) in the zinc plate. 
 The iron and the zinc assume different potentials in the 
 exciting acid, and each particle of iron and the zinc sur- 
 rounding it form the plates of a closed battery, the circuit of 
 which is completed by the solution in which they are im- 
 mersed, the result being that chemical action is always going 
 on, whether the poles of the battery cell are open or closed, 
 and the zinc, which is the electropositive metal, is consumed 
 to no purpose. 2 
 
 To prevent this cause of local action, battery zincs which 
 have to be immersed in strong acids are amalgamated, with 
 the object of rendering their surface uniform. 3 
 
 This precaution is not necessary in the case of the 
 Minotti battery, in which the zinc is immersed in water, 
 subsequently converted by the action of the current into 
 sulphate of zinc ; local action, however, occurs in the 
 Minotti, from the tendency of the liquids to mix, which 
 can never be prevented altogether in any two -fluid battery, 
 though care in the first preparation and in the subsequent 
 handling of the battery will reduce this source of waste of 
 material and loss of current to the lowest possible limit. 
 Amalgamation would, of course, be no remedy for this. 
 
 34. A battery sponge is useful for removing the water 
 from cells in which it has become impure or saturated. 4 
 
 Syringes may, under certain circumstances, be convenient 
 for this purpose, but if so, they should not be used also for 
 replenishing the cells with fresh water ; for there would be 
 a risk of re-inserting impurities, particularly copper solution, 
 which would do as much harm as shaking the cell. A metal 
 vessel should not be used for replenishing cells, unless it be 
 made of zinc. 
 
 35. White crystals are sometimes seen on the zinc 
 discs as well as on the jars of working cells, caused by the 
 water becoming saturated with the sulphate of zinc formed 
 by the chemical action of the cell, the principal danger 
 
 D 2 
 
 35 
 
 Def. 33. 
 
 5- 
 
MANUAL OF TELEGRAPHY. 
 
 Composition 
 to prevent 
 Corrosion. 
 
 To clean 
 Coppers and 
 Zincs. 
 
 Leakage from 
 Cell to Cell. 
 
 attending the formation of which is that it is likely to reach 
 the junction between the brass terminal screw and the neck 
 of the zinc disc, and being when dry a very bad conductor, 
 it would introduce a large unnecessary resistance into the 
 circuit. 
 
 Secondly, it is liable to form on the sides and edges of 
 the cell, and cause leakage by drawing off the liquid of the 
 cell by capillary action. 
 
 Prevention. (i) As the crystals are caused by satura- 
 tion, keep the cells replenished with clean water, removing 
 the zinc solution when necessary. 
 
 (2) The first fault is prevented by keeping the water 
 below the junction of the zinc disc and brass terminal screw. 
 
 (3) The second by keeping the jars dry and clean, but 
 where this is impracticable, by coating their rims with 
 paraffin. l 
 
 (4) A mixture of coal tar and resin (or sealing wax), 
 painted over the junction between the neck and brass ter- 
 minal of the zinc disc, prevents the formation of sulphate 
 of zinc at this point. 
 
 36. To clean copper discs. (i) Keep them from 
 exposure to the air. 
 
 (2) On finding them dirty or tarnished, immerse them 
 at once into boiling water with tamarinds in it. 
 
 (3) When bright and clean, wash them with fresh water, 
 and leave them under water till required for use. 
 
 (4) If the above is not sufficient, the leading wires must 
 be taken off, and the disc must be heated in a charcoal fire 
 and plunged, while hot, into cold water. 
 
 To clean zinc discs : 
 
 [Zincs which are much eaten away should be re-cast.] 
 
 Those which are only slightly worn should be brushed 
 until they assume a bright smooth surface. This can be 
 done either with a battery brush of iron or steel wire, or the 
 surface may be ground smooth on sandstone. 
 
 Zincs must not be kept in water. 
 
 37. Another point in the conservancy of batteries to be 
 observed, with a view to its prevention, is leakage taking 
 place by capillary action through the wires which connect 
 the cells with one another, inside the insulated covering of 
 the wire, caused by the latter not adhering as closely to the 
 wire as it should. 
 
 1 This is best 
 done by 
 melting the 
 paraffin in a 
 shallow dish, 
 and dipping 
 the inverted 
 battery jar into 
 it, so that the 
 rim becomes 
 covered to a 
 depth of about 
 a quarter of an 
 inch. 
 
Adaptation of 
 ' Battery Power 
 
 TELEGRAPH BATTERIE 
 
 This affects the working of the cell seriously, 
 the copper solution of one cell to be conveyed to the zinc 
 solution of the next. 
 
 Remedy. Seal up the ends with wax, guttapercha, 
 paraffin, or Chatterton's compound. 
 
 To prevent the same happening through the outer surface 
 of the connecting wires, soak them before use in melted wax. 
 
 38. Leakage may be caused by a damp battery stand, or by 
 cracked jars, or by cells touching one another, the result being 
 that the cells are partially short circuited ; their current is 
 
 thus shunted, 1 and a portion of it is consequently lost to ' Defs. 21, 22. 
 practical use. The insulation of the battery stand may be 
 effected by standing it on insulators, these being wiped 
 daily. 
 
 39. When a battery is worked out, as shown by the signs 
 of exhaustion referred to in paras. 27 and 28, it should be 
 dismantled and the contents of the cell disposed of as 
 follows : 
 
 Copper deposit, when it can be detached from the 
 disc, should be collected, as it is pure metallic copper ; 
 when adhering to the disc so firmly that it cannot be 
 hammered off without injuring the disc, clean with tamarind, 
 if necessary, 2 and use again. 2 36. 
 
 Sulphate of copper remaining should be washed and 
 used again, its chemical state being unchanged by the action 
 of the battery. 
 
 The diaphragm, on the contrary, becomes saturated 
 with mixed solutions which, in the case of a sawdust dia- 
 phragm, can never be removed. A sand diaphragm may 
 be cleansed by being thoroughly well washed ; in either case, 
 however, it is safer not to use an old diaphragm again. It 
 the cell has been carelessly handled metallic copper may be 
 found in the diaphragm. 
 
 40. When a cell is dismantled, it can be seen by 
 the state of its contents whether it has been care- 
 fully prepared and looked after. 
 
 First) by the regular deposit of metallic copper, which 
 should be evenly distributed over the copper plate. 
 Second, by the absence of copper in the diaphragm. 
 Third, by the cleanness of the zinc disc. 
 
 41. Battery power for line work is regulated as 
 follows : 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 
 
 'No. 
 
 ll 
 
 B. 
 
 
 
 
 
 RULE 
 
 5> 
 
 3 
 
 to Length and 
 
 
 
 A 
 
 Gauges and 
 Climatic 
 Changes. 
 
 Use for 
 every 100 
 
 " 
 
 " 
 
 *f 
 
 Si 
 
 8 
 
 
 miles of 
 
 55 
 
 _ i 
 
 rNo. iB.W.G.=5o I.G. Wire, 4 cells in series 
 
 Arrangement 
 of Cells for 
 Local Circuits. 
 
 Standard Cell. 
 
 Summary of 
 Rules for the 
 Preparation 
 of Battery 
 Cells. 
 
 Conservancy. 
 
 = 24 
 = 16 
 
 = 12 
 
 = 6 
 
 5 
 6 
 8 
 
 12 
 
 16 
 13 
 
 during the dry season ; and during the wet, so many more 
 cells as the state of the line may require. 1 
 
 42. Local batteries are arranged as follows : 
 The average resistance of a departmental sounder 2 being 
 
 about 30 units, it is worked to the best effect with four local 
 cells joined in series ; 3 if more sounders than one are to be 
 worked, they are grouped in couples, and one local battery 
 of four cells in series and two parallel 4 is connected up with 
 each group according to the diagram shown in fig. 46. 5 
 
 43. A standard cell is kept in every office as a unit of 
 E.M.F., and the E.M.F. of any battery or cell is estimated 
 relatively to the standard cell which is taken as unity 
 
 (=!).* 
 
 It is prepared like any other cell, with particular care^ 
 and is never used for any purpose other than testing. 
 
 44. The following are the most essential points to be re- 
 garded in setting it up, and may be considered a summary 
 of rules for guidance in the preparation of any battery cell : 
 
 (1) Take care that the jar and all that goes into it are 
 dean. 
 
 (2) That the leading wire is well insulated. 
 
 (3) That the contact between this and the copper plate 
 is clean and firm. 
 
 (4) That the zinc terminal screw bites. 
 
 (5) That leakage is not caused by the wire, allowing of 
 capillary action within the insulated covering. 
 
 (6) That the diaphragm is of right consistency, not too 
 tight nor too wet, and not offering unnecessary resistance in 
 the shape of paper or other discs above and below it ; nor 
 too loose, so as to admit of the liquids mixing. 
 
 45. To preserve the standard cell. (i) Most care- 
 fully avoid shaking it. 
 
 (2) Keep the zinc disc covered with water, but keep this 
 below the brass terminal. 
 
 1 This is dis- 
 covered by 
 measuring the 
 arriving 
 current, 290. 
 
 2 73- 
 
 5 Law 15, 
 App. A. 
 
 4 19- 
 
 5 181. 
 
 * The result in 
 volts would be 
 E. x i '079, as 
 the E.M.F. of 
 a Minotti = 
 1*079 volts 
 5- 
 
R.ough 
 tandards of 
 efficiency for 
 Line,' 
 Local/ 
 Testing,' and 
 Standard ' 
 Jells. 
 
 'ortable 
 Battery. 
 
 TELEGRAPH BATTERIES. 
 
 (3) Remove froth and all dirt from the surface of the 
 water. 
 
 (4) Change the zinc whenever it becomes discoloured. 
 
 (5) Keep the circuit open, and only use the cell for 
 testing. 
 
 Criterion of efficiency of the standard cell : 
 
 Its resistance should be about 20 ohms, and its current 
 
 should give a deflection of not less than 60 through the 
 
 thin coil of the tangent galvanometer. 1 
 
 46. The following may be considered standards of 
 
 efficiency (useful, though rough) of the various kinds of 
 
 batteries used : 
 
 39 
 
 
 Best 
 
 resistance 
 
 Best deflection (tangent galvanometer) 
 
 Standard cells 
 
 Line 
 Local 
 Testing 
 
 20 ohms 
 
 30 
 
 IO 
 20 ,, 
 
 6 o / (not less} through thin coil, 
 \and no resistance. 
 
 cc 
 55 " 
 
 65 
 60 
 
 152. 
 
 47. The departmental form of portable battery con- 
 sists of a small box divided into cells by the plates them- 
 selves copper and zinc which are soldered together in 
 pairs, the rims being covered with pitch to prevent local 
 action between them. 2 
 
 It is filled with sand and powdered salammoniac 
 moistened with water (or cotton cloth, in pieces, saturated 
 
 with a solution of salammoniac). 
 
 /, . . r , 
 
 Zinc is dissolved and chloride of zinc formed. 
 
 [NOTE. For < Battery Testing' see Section F, Paras. 
 267-288. 3 ] 
 
 33- 
 
 3 The faults to 
 w hi c h the 
 battery is liable 
 
 216-224. 
 
SECTION C. 
 
 TELEGRAPH INSTRUMENTS 
 
 PART I. SIGNALLING INSTRUMENTS 
 
 48- 57. PRINCIPLES ON WHICH SIGNALLING AND TESTING INSTRU- 
 MENTS ARE CONSTRUCTED 
 
 58- 97. RECEIVING INSTRUMENTS : THEIR DESCRIPTION, ACTION, 
 EFFICIENCY, MODE OF ADJUSTMENT AND ADAPTATION TO 
 FORM MORSE CHARACTERS. THE MORSE ALPHABET AND 
 RULES FOR ACCURATE SIGNALLING 
 
 98-103. ELECTRIC BELLS 
 
 104-108. TRANSMITTING APPARATUS: SINGLE AND DOUBLE CURRENT 
 KEYS 
 
 109-121. EFFECTS OF ELECTRO-STATIC INDUCTION AS SHOWN BY 
 PHENOMENA OF 'CHARGE,' 'DISCHARGE,' AND 'RETURN 
 CURRENT OF DISCHARGE.' SIMILITUDE OF A TELEGRAPH 
 LINE TO A LEYDEN JAR OR CONDENSER. EXTRA CURRENTS, 
 THE EFFECT OF VOLTAIC OR ELECTRO-DYNAMIC INDUCTION. 
 SUMMARY OF CAUSES AFFECTING SPEED 
 
 122-124. THE ELECTRO-MAGNETIC SHUNT : ITS USE AS A CURE FOR THE 
 EFFECTS OF VOLTAIC INDUCTION, ALSO AS EXERTING A 
 COUNTER INFLUENCE ON THE RETARDING AND PROLONGING 
 EFFECTS OF ELECTRO-STATIC INDUCTION 
 
 125-132. VARIOUS FORMS OF DISCHARGING ARRANGEMENTS, FOR NULLIFY- 
 ING THE EFFECTS OF ELECTRO-STATIC INDUCTION 
 
 133-134. LIGHTNING DISCHARGERS 
 
 135-141. VARIOUS FORMS OF SWITCHES OR COMMUTATORS 
 
 PART II. TESTING INSTRUMENTS 
 
 142-165. DESCRIPTION OF VARIOUS FORMS OF GALVANOMETERS ; THE 
 WHEATSTONE BRIDGE, THE DIFFERENTIAL GALVANOMETER, 
 SHUNTS AND RESISTANCE COILS. THEIR MODE OF USE 
 
 AND PRINCIPLE OF ACTION 
 
 PART III. MAGNETO-ELECTRIC INSTRUMENTS 
 
 166-172. MAGNETO-ELECTRIC MACHINES AND THE PRINCIPLE OF THEIR 
 ACTION : INSULATOR AND JOINT DETECTOR, WHEATSTONE'S 
 ABC DIAL INSTRUMENTS. THE TELEPHONE AND MICRO- 
 PHONE 
 
Signalling and 
 Testing 
 Instruments. 
 Electro - 
 magnetism. 
 
 Action of 
 Current on 
 Soft Iron Core. 
 
 TELEGRAPH INSTRUMENTS. 
 
 PART I. SIGNALLING INSTRUMENTS. 
 
 48. TELEGRAPH instruments may be classed under two 
 heads viz. ' signalling ' and ' testing ' instruments. 
 
 49. In both cases the property of electro-magnetism 
 is mainly made use of i.e. the desired effect is produced 
 by the operation of a galvanic current upon soft iron or a 
 permanent magnet. 
 
 50. The ordinary Morse sounder ] is an example of the 
 electro-magnetic action of a current on soft iron. 
 
 In the following diagram A B may be considered to 
 represent one of the soft iron cores of the sounder, round 
 which the current circulates in the direction indicated by the 
 arrows. 2 When the current traverses the coil, the soft iron 
 
 SECTION 
 C. 
 
 73- 
 
 2 In speaking 
 of a current, a 
 + current, i.e. 
 copper to line, 
 is always 
 referred to in 
 the absence of 
 remark to the 
 contrary. 
 
 FIG. i. RIGHT-HANDED HELIX. 
 
 core becomes a magnet with its north pole at B and its 
 south pole at A. 
 
 If in the same circuit the battery be reversed, 3 the end 5 138. 
 of the core, which was before a north pole, becomes a south, 
 and vice versa. 
 
 Or if with the same current as in fig. i, the wire round 
 the coil be wound in the opposite direction, as shown in 
 
44 
 
 SECTION 
 C. 
 
 MANUAL OF TELEGRAPHY. 
 
 fig. 2, then j which was a north pole before, becomes south, 
 and A north (see fig. 2). This polarity, according to the 
 direction of the current and the winding of the coil, is 
 determined by Ampere's law, 1 and a means is thus pro- i Law 13 and 
 vided of winding the coils of electro-magnets according to 53> A PP- A - 
 the magnetic effect they are required to produce. 
 
 FIG. 2. LEFT-HANDED HELIX. 
 
 Polarised 
 Instruments. 
 
 Effect of a 
 Current on 
 Magnet. 
 
 Oerstedt's 
 Discovery. 
 
 Ampere's Law. 
 
 Directly the current ceases, the magnetism in the core 
 ceases also, presuming the absence of coercive force 2 in soft, 
 i.e. pure untempered iron, a property which renders it of 
 the greatest use in the manufacture of signalling instru- 
 ments. 
 
 When the core of an electro-magnet in its normal state 
 is under the influence of a permanent magnet, as in the 
 case of a Siemens' relay, 3 it is said to be polarised, and 
 the current exercising its effect on the core either increases 
 or reduces its polarity according to the direction in which 
 the electro-magnet is wound, or reverses the polarity of the 
 core altogether, according to the strength of the permanent 
 magnet. 
 
 51. The effect of a current on a permanent magnet is 
 illustrated by any galvanoscope or galvanometer needle. 4 
 
 The direction in which the needle deflects is determined 
 by the direction of the current. 
 
 52. The simplest case to consider is that of a mag- 
 netic needle suspended over a straight wire. Immediately 
 a current is sent through the wire the needle is deflected 
 so as to stand across it. 
 
 53. Professor Ampere developed this discovery, and 
 enunciated his famous law by which the direction in which 
 the needle will deflect under the influence of a current is 
 known. 
 
 He imagines an observer swimming with the current 
 
 Def - 43- 
 
 59 and 60. 
 
Effect of a 
 Current on a 
 Magnet Pro- 
 portional to 
 Current 
 Strength and 
 Number of 
 Convolutions. 
 
 Principles on 
 which Galvano- 
 meters are 
 Made. 
 
 Application of 
 Electro- 
 magnetism to 
 the Construc- 
 tion of 
 Signalling 
 Instruments. 
 
 Attractive 
 Force of 
 Electro- 
 magnets. 
 
 Sounders and 
 Relays. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 with his face always turned towards the needle ; the north 
 pole is always deflected to his left. 
 
 54. The force of the deflection depends principally 
 upon the strength of the current and the distance between 
 the wire and the magnet ; and if the wire be bent round 
 the magnet so as to be above as well as below it, the 
 effect of the current is doubled ; by an additional turn it is 
 trebled and so on, the action of the current on the magnet 
 being directly multiplied by the number of convolutions 
 round it. 
 
 55. The above facts illustrate the essential principles 
 upon which the construction of galvanoscopes or galvano- 
 meters is based. 
 
 It is also evident that by multiplying the convolutions 
 of wire round the needle, increased sensitiveness can be 
 obtained.' 1 
 
 Other circumstances, however, which will be fully ex- 
 plained in treating of the various forms of galvanometers, 2 
 come into play, and there is a limit to the multiplying effect 
 of the convolutions, owing to the fact that the extra resist- 
 ance introduced by the increased number of coils opposes 
 the intensity of the current. 3 
 
 56. To return then to signalling instruments, the 
 first matter for consideration is how the property of electro- 
 magnetism is to be applied so as to produce signals, either 
 audible or visible, with the greatest effect. 
 
 In either case signals are made by the attraction of a soft 
 iron armature by the core of an electro-magnet under the 
 influence of a current. 
 
 57. A certain force 4 is required to allow of the attrac- 
 tion being sufficient to record signals or produce readable 
 sounds. 5 
 
 58. The instrument which performs this office is usually 
 called a sounder, various forms of which are described in 
 paras. 73-87 ; but as these require a strength of arriving 
 current seldom obtainable through a long and imperfectly 
 insulated line, a more sensitive instrument, called a relay, 
 is placed in the line circuit as the receiver, and is so con- 
 structed as to work with the faintest currents, and, in so 
 doing, to complete a local circuit 6 containing the sounder 
 and a local battery, which can be adjusted to any required 
 strength. 
 
 45 
 
 SECTION 
 C. 
 
 1 Galvano- 
 meters are for 
 this reason 
 also called 
 multipliers. 
 
 * 143-165. 
 
 Def. 3. 
 
 4 In practice 
 not less than 
 twelve ounces. 
 
 5 This force of 
 attraction de- 
 pends upon 
 the strength of 
 the current, 
 the number of 
 convolutions 
 round the core, 
 and the size of 
 the core. 
 
 The larger 
 the core, how- 
 ever, the 
 greater the 
 1 Magnetic 
 Inertia.' See 
 remarks on 
 Speed of 
 Signalling, 
 
 131. 
 
 6 181. 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 Siemens' 
 Relay : 
 Description. 
 
 Siemens' polarised relay is the form universally 
 adopted for departmental use. 
 
 59. Its essential parts are a strong permanent L- 
 shaped magnet, NS (fig. 3), two soft iron cores and shoes, n, n' 
 (the latter adjustable), a soft iron tongue, s (delicately 
 pivoted), and working between two contacts one, platinum, 
 p, which is adjustable and regulates the play of the tongue ; 
 the other, agate, A, which is fixed (fig. 4) ; a micrometer 
 screw, M, and the coils. The frame in which the contacts 
 p and A are fixed is called tjie carriage, the adjustment of 
 which, by means of the screw M, determines the position 
 of the tongue with regard to the shoes n and n'. In the 
 newest pattern of relay the frame is fixed, and the screw M, 
 which is attached to the nut n', serves to increase or decrease 
 
 Electrical 
 Action of 
 Siemens' 
 Relay. 
 
 FIG. 3. FIG. 4. 
 
 SIEMENS' POLARISED RELAY. 
 
 the mass of iron acting on the tongue, the result being the 
 same in either case. 
 
 A magnetic indicator, consisting of a small magnetic 
 needle finely balanced on a vertical pivot, which is sup- 
 ported by a small flat base of brass, is placed on the glass 
 cover of the relay, and acts as a galvanoscope, showing the 
 presence of any current, however faint, passing through the 
 coils. 
 
 60. By magnetic induction 1 the tongue s assumes the * Def. 41. 
 same magnetism as the upper or south pole of the per- 
 manent magnet, and the cores and shoes, n, n', the same as 
 the lower or north pole ; so that when no current is flowing 
 s is attracted either by n or n', whichever is nearer. 
 
 Suppose this to be n', and the tongue to be against the 
 
Bias. 
 
 Working 
 Force. , 
 
 Rest Force, 
 and Force of 
 Restitution. 
 
 Weak Point of 
 
 Siemens' 
 
 Relay. 
 
 Play. 
 
 Force of 
 Restitution 
 accelerated by 
 a Reverse 
 Current or 
 E.M. Shunt. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 rest contact A (fig. 4), and suppose a current to be sent 
 through the coils, tending to make n' a south pole, and, 
 consequently, n a north pole, then the original force of n' 
 becomes decreased or reversed, and that of n correspond- 
 ingly increased, so that the tongue is drawn over towards 
 n (against the platinum contact />), and remains there till 
 the current ceases. When this happens, the soft iron cores 
 resume their original condition, and the tongue falls back to 
 A by reason of the nut n' being nearer to it than n. Thus 
 the relay works by the polarity of n and n' produced by the 
 making and ceasing of the current, and hence it is termed 
 polarised. 
 
 61. In order that the tongue may always have a bias 
 towards the insulated or rest stop A, it is necessary that the 
 adjustment be so made that the magnetism of the shoe n 
 can never exert a stronger force on the tongue than n' when 
 no current is flowing ; so that on the cessation of the force 
 (called the l working force'\ which tends to draw the tongue 
 against the working contact />, under the influence of a 
 current, the tongue will readily return to its initial position, 
 against the rest stop A. 
 
 62. The force which holds the tongue, in its position of 
 rest, against A, under the influence of the magnetism of the 
 nearer shoe ', is called the restforce, and that which draws 
 it back to that position on the cessation of the working force 
 is called the force of restitution. Now it is obvious that in 
 any one adjustment of the relay the force of restitution 
 tending to draw the tongue back to A after it has been 
 attracted to p must be smaller than that exerted on the 
 tongue when already against A. 
 
 63. This fact presents an obstacle to sensitiveness in all 
 polarised instruments ; a defect which consequently applies 
 to the Siemens' relay, and which is, in fact, the weak point 
 of that instrument. 1 
 
 64. The effect of this disparity between the rest force 
 and force of restitution is, however, reduced to a minimum 
 by making the play of the tongue as small as possible by 
 means of the adjustable contact screw p. 
 
 65. From the above it will be clear how the system of 
 double current working 2 in which the force of restitution is 
 no longer effected simply by the bias of the tongue, but by 
 an opposite current drawing it back with a force equal to or 
 
 47 
 
 SECTION 
 C. 
 
 1 It will be ob- 
 served also that 
 in one adjust- 
 ment the force 
 of restitution is 
 the same what- 
 ever the 
 strength of the 
 working cur- 
 rent may be. 
 
MANUAL OF TELEGRAPHY. 
 
 Adjustment. 
 
 Sensibility 
 dependent on 
 Play of 
 Tongue. 
 
 Use of Micro- 
 meter Screw. 
 
 Limit to Small- 
 ness of Play. 
 
 Sticking. 
 
 62-64. 
 
 2 See fig. 46, 
 181. 
 
 Remedy for 
 Sticking. 
 
 greater than the working force remedies the above-men- 
 tioned defect in the Siemens' relay, rendering it far more 
 sensitive, and reducing the necessity for readjustment. 
 
 The same object is effected by the application of an 
 electro -magnetic shunt, the action of which is explained in 
 para. 122. 
 
 66. Adjustment of the relay. The shoes having 
 been securely fixed with their faces parallel and equidistant 
 from the tongue, the more delicate adjustment of the relay 
 is effected as follows. 
 
 67. To obtain the greatest sensibility, make the play of 
 the tongue between the metal and agate points as small as 
 possible, l provided it is sufficiently large to allow the contact 
 to be opened on the cessation of the current, and not to be 
 jammed by the screwing of the micrometer screw. 
 
 This is best done by screwing the local contact screw 
 towards the tongue until a beat on the sounder shows the 
 local circuit to be completed, 2 then gradually unscrew the 
 contact again until the instant when the sounder armature 
 is released. 
 
 Further adjustment to suit variations in the received 
 current is effected by means of the micrometer screw, which 
 alters the relative position of the tongue and the shoes as 
 circumstances require. 
 
 68. There is a limit to the smallness of the play of the 
 tongue, for it is necessary to prevent its being jammed 
 against both contacts when its position is altered by the 
 micrometer screw. 
 
 Further, the play must be so large that the circuit shall 
 not be closed by the spark produced by the extra current, 3 3 See 119 
 and so large also that the motion of the tongue shall by its and 226 ( F )- 
 momentum be sufficiently strong to cause a firm contact, and 
 consequently a readable signal on the sounder. 
 
 69. The tongue sticks when the play is too large, or when 
 it is placed too near the attracting, and too far from the re- 
 pelling pole, under the influence of a current ; in either case 
 this fault is due to the fact that the demagnetisation of the 
 shoe or the reversal of its magnetism caused by the cessation 
 of the current does not cause the tongue to return to the 
 position from which it was moved by the current of magne- 
 tisation. 
 
 70. Sticking is thus prevented in the first instance by 
 
Efficiency. 
 Range. 
 
 Sounders 
 inserted in 
 Local Circuit. 
 
 Siemens' Morse 
 Sounder. 
 
 Electrical 
 Action of 
 Siemens' 
 Sounder. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 careful adjustment of the play of the tongue, and further by 
 the adjustment of the micrometer screw. 
 
 71. The efficiency of a relay is tested by its range ; 
 that is, the extent to which the strength of the working current 
 may be altered without necessitating alteration in the adjust- 
 ment of the instrument. 
 
 To find this : Adjust the relay with minimum play, with 
 the tongue as far from the metallic stud as regular working 
 will admit of, so that it will work with one cell through a, re- 
 sistance equal to its own ; then apply 10 cells without any 
 external resistance : the relay should work without readjust- 
 ment, thus displaying a range of 20, the minimum range a 
 Siemens' relay should exhibit. 1 
 
 The above results may of course be effected under diffe- 
 rent conditions of E.M.F. and resistance, as circumstances 
 may suggest. 
 
 As a rule, for relays of 1,000 units and under, use i and 
 10 cells ; for those of greater resistance, use 2 and 20 cells 
 respectively. 
 
 Many of the newer relays exhibit a range of 25 and up- 
 wards ; any range, however, can be measured and calculated, 
 as shown in para. 344, Sect. F. 2 
 
 72. Sounders. It has been mentioned that the attrac- 
 tion of the tongue of the relay against the working contact 
 p 3 causes a sounder to work. 
 
 The circuit in which the sounder is placed and its mode 
 of action being explained in Section D on circuits^ it will be 
 only necessary here to describe the instrument itself. 
 
 73. The form of sounder most generally used is the 
 Siemens' Morse, represented in fig. 5. 
 
 74. Like the relay, it works on the principle of electro- 
 magnetism, its coils being wound round soft iron cores which 
 become magnetic under the influence of a current. Unlike 
 the relay cores, however, they become altogether demag- 
 netised by the cessation of the current (presuming the iron 
 to be perfectly pure), as the sounder is not a polarised 
 instrument. 5 
 
 The soft iron armature A is attracted by the cores when- 
 ever a current traverses the coils, imparting its motion to the 
 brass lever L L to which it is attached, one extremity of 
 which plays between the contacts 1 and 2. 
 
 In its position of rest, L L is held against contact 2 by 
 
 E 
 
 49 
 
 SECTION 
 C. 
 
 1 That the 
 current in the 
 second case is 
 twenty times as 
 strong as in the 
 first is appa- 
 rent from 
 Ohm's law, as 
 follows : 
 
 Calling the 
 E.M.F.ofeach 
 cell used e ; 
 resistance of 
 relay R 
 current in first 
 case c ; cur- 
 rent in second 
 case C. Then, 
 in the second 
 case, 
 
 C = * oe 
 ~~R~ 
 
 And, in the 
 first case, 
 
 R 
 
 = 20 
 
 The battery 
 resistance is 
 neglected in 
 the above, 
 being insignifi- 
 cant, in com- 
 parison with 
 the external 
 resistances 
 employed. 
 
 2 The faults to 
 which the relay 
 is subject are 
 described in 
 226. 
 
 5 59, fig. 4- 
 
 4 181. 
 
 5 50. 
 
SECTION 
 C 
 
 MANUAL OF TELEGRAPHY. 
 
 means of the spring 5 s, the force of which is adjusted by 
 the screw c. 1 
 
 When a current flows through the coils, the magnetism 
 of the cores produced thereby overcomes the force of the 
 spring and attracts the armature A, causing the end of the 
 lever to be drawn from contact 2 and pressed against 1. 
 
 Here the 
 
 is a non- 
 polarised 
 instrument, 
 differs from 
 that of the 
 polarised relay, 
 the armature 
 being drawn 
 back by an 
 antagonistic 
 spring, and not 
 by the polarity 
 of a magnet. 
 
 FIG. 5. SIEMENS' MORSE SOUNDER. 
 
 Translation 
 Springs for 
 Prolonging 
 Contacts. 
 
 Adjustment of 
 the Siemens' 
 Morse 
 Sounder. 
 
 75. The contact between L and 1 is prolonged by means 
 of the translation spring /, and the screws x and Y regulate 
 the play of the lever between the contacts 1 and 2. 
 
 76. The adjustable parts of the sounder are as 
 follows : 
 
 (1) The two screws x and Y above and below the arma- 
 ture lever L L, which limit its play. 
 
 (2) Their clamping screws. 
 
 (3) The spiral spring (acting against the current), which 
 draws back the lever into its position of rest. 
 
 Rules for adjustment : 
 
 (i) Adjust the lower limiting screw FSO that the arma- 
 ture (under the influence of a current) does not quite touch 
 the small rivet at the top of the core ; a piece of common 
 note paper should pass between them with slight friction. 
 
TELEGRAPH INSTRUMENTS : SIGNALLING. 
 
 Efficiency. 
 
 Electrical 
 Circuit of 
 Siemens 
 Sounder and 
 Relay Com- 
 bined. 
 
 When thus adjusted, clamp up this lower screw by its jam 
 nut. 
 
 (2) Adjust the upper limiting screw A- making the play 
 as small as possible so as to admit of maximum speed and 
 clearness of sound. 
 
 (3) Adjust the spiral spring s s by the following process: 
 Work the sounder electrically, and tighten the spring until 
 the magnet is unable to attract the armature ; mark the posi- 
 tion of the screw on the stem ; now loosen the spring until the 
 armature falls on the electro-magnet by its own weight ; 
 mark the second position on the screw, then tighten the screw 
 to about midway between the two marks. 
 
 N.B. Fix all jam nuts securely. 
 
 77. A Siemens sounder should give a range of 20. 
 
 78. Fig. 6 represents, symbolically, the complete circuit 
 of a Siemens' Morse sounder and relay combined. 
 
 SECTION 
 C. 
 
 T C Z 
 
 FIG. 6. SIEMENS SOUNDER AND RELAY. 
 
 Platinum 
 Points and 
 their Use. 
 
 In the above there are four distinct electrical circuits 
 through the instrument : 
 
 (1) That of the arriving line current entering at L and 
 leaving at E. 1 
 
 (2) That of the local current entering at c, traversing the 
 sounder coils and relay tongue, and leaving through B and z. 2 
 
 (3) That of the received translating current entering at 
 T and leaving at 2. 3 
 
 (4) That of the sending translation current entering at 1 
 and leaving at r. 3 
 
 79. pppp represent platinum points ; these are placed 
 between the tongue and metallic contacts in the relay, be- 
 cause they close the circuit of the local current ; and as the 
 
 E 2 
 
 1 See ' Circuit 
 of S Working, 
 180, Sect. D 
 
 2 See ' Local 
 Circuit,' 181, 
 Sect. D. 
 
 3 See ' Trans- 
 lation Circuit,' 
 183, Sect. D. 
 
SECTION 
 C. 
 
 Douglas State 
 
 Railway 
 
 Sounder. 
 
 MANUAL OF TELEGRAPHY. 
 
 spark, due to the passage of such strong currents, takes place 
 at these points, a hard metal such as platinum is necessary. 
 
 Platinum points are also placed, to ensure conductivity, 
 where the sounder lever makes contact, both with the upper 
 and lower pillar, as both these contacts close the translation 
 circuit. 
 
 80. Another form of sounder in use in the department 
 is that known as the Douglas sounder. 
 
 Its electrical principle is precisely the same as that of the 
 Siemens sounder. 
 
 In fig. 7 its various parts are lettered so as to agree with 
 
 FIG. 7. DOUGLAS SOUNDER, with Key and connections complete, for 
 direct working on State Railways, &c. 
 
 the corresponding parts of the Siemens sounder shown in 
 ng. 6. 
 
 The main difference between the two instruments is in 
 the shape of the lever L L, the position of the contact screws 
 1 and 2 and their pillars, as also that of the antagonistic 
 spring s s. 
 
 The Douglas sounder is now chiefly used for direct 
 working on short circuits, and particularly for State railway 
 working in closed circuit l without a relay. 
 
 Its complete electrical circuit and internal and external 
 
 189. 
 
Adjustment. 
 Efficiency. 
 
 Dubern 
 Sounder. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 connections as a State railway sounder are shown in fig. 7, 
 which includes the key. 
 
 81. The Douglas sounder is adjusted in the same way as 
 the Siemens ; its range should be 25. 
 
 It will be observed from figs. 6 and 7 that the armatures 
 of both the Siemens and Douglas sounders are split longitu- 
 dinally. 
 
 This is done with the object of preventing a circuit for 
 extra currents l which would be formed by the rapid magne- 
 tising and demagnetising of the cores. 
 
 82. Fig. 8 represents a Dubern sounder, which is 
 
 53 
 
 SECTION 
 C. 
 
 "9- 
 
 FIG. 8. DUBERN SOUNDER. 
 
 designed either for direct working, or for use, like other 
 sounders, in a local circuit. 
 
 Its principle is that of placing the armature in the 
 strongest magnetic field, 2 with which object it is placed 
 across the coils, as shown in fig. 8, and it is relieved of fric- 
 tion as much as possible by means of the spiral spring s t 
 inside which is a vertical pin to which me armature is fixed. 
 This pin is pivoted at its lower end below the armature, and 
 at its upper end below the screw M. 
 
 The weight of the armature on its lower bearing is regu- 
 lated by the position of the screw R on the pin, by which the 
 spiral screw 5 is tightened or loosened, the bottom end of 
 the spiral being permanently fixed to the small screw B ; the 
 
 Def. 40. 
 
54 
 
 SECTION 
 C. 
 
 Adjustment. 
 
 Efficiency. 
 
 Portable 
 Sounder. 
 
 Adjustment. 
 
 MANUAL OF TELEGRAPHY. 
 
 top end is fixed to the circular screw M, the adjustment 
 of which imparts a lateral motion to the spiral screw and 
 armature, thus serving to give the latter the necessary bias 
 against the rest stop, and playing the part of the screw c 
 (figs. 5 and 7) which regulates the force of the antagonistic 
 spring. 
 
 The armature (prolonged) plays between the limiting 
 screws x and y. 
 
 When the instrument is to be used for direct working, 
 the coils are wound to a resistance of 500 ohms. 1 When 
 used as a local sounder, they are made of the usual resist- 
 ance of 30 ohms, and the armature is furnished with a 
 translation spring. 
 
 83. The play of the instrument is adjusted by means of 
 the screws x and F, in the same way as that of the Siemens 
 and Douglas sounders is done by the screws similarly lettered. 
 See figs. 5 and 7. 
 
 84. A Dubern sounder should have a range of 25. 
 
 85. Fig. 9 explains the essential parts of a portable 
 sounder. 
 
 The whole of the apparatus, including key and connec- 
 tions, is enclosed in a small box ; the soft iron tongue, s, 
 which produces the sound, is pivoted on the south pole, 5, 
 of a permanent L-shaped magnet, the soft iron cores of the 
 electro -magnet being attached to the north pole (hidden in 
 the figure) of the permanent magnet. 
 
 The portable sounder is a polarised instrument, 2 and its 
 electrical action is precisely similar to that of the polarised 
 relay explained in paras. 59 and 60, and will be understood 
 by reference thereto, the essential parts, s, s, n, n' in fig. 9 
 corresponding with the similarly lettered parts in fig. 3. 
 
 The terminals z, L, c (fig. 9) are connected outside the 
 instrument to earth, line, and copper respectively. 
 
 Inside, z is connected to one end of the coils, L to the 
 other end (through the front contact of the key), and c 3 to 
 the back contact of the key. 
 
 The resistance of the coils is usually about 500 ohms, 
 and the sounder is worked direct by the current from the 
 line. 
 
 86. The play of the tongue is adjusted by means of the 
 screws x and y, which are kept in position by their jam 
 nuts. 4 
 
 1 250 ohms 
 each. 
 
 105. 
 
 1 The faults to 
 which the 
 various forms 
 of sounders are 
 liable are 
 described in 
 Sect. E, 227. 
 
TELEGRAPH INSTRUMENTS: 
 
 Efficiency. 87. The portable sounder should exhibit a range of 20 
 
 and should work with a current of i milli-oersted. 1 
 
 55 
 
 1 Defs. 2 and 
 34, and 289. 
 
 FIG. 9. PORTABLE SOUNDER. 
 
 Morse 
 Characters. 
 
 88. The various kinds of sounders described above pro- 
 duce, by the attraction and release of their armatures, 
 audible signals, which are distinguished from one another 
 by their duration. 
 
 A short signal, or a long one, or a combination of both, 
 represents a letter of the alphabet. 
 
 The length of each signal is determined by the dura- 
 tion of time between the sound caused by the attraction 
 of the armature and that caused by its withdrawal under 
 the influence of the antagonistic spring, when the current 
 ceases. 
 
 These signals are separated from one another by pauses, 
 the length of a pause being the duration of time between 
 
MANUAL OF TELEGRAPHY. 
 
 The Morse 
 Alphabet. 
 
 the return of the armature to its position of rest and its next 
 attraction to the cores of the electro-magnet. 
 
 The pauses or spaces between the signals are as impor- 
 tant as the signals themselves ; for their duration indicates 
 whether they separate from one another individual characters, 
 or letters, or words. 
 
 89. Representing a short signal by a dot (-), and a long 
 one by a dash ( ), and combining the two thus (- ), the 
 letter * a ' is formed, which consists of two signals, viz. a dot 
 and a dash, separated by a space. 
 
 Various combinations of the dot with the dash, as shown 
 below, compose the conventional code known as the Morse 
 alphabet 
 
 LETTERS 
 
 A 
 
 Units 
 
 N 
 
 Unity 
 
 1 
 
 
 
 a 
 
 3 
 
 
 
 4 
 
 
 
 5 
 
 
 
 6 
 
 7 
 
 8 
 
 9 
 
 10 
 
 n 
 
 12 
 
 13 
 
 Lt 
 
 15 
 
 16 
 
 17 
 
 18 
 
 i 
 
 
 
 2 
 
 
 3 
 
 
 
 ! 
 
 3 
 
 
 
 6 
 
 7 
 
 8 
 
 9 
 
 10 
 
 n 
 
 12 
 
 13 
 
 ]-t 
 
 lo 
 
 te. 
 
 17 
 
 1819 
 
 B 
 
 
 
 
 
 B 
 
 
 B 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 C 
 
 B 
 
 
 
 
 
 
 
 
 
 
 
 
 
 HI 
 
 
 
 
 
 
 
 
 
 
 
 p 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 K 
 
 
 
 
 
 
 
 
 D 
 
 
 
 
 
 
 
 
 K 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Q 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 B 
 
 m 
 
 m 
 
 
 
 
 
 
 E 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 R 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 F 
 
 
 
 
 
 
 
 m 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 S 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 C 
 
 
 
 
 
 
 
 
 m 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 T 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 H 
 
 
 
 
 
 
 
 
 
 , 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 U 
 
 
 
 
 
 
 
 
 
 
 
 m 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 V 
 
 
 
 
 
 
 
 
 
 
 m 
 
 
 
 
 
 ' 
 
 
 
 
 
 
 
 
 
 J 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 w 
 
 
 
 
 
 
 
 
 
 
 
 m 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 K 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 X 
 
 
 
 
 
 
 
 
 
 
 
 m 
 
 
 
 
 
 
 m 
 
 
 
 
 
 
 
 
 L 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Y 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 n 
 
 
 m 
 
 m 
 
 
 
 
 
 
 
 
 M 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 z 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 B 
 
 I 
 
 
 
 
 
 
 
 ( 
 
 Ch 
 1 U 
 
 6 
 
 
 R 1 
 
 
 
 
 
 
 
 
 E 
 
 
 
 
 
 
 s 
 
 
 
 m 
 
 
 E 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 2 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 7 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 3 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 8 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 a 
 
 
 
 
 4 
 5 
 
 
 
 
 
 
 
 
 
 
 " 
 
 
 H 
 
 
 
 
 
 
 
 
 
 
 
 
 9 
 O 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 
 m 
 
 m 
 
 
 . 
 
 
 
 
 EB 
 
 
 
 
 
 
 B 
 
 
 E, 
 
 B 
 
 ' 
 BB 
 
 THE MORSE ALPHABET. 
 
 Rules for 
 Accurate 
 Signalling. 
 
 Correct signalling depends mainly upon the accurate 
 duration of signals and spaces. 
 
 90. The following rules will be found useful as a guide 
 for giving to each signal and space its correct value : 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. c- 
 
 1. A dot is taken as the unit of time or (when repre- SECTION 
 sented on paper) length. , ^ ^ 
 
 2. A dash is three times the length of a dot, and is thus 
 equal to 3 units. 
 
 3. The space between each signal or character forming a 
 letter should be i unit, i.e. equal to i dot. 
 
 4. The space between each letter of a word should be 
 3 units. 
 
 5. That between each word of a sentence 5 units. 
 
 It should further be observed that no letter of the 
 Morse alphabet can contain more than four cha- 
 racters ; and that every figure must contain exactly 
 five. The observance of this fact and of the regular system 
 on which the figure code is arranged, progressing by dots 
 from i to 5 and by dashes from 5 to o ; and the check each 
 figure exercises on itself by containing so many dots followed 
 by dashes making up the balance of 5, or vice versa, should 
 render errors in the signalling of the figure code easy of 
 detection and prevention ; and it may not be out of place 
 here to add, by way of warning, that the chief, if not the 
 only cause of errors in the signalling of figures (on which, it 
 must be remembered, the most important issues may depend), 
 is due to a practice, unfortunately too common, known as 
 ' exaggerating signals,' by which a letter is given more cha- 
 racters than it really possesses : for example, the letter h, a 
 most common victim of this ill-treatment, is, by the addition 
 of an extra dot, mutilated into the figure 5- 
 
 Thus it behoves every telegraphic operator, not only to 
 bear in mind, but to exercise, the practical check that no 
 letter can contain more than four characters, and no figure 
 less than five ; and positively to desist from taking down any 
 word containing a letter or figure in which a departure from 
 this rule is observed. 
 
 ink-writers. pi. It has been shown how, by means of various kinds 
 of sounders, the signals representing characters of the Morse 
 alphabet are produced. and distinguished by the ear. 
 
 In order, however, that these signals should leave some 
 visible trace of their existence, as is necessary in the case of 
 foreign messages, special contrivances are adopted by which 
 the Morse characters are recorded on tape, by the move- 
 ment of the armature of the sounder. 
 
 This is effected by the addition to the armature of a 
 
SECTION 
 C. 
 
 Siemens 
 Ink-writer. 
 
 MANUAL OF TELEGRAPHY. 
 
 light lever, with a small thin brass wheel at the end of it, 
 which is made to revolve by clockwork. 
 
 The fulcrum or axis of the lever is between the armature 
 and the wheel. 
 
 When the armature is at rest the lower edge of the wheel 
 dips into a vessel of prepared ink : at each attraction of the 
 armature the wheel is raised, and lightly touches a paper 
 tape which is made to pass over small grooved brass rollers 
 revolving at a uniform rate ; l each movement of the arma- * The tape 
 ture is thus communicated to the tape, on which dots and te rate'of not 
 dashes are printed in ink, resembling the Morse characters less than six 
 shown in the preceding table. minute' 
 
 Such a recording instrument is called an ink-writer. 
 
 It is joined up in the local circuit like any other sounder, 
 the circuit being, as usual, closed by a relay under the in- 
 fluence of a current from the line. 
 
 Ink-writers are sometimes fitted up together with a relay, 
 galvanoscope, key and switch, all on one board. 
 
 92. Fig. 10 represents such an arrangement. 
 
 FIG. 10. SIEMENS INK-WRITER (Relay, &c., combined). 
 
 The isolated arrangement first mentioned is preferable ; 
 as, however, the ink-writer with relay combined is sometimes 
 used, and its connections are somewhat complicated, its 
 terminals and the mode of joining them up may, with ad- 
 vantage, be explained, as follows : 
 
Ink- writer 
 Terminals and 
 Connections 
 
 Embossing 
 Recorders. 
 
 Needle Instru- 
 ments. 
 
 Single Needle. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 93. Ink- writer terminals (from right to left). 
 
 59 
 
 Marked 
 
 Joined inside instrument to 
 
 Joined outside instrument to 
 
 L 
 
 Galvanoscope 
 
 Line 
 
 E 
 
 V | 
 
 Earth and K$ of discharger i 
 
 D 
 S 
 T or T 
 
 f By plugs to bar short-cir- ; 
 1 cuiting galvaroscope 
 
 Z of ink-writer 
 K of key (middle) 
 (For translation work) to { of 
 
 i or i 
 
 \ 
 
 other ink-writer 
 
 I 
 
 I of sounder 
 
 (For translation work) to T or 
 I of other ink-writer 
 
 II 
 
 2 of sounder 
 
 (For translation work) to IJL of 
 other ink-writer 
 
 II 
 
 2 of key (back) and L of( 
 relay 
 
 E of discharger l 
 
 III 
 
 3 of sounder 
 
 (For translation work) to copper 
 of battery of other instrument 
 
 Z 
 
 ZE of relay 
 
 D of ink-writer, and by plug 
 between D, E to earth 
 
 C 
 
 i of key front contact 
 
 L of discharger 1 
 
 e 
 
 1 Metal contact of relay 
 
 Zinc of local battery 
 
 c 
 
 f Sounder coils 
 
 Copper of local battery 
 
 The faults to which the ink-writer is subject are described 
 in Section E (para. 228). 
 
 94. Before the invention of recording the characters in 
 ink was thought of, the tape was marked by means of a sharp 
 style (instead of a wheel) at the end of the lever ; such in- 
 struments are still in use, and are called ' embossers.' 
 
 Characters thus formed, however, are neither so perma- 
 nent nor so legible as those recorded in ink. Both ink- 
 writers and embossers are also made for direct working with 
 a resistance of about 500 units, and as such are adapted for 
 use on railway telegraphs. 
 
 95. The instrument most commonly used on railways for 
 open circuit working is the needle instrument. 2 
 
 96. The single needle is the simplest form of signal- 
 ling instrument made, and the most inexpensive. 
 
 The receiving part of the instrument is simply a gal- 
 vanoscope consisting of two coils of wire and a small needle 
 between them with its north end downwards. 
 
 Signals are formed by the left and right deflections of 
 the needle under the influence of reverse currents ; a left 
 deflection representing a dot, and a right a dash of the 
 Morse alphabet. 
 
 The ivory stops / and y limit the deflections of the 
 needle on either side. 
 
 1 As ink- 
 writers are 
 usually em- 
 ployed on long 
 circuits on 
 which dis- 
 charging 
 arrangements 
 are necessary, 
 the connections 
 shown in the 
 third column 
 explain how 
 the discharger 
 is joined up 
 with the ink- 
 writer. Fig. 
 24, 126, 
 shows a dia- 
 gram of the 
 discharging 
 instrument. 
 
 190. 
 
6o 
 
 MANUAL OF TELEGRAPHY. 
 
 Fig. 1 1 represents the circuit of the instrument in its state 
 of rest for receiving. 
 
 Its own battery is insulated by the centre of the handle 
 H, which is generally made of ebonite or some hard wood, 
 and the circuit of the line current entering at B is completed 
 by the movable metallic springs L and R and the contact 
 T against which they press ; thence it traverses the coils, 
 and proceeds through A to the earth. 
 
 FIG. ii. SINGLE-NEEDLE INSTRUMENT. 
 
 Double-needle 
 Instrument. 
 
 The vertical handle H is the sending part of the instru- 
 ment ; its upper and lower parts, c and z, being insulated 
 from one another at H, and, when it is turned so that z 
 presses against x, c also- presses against L, breaking the 
 contact 7 1 , and sending a zinc current to line. 
 
 When turned in the reverse direction a copper current 
 is sent to line. + 
 
 Any convenient form of battery reverser might take the 
 place of H and the springs L and R. 
 
 A newer form of single-needle key is that known as the 
 tapper or pedal, the action of which is explained at para- 
 graph 107, and represented in fig. I5. 1 
 
 97. The double-needle instrument is simply the single- 
 needle instrument doubled, having two pairs of coils and ment is subject 
 two handles. 
 
 The electrical action of the two instruments is precisely 229. 
 the same. 
 
 1 The faults to 
 which the 
 needle instru- 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 6l 
 
 98. Bells are sometimes used in connection with 
 receiving instruments for the purpose of arresting atten- 
 tion. 
 
 The alarum or trembleur (the form of bell most 
 used in departmental offices) consists of an electro -magnet, 
 D D, to the armature M of which is connected a flexible 
 steel arm with a small leaden hammer H at its extremity, 
 which, under the influence of a current, strikes the bell E. 
 A steel spring, s, breaks and makes contact with the ad- 
 
 SECTION 
 C. 
 
 FIG. 12. ALARUM OR TREMBLING BELL. 
 
 justable screw F according as the armature is attracted 
 towards the cores or not. 
 
 The instrument can be short circuited by a plug between 
 A and B. 
 
 The electrical circuit of the instrument will be at once 
 understood from fig. 12. 
 
 All that it is necessary to remark upon is the spring s at 
 which contact is made and broken. 
 
 In its position of repose the armature rests against this 
 
 ng, but immediately a current passes through the coils 
 the armature is drawn away from the spring towards the 
 core of the electro-magnet. 
 
 When such occurs, however, it is manifest that the 
 circuit is no longer complete, being broken at s ; and there- 
 
 Note. The 
 short-circuiting 
 chattering bell 
 is a modifica- 
 tion of this 
 instrument, 
 and is so con- 
 trived that each 
 attraction of 
 the armature 
 short-circuits 
 the battery. 
 The current in 
 the coils being 
 thus shunted, 
 the armature is 
 released, which 
 breaks the 
 shunt circuit, 
 causing re- 
 attraction of 
 the armature 
 under the influ- 
 ence of the 
 current, which 
 now follows 
 the direct cir- 
 cuit of the coils, 
 so that attrac- 
 tion and release 
 continue 
 successively. 
 
62 
 
 Ad ! ustment. 
 
 Efficiency. 
 
 Single-stroke 
 Bel). 
 
 Adjustment. 
 
 Efficiency. 
 
 Transmitting 
 Apparatus. 
 
 l The defect to 
 
 which this 
 
 spring is 
 
 MANUAL OF TELEGRAPHY. 
 
 fore immediately the current has done its work in causing 
 the attraction of the armature, the circuit is broken ; the 
 magnetism of the soft iron core disappears, and the tongue 
 falls back on the spring s. 1 
 
 Immediately it does so, however, the circuit is closed 
 again, and so long as a current is flowing, the above action 
 is repeated, and contacts are alternately made and broken Sect. '" 230. 
 in rapid succession, the bell being struck by the hammer at 
 each attraction of the armature. 
 
 99. To adjust the instrument, make the distance 
 between the armature and core as large as possible while 
 yet admitting of its working. 
 
 Then reduce this distance to one-half by adjusting the 
 screws at c at the back of the armature lever. 
 
 Then adjust the spring till contact is made at s, and 
 until the best ring of the bell is obtained. 
 
 The play, and consequently the speed, of the hammer is 
 regulated by the screw F. 
 
 Further, by turning the bell itself round, a position is 
 found when the ring is most regular and clear. 
 
 100. The alarum should exhibit a range of 25. 
 
 101. The single-stroke bell, commonly used in 
 railway offices, consists of an ordinary horse-shoe electro- 
 magnet, with a soft iron armature carrying a wire with a 
 hammer at its extremity, so arranged as to strike the inside 
 of the bell at each attraction of the armature under the 
 influence of a current. 
 
 On the cessation of the current, the armature (like that 
 of a sounder) is drawn back by an antagonistic spring, s. 2 
 
 The instrument is provided with a plug, the removal of 
 which breaks the circuit of the coils, and thus throws the 
 bell out of circuit. 
 
 102. Its play is adjusted by a screw at the back of the 
 hammer furnished with a jam nut, and the bell is turned 
 round till the best ring is ascertained by experiment. 
 
 The force of the antagonistic spring is also regulated by 
 an adjusting screw. 
 
 103. The bell should give a range of 25. 
 
 104. The foregoing remarks on the subject of instru- 
 ments have been confined to those parts of the signalling 
 apparatus which indicate received signals ; in other words, 
 receiving instruments^ whether relays, sounders, recorders, 
 
 2 8o > fi g- ? 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 indicators, or bells ; all of which are actuated by the influ- 
 ence of currents on electro-magnets. 
 
 The transmitting apparatus is far more simple and less 
 varied ; the main object being the contrivance of a simple 
 means of causing a current to flow through a telegraph 
 circuit at given intervals, the duration of which can be so 
 controlled as to indicate the dots and dashes of the Morse 
 alphabet. 
 
 105. This is effected by means of the key or handle, 
 the general simple form of which is represented conven- 
 tionally in fig. I3- 1 
 
 Line 
 
 U 
 
 Copper 
 
 175- 
 
 A B is a continuous brass lever, working vertically on the 
 pivot c, a connection at which keeps the lever always joined 
 to the line ; 7 and 2 represent the points at which the lever 
 makes contact with brass studs (all four contact points being 
 platinum-tipped) fixed to an insulated base-board. 
 
 The knob at A is also made of insulating material. 
 
 The spiral spring s, joined to the lever and base-board, 
 keeps the end B of the lever drawn down, so that the key 
 in its position of rest forms a conducting circuit, through 
 contact ^, between the line and the receiving instrument. 2 
 
 When A is pressed by the hand, however, contact 2 is 45 
 broken and 7 is closed, allowing a battery current to flow 
 into the line and work the receiving instrument at the 
 distant end, just so long as A is held down, the antagonistic 
 spring s opening the contact 7 directly the pressure of the 
 hand is withdrawn. 
 
 Thus dots and dashes are signalled at will. 3 
 
 106. It will be observed that the above operation only 
 sends a single current that is to say, a current from the 
 same pole of the battery at each depression of the key, 
 which is all that is desired in the case of non-polarised 
 instruments. 4 4 
 
 The case of the needle instrument, however, is different, 
 
 180, fig. 
 
 5 The faults to. 
 keys art- 
 are 
 
 described in 
 Sect. E, 232. 
 
6t 
 
 SECTION 
 C. 
 
 MANUAL OF TELEGRAPHY. 
 
 in which signals are formed by opposite deflections of the 
 needle. 1 
 
 Fig. n shows how the handle of the needle instrument 
 effects this by reversing the current. 
 
 In para. 65 it was explained how a reverse current 
 would accelerate the return of the relay tongue to its posi- 
 tion of rest ; in other words, accelerate its force of restitu- 
 tion. 
 
 In order to do this, a key differing from the simple 
 Morse key represented in fig. 13 is necessary. 
 
 Keys by which positive and negative currents are sent 
 alternately are called double current or reversing keys. 
 
 Fig. 14 shows a simple arrangement by which this can 
 be done. 
 
 96. 
 
 The Tapper or 
 Pedal Key. 
 
 FIG. 14. PLAN OF REVERSING KEY. 
 
 A and B are springs, both having a tendency to press 
 towards c, a fixed contact. 
 
 When the handle H is in its position of rest (as in the 
 figure), A makes contact with c. When the handle is de- 
 pressed and contact 1 closed, B presses against c, and H 
 against A ; contact 2 being opened. 
 
 Supposing the line and battery connections completed as 
 shown by the dotted lines, it is evident from the diagram 
 that when the key is in a position of rest a zinc current 
 flows to line through A ; and that every time the handle is 
 depressed a copper current goes to line (through #). Further, 
 when the key is released again, the current is at once re- 
 versed and a zinc current flows to line (through A). 
 
 107. Various forms of reversing keys are employed. 
 
 Fig. 15 represents that known as the tapper or pedal 
 referred to in para. 96 ; and which, in the newer forms of 
 needle instrument, takes the place of the upright handle. 
 
 L and E are the terminals of two springs, which in their 
 position of rest press against the upper bar c. 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 When either is depressed, it forms metallic contact with 
 the lower bar z. 
 
 From the diagram it is apparent that when the key is at 
 rest the battery is in open circuit. 1 1 179. 
 
 XtM 
 
 When the handle 7 is depressed, a copper current flows 
 to line (zinc to earth) ; and when 2 is depressed a zinc 
 current flows to line (copper to earth). Thus, copper and 
 zinc currents can be sent in rapid succession, and the former 
 may be employed to indicate the dashes, and the latter the 
 dots composing Morse characters, or vice versa. 
 
 I08. Fig. 1 6 gives a symbolic representation of a con- 
 stant resistance key, joined up for sending a momentary 
 reverse current after each direct current. 
 
 FIG. 16. CONSTANT RESISTANCE KEY. 
 
 When the key is at rest, the line is joined to earth, and 
 contact z is closed. The play of contacts z and R is so 
 adjusted, by means of springs, that when the key is depressed 
 R is closed just before z is opened, and z is opened before c 
 is closed ; so an instantaneous zinc current from the battery 
 e precedes and follows each + signal from the battery . 2 
 
 109. The above form of key (fig. 16) is commonly used 
 
 196. 
 
66 
 
 SECTION 
 C. 
 
 MANUAL OF TELEGRAPHY. 
 
 as a discharging key, but before describing its action as such, 
 it will be advisable to describe those phenomena observed 
 in telegraph lines and cables known as charge and dis- 
 charge, with a view to explaining why discharging instru- 
 ments are used. 
 
 Charge : 
 Variable and 
 Permanent 
 State. 
 
 FIG. 17. PHENOMENA OF 'CHARGE.' 
 
 Suppose A B to be a telegraph line with its distant end 
 B to earth. At A is a simple key, by pressing which contact 1 
 causes a current to flow through the line. a, b, are galva- 
 nometers of equal sensitiveness inserted in the line at A and B 
 respectively. 
 
 When contact 1 is closed a current flows through the 
 line traversing the galvanometers a and b almost simul- 
 taneously. 
 
 At the instant of contact, however, the galvanometers, 
 though equally sensitive, manifest very different deflections, 
 a being violently deflected to its full extent before b shows 
 any movement at all (see fig. 17). 
 
 Suppose the key to be still held down, the needle of a 
 will be observed to settle down at a point on the scale rather 
 below its first deflection, while b will deflect more and more 
 till it reaches the point at which a remains steady. * 
 
 110. This shows that it takes a certain appreciable time 
 before the current arriving at the distant end of a line attains 
 the same strength which it has when leaving the battery, 
 and that during this time, which is termed the variable 
 state, the strength of current goes on increasing in the more 
 distant portion of the line until it is the same at all points, 
 when it is said to have assumed the permanent state. 
 
 Supposing 'b (fig. 17) to be a receiving instrument, it is 
 evident that the speed with which it would indicate signals 
 
 1 It is supposed 
 here that the 
 insulation of 
 the line is 
 perfect, and 
 that the 
 strength of 
 current is the 
 same at every 
 point through- 
 out its length. 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 is very much influenced by the time the line takes to receive 
 a charge. 
 
 Now the duration of this variable state is in direct pro- 
 portion to the length of the line, which accounts for the fact 
 that the effects of charge, though often imperceptible on 
 short lines, vastly impede rapid signalling on long ones. 
 
 Any increase of resistance in the line, battery, or earth 
 circuits increases the duration of the variable state, as it 
 impedes the circulation of the current through the line. 
 
 Leakage has the same effect, by diminishing the potential 
 of the current arriving at the distant end of the line. 
 
 The extra currents of electro-magnets in circuit also 
 prolong the duration of the variable state. 
 
 The above impediments to rapid signalling are due to 
 the effects of charge, by which signals not only take an 
 appreciable time to arrive at the distant end of a line, but 
 are formed by an inconstant current increasing in strength 
 until it arrives at its maximum, or even sufficient strength to 
 work the receiving instrument. 
 
 From this it follows that the first evidence of a signal is 
 weak and undefined, owing to the gradual appearance of the 
 current, under the influence of the charge (or static charge, 
 as it is called) of the wire. 
 
 III. We have also to consider the effects of discharge, 
 to illustrate which the diagram represented in fig. 17 is here 
 reproduced (fig. 18), with the addition of a second earth 
 
 SECTION 
 C. 
 
 
 FIG. 18. PHENOMENA OF DISCHARGE. 
 
 connection at the sending end A, which can* be made by 
 closing contact 2 of the key. 
 
 In the above diagram, suppose the handle 1 to be first 
 depressed so that a current flows from A to , deflecting the 
 
 F 2 
 
68 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 Return Current 
 of Discharge. 
 
 Induction. 
 
 Electro-static 
 Induction. 
 
 The Leyden 
 Jar. 
 
 galvanometer needles a and b to the right, as shown by the 
 arrows. 
 
 Let contact 1 now be opened and 2 immediately closed, 
 then the needle of a will be suddenly deflected to the left, 
 showing that the whole of the charge has not passed to earth 
 through />, but that a considerable portion returns through a 
 to earth, by way of contact 2, deflecting the needle in the 
 opposite direction to the charge current as shown by the 
 dotted arrow. 
 
 The longer and better insulated the line, and the greater 
 the resistance in circuit, the greater will be this discharge 
 current, as all the above circumstances tend to impede the 
 flow of the original or direct current of charge to earth at 
 the further end. 
 
 112. It is evident that if another galvanometer were 
 inserted between 2 and the earth, it would be deflected 
 similarly to a by this current of discharge, or return 
 current, as it is appropriately called. 
 
 Now this is the position that the receiving relay takes in 
 ordinary open circuit (or s] working, 1 so that after each 
 signal sent by the closing of contact 1 a return current 
 flows to earth through 2, causing a momentary beat on the 
 relay. 2 
 
 This return current of discharge must not be confounded 
 with the extra current (described in para. 119) which occurs 
 momentarily at each closing and interruption of the battery 
 circuit, and which is due to the inductive effect of the 
 current on itself. 
 
 113. Both effects are due to induction, but of different 
 kinds and exhibiting different phenomena. In the former 
 case the line has held a charge by virtue of the properties of 
 electro-static induction. 
 
 The ' extra current ' in the second case is the result of 
 electro-dynamic induction. 3 
 
 114. The phenomena of electro- static induction may be 
 most simply understood from the action of a Leyden jar. 
 
 115. Fig. .19 represents a common form of this instru- 
 ment. 
 
 The glass 1 jar A is coated inside and out to the same 
 height with tin-foil, which is not carried up to the top of 
 the jar, for the better insulation of the coatings from one 
 another. 
 
 1 180 and 
 
 fig. 45. 
 
 2 Special instru- 
 ments used to 
 remedy the 
 effects of 
 discharge are 
 explained in 
 125-132. 
 
 118-119. 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 In the mouth of the jar is fixed a stopper of dry wood, 
 or any good insulator, which holds a metal rod, the lower 
 end of which is in connection with the inner coating of the 
 jar, and the upper end terminates in a knob B outside. If 
 
 FIG. 19. THE LEYDEN JAR. 
 
 the conductor of an electrical machine in action be placed 
 close to , and the outer coating of the jar be connected 
 with the ground, the inner foil will become charged with 
 positive electricity and the outer with negative, sparks pass- 
 ing from the conductor to the jar till the latter is charged : 
 the greater part of the charge thus communicated to the 
 jar will, if the coatings of foil are well insulated from one 
 another, be retained for many days. To discharge the jar a 
 metallic connection is made between the outer foil and the 
 knob ; the opposite electricities which were accumulated on 
 the inner and outer coatings immediately combine with a 
 force infinitely greater than that which charged the jar, as 
 judged by the sparks it emits, or by severe shocks if the 
 body form part of the circuit. After the first discharge, the 
 jar may be found possessed of a residual charge, as evidenced 
 by faint sparks on approaching a conductor from the outer 
 foil to the knob B. 
 
 The above phenomena prove that the jar is thus 
 capable of receiving and storing up a charge of 
 electricity, and of retaining it for a considerable 
 time. 
 
 The action which takes place to produce this effect is 
 explained on the principle of static induction, based on 
 the fact that if a conductor, A, connected with the ground, 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 be placed near another conductor, B, insulated and charged 
 with electricity (fig. 20), a mutual inductive action takes 
 place, by which the charge of B is greatly increased. 
 
 FIG. 20. PHENOMENA OF STATIC INDUCTION. 
 
 The Con- 
 denser. 
 
 To explain this, suppose B to be positively electrified. 
 Its charge then induces a certain quantity of electricity 
 to the side of A next to B, repelling a corresponding quantity 
 of + electricity from A to the earth. 
 
 Now suppose B also to be connected with the ground, as 
 shown by the dotted line ; the electricity developed in A 
 reacts upon B, augmenting B'S 4- charge by a certain amount 
 and repelling a corresponding amount of electricity from 
 B to earth. l 
 
 This reflex action continues indefinitely, causing accu- 
 mulated charges of electricity to be condensed on B. 
 
 Il6. On this principle condensers are made, and are 
 so termed because they are used to condense or store up 
 electricity. 
 
 Fig. 21 represents the principle of any form of con- 
 denser. 
 
 It consists of a number of layers of tin-foil separated 
 from one another by thin sheets of paper saturated with 
 paraffin, or sometimes sheets of mica. 
 
 These layers of tin-foil are arranged to form two series, 
 A and B, which by induction act in a precisely similar way 
 to the inner and outer coatings of a Leyden jar, 2 or to the 
 plates A and B in fig. 20. The plates of the A series (fig. 21) 
 are joined together ; likewise those of B, but the two series 
 are kept insulated from one another. 
 
 Thus a + current entering the B series charges these 
 
 1 Electricity 
 developed in 
 this manner, by 
 induction, is 
 called Bound, 
 Dissimulated, 
 or Latent, be- 
 cause it exhibits 
 the remarkable 
 property of not 
 passing off, 
 even when the 
 conductor on 
 which it is con- 
 densed is con- 
 nected to earth. 
 Should the 
 other con- 
 ductor be 
 removed, how- 
 ever, the elec- 
 tricity of the 
 first no longer 
 remains bound, 
 but passes 
 away to earth. 
 This latter 
 natural condi- 
 tion of electri- 
 city is called 
 Free. 
 
TELEGRAPH INSTRUMENTS : SIGNALLING. 
 
 plates with electricity of the same kind, inducing in the A 
 plates an equal amount of electricity, and causing a cor- 
 responding amount of + electricity to pass through A to 
 earth. 
 
 SECTION 
 
 FIG. 2i. THE CONDENSER. 
 
 Resemblance 
 
 Leyden jar or 
 
 Voltaic or Elec- 
 induction 110 
 
 From the above process, which is called condensation of 
 electricity, it is easy to understand how a small charge of 
 electricity from the battery c (fig. 21), communicated to the 
 B series (the A series being to earth), can be accumulated in 
 a condenser, and thus be increased to any extent according 
 to the capacity l of the condenser. 
 
 The quantity of the charge depends upon the number 
 of cells used in the battery, upon the size of the dielectric 
 sheets, 2 their thinness, and the smoothness of their surface. 
 
 The use and application of condensers to telegraph 
 work will be explained hereafter. 3 
 
 The object of the foregoing remarks has been to explain 
 how, by virtue of electro-static induction, a charge of elec- 
 tricity is communicated to and retained by a Leyden jar 
 or condenser. 4 
 
 1 17. Now a telegraph line may be regarded in the light 
 of either : the wire itself playing the part of the inner coat- 
 
 x j *-' 
 
 ing of the Leyden jar ; the ground and all connected to it, 
 such as posts, &c., representing the outer coating ; and the 
 air, the dielectric, acting like the glass jar, and admitting 
 of induction between the wire and the earth. 5 
 
 Electro-static induction, however, is not the only force 
 at work which affects the transmission of signals along a 
 telegraph wire ; 6 and in order that the important part played 
 also by voltaic or electro-dynamic induction may be 
 
 ,. J . .... 
 
 clearly understood, the phenomena resulting therefrom will 
 
 be briefly described as follows : 
 
 J , 11-11 
 
 n8. A galvanic current sent through a wire has the 
 power of inducing a momentary current in a neighbouring 
 
 1 Def. 15. 
 
 2 Def. 17. 
 5 202. 
 
 4 The faults 
 
 condensers 
 liable are 
 
 described in 
 
 231. 
 
 5 Bearing this 
 
 ^indTus 111 
 obvious that 
 
 telegraph line 
 
 the less facility 
 j s offered to 
 induction, and 
 
 consequently 
 
 to 'charge.' 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION wire, this action being called induction ; and the mo- 
 _ c ' __, mentary current produced, the induced or secondary 
 current. 
 
 The battery current is called the inducing or primary 
 current. 
 
 Suppose the ends of the primary wire A, B (fig. 22) to be 
 joined to a battery and key, and those of the secondary 
 wire a, b joined to a galvanoscope, it will be observed that in 
 making battery contact 7 the primary current will cause a 
 secondary current in the wire #, b, as indicated by a move- 
 ment of the needle ; the deflection, however, only lasts an 
 instant, and the needle at once falls back to zero, this induced 
 current being opposite in direction to the primary current. 
 
 FIG. 22. PHENOMENA OF VOLTAIC INDUCTION. 
 
 When the primary circuit is broken^ another momentary 
 deflection is observed in the galvanoscope, this time showing 
 the induced current to be the same in direction as the 
 battery current. 
 
 The action thus observable in single wires is greatly 
 increased when they are wound in coils. 
 
 If, when a current is flowing in the primary coil, it be 
 brought nearer to the secondary coil, or if the strength of 
 the primary current be increased, a secondary current will 
 be induced similar in direction to that when contact is made ; 
 and if the reverse be done, a current will be induced in the 
 same direction as would be caused by breaking the primary 
 circuit. 
 
 These principles will be found detailed among the laws 
 of induced currents, 1 and a knowledge of them is of great i Laws 20-22, 
 importance. App - A - 
 
 If a magnet take the place of the primary current in the 
 above instances, the same effects of induction will be produced ; 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 and this may be readily understood from Ampere's theory ', that 
 a magnet is simply a solenoid, 1 or rather a bundle of solenoids, 
 i.e. each molecule of a magnetic substance is traversed by a 
 closed electric current. 
 
 Extra Current. lip. Another phenomenon of induction is that known 
 as an extra current, which is caused by the inductive 
 action of a current on itself. 
 
 At the moment a current commences to flow through 
 a long coil of an electro-magnet, it induces in it an extra 
 current lasting but an instant, flowing in a reverse direction 
 to that of the battery and opposing it, so that if there were a 
 soft iron core in the coil its magnetisation would be delayed. 2 2 The strength 
 
 On breaking the battery current, an instantaneous direct curren ts is 
 current is induced, which would therefore tend to delay the ^^^ of 
 demagnetisation of the same core. a soft iron core 
 
 There are thus two kinds of extra current, viz. the jjj^jjl^ 
 inverse caused by making contact which opposes the battery, App. A. 
 and exists when the magnetism of the iron is increasing 
 and the direct, caused by breaking contact, and which exists 
 when the magnetism is decreasing. Although the E.M.F. of 
 both the extra currents is the same, the effect of the direct 
 (caused by breaking contact) is the more observable, as it 
 acts in the same direction as the battery current which it 
 momentarily increases. The spark observed in relays at the 
 moment contact is broken in the local circuit is an illustra- 
 tion of this fact. 3 5 La w 21, 
 
 It is thus obvious that the extra current in the coils of App- A 
 electro-magnets tends to increase the variable state, 4 and * no. 
 consequently to diminish the speed of signalling. 
 Extra Current 120. To counteract the retarding effects of extra currents 
 
 Cotis^eutrai in the coils of electr - ma g nets > the Y are sometimes wound 
 
 ised by joining parallel instead of in series. 5 5 l62 - 
 
 them parallel. The effect of ^ j s to cause ^g extra current in the 
 
 coils to be opposite in direction to one another, and thus 
 
 they neutralise each other instead of combining, as they do that by joining 
 
 when the coils are wound in series, to oppose the primary para n e i t h e i r 
 
 current at its commencement, and to augment and prolong ^^"^ g e ~ 
 
 it at its Close. 6 App. A), and' 
 
 Summary of I2 i. Induction is the main cause which diminishes the further, that 
 
 Cruises iffcct- trie magnetic 
 
 ing the Speed speed of signalling ; first, by accumulating on the surface force of the 
 ?I) S Srda S : f the Wire a P rtion f the current which W0uld Othe 
 
 tfon by Electro- wise pass on to form signals ; the quantity accumulated A PP . A). 
 
74 
 
 SECTION 
 C. 
 
 static Induc- 
 tion, ' Charge 
 and Discharge. 
 
 (&) Magnetic 
 Inertia. 
 
 (c] Extra 
 Currents. 
 
 (d) Mechanical 
 Inertia. 
 
 (e) Residual 
 Magnetism. 
 
 MANUAL OF TELEGRAPHY. 
 
 depending upon the length and surface of the wire, 1 upon 
 its proximity to the earth, and upon the insulating medium 
 which separates it from the earth. 
 
 For this reason the electro-static capacity 2 of cables is 
 much greater than that of land lines i.e. they hold a much 
 greater charge. 
 
 The greater this electro-static capacity, the less the 
 speed, for it causes the first portion of a sent current to be 
 absorbed or accumulated as shown above, and thus it retards 
 the first appearance of the signal at the distant end. 3 
 
 Again at the cessation of the signal, the accumulated 
 charge takes an appreciable time to discharge, and conse- 
 quently each signal is prolonged. 4 
 
 Thus ' charge ' produces retardation, and c discharge ' pro- 
 longation. 
 
 This of course necessitates slower signalling to prevent 
 dots from running one into another. 
 
 Again, an electro-magnet cannot be magnetised and 
 demagnetised with infinite rapidity. The core takes time 
 to magnetise and to lose its magnetism ; 5 and, moreover, 
 the currents which pass through the coils induce other 
 currents in the same coils which retard their demagnetisa- 
 tion. 
 
 These extra currents, as they are called, and which 
 are created in the line as well as in the coils of electro- 
 magnets, 6 influence speed by retarding the commencement 
 of a signal and delaying its termination, and their strength 
 is proportional to the number of turns of wire in the electro- 
 magnet, as well as to the mass of iron in the core. 
 
 The sensitiveness of receiving instruments is an important 
 point affecting the speed of signalling, and the play of the 
 armatures of electro-magnets should for this reason be made 
 as small as possible. 
 
 Further, the cores of the electro- magnets should be made 
 of the purest iron possible, in order to allow of their be- 
 coming demagnetised immediately on the cessation of the 
 current. 7 
 
 The two last-mentioned causes affecting speed, viz. me- 
 chanical inertia and residual magnetism, do not present the 
 obstacles to rapid signalling which the effects of induction 
 do, to remedy which it is necessary to overcome the retard- 
 ing and prolonging effects of charge and discharge respec- 
 
 1 Law 12, and 
 114-117. 
 
 Def. 15. 
 
 109 and 
 
 in and 
 
 5 Defs. 44 and 
 48. 
 
 119. 
 
 7 Defs. 43 and 
 44- 
 
TELEGRAPH INSTRUMENTS : SIGNALLING. 
 
 tively, and to counteract the influence of extra currents ; in 
 the first case by discharging the line as rapidly as possible, 
 and in the second by annihilating the effects of extra cur- 
 rents by counter extra currents more powerful than those 
 from the line. 
 
 122. The latter object is fulfilled by the electro- 
 magnetic shunt, which consists of a soft iron core in the 
 shape of a horse-shoe, wound with insulated wire, the ends 
 of which are connected to the two terminals of the receiving 
 relay x, as represented in fig. 23. 
 
 75 
 
 FIG. 23. RELAY WITH E.M. SHUNT.! 
 
 From what has been said of extra currents, it will be 
 understood that a momentary extra current of demagnetisa- 
 tion will be formed in the coils of the relay and shunt, in 
 the direction shown by the dotted arrows, at the termination 
 of each signal received. 2 
 
 It has also been shown that the strength of the extra 
 currents formed in electro-magnets depends upon the number 
 of turns of wire and the mass of iron in the core, so that it 
 is quite possible to construct the E.M. shunt s so that its 
 extra current shall be equal to or greater than that passing 
 through R. 
 
 It will be observed, by reference to the dotted arrows in 
 the figure, that the extra current of s opposes that of R at 
 the point E ; and if the two ' currents be equal in strength, 
 that of s will exactly counteract and neutralise that of R. 
 
 If, then, the magneto-inductive capacity of the shunt be 
 greater than that of the relay (as it is made in practice), the 
 stronger extra current of s will not only counteract that of R, 
 
 1 To increase 
 the inductive 
 effect of the 
 core, the two 
 extremities of 
 the horse-shoe 
 are joined by a 
 fixed armature. 
 
 2 119 and 
 121 (c). 
 
7 6 
 
 SECTION 
 C. 
 
 Results of the 
 E.M. Shunt. 
 
 MANUAL OF TELEGRAPHY. 
 
 but overcome it, and will tend to press the tongue against 
 the insulated stud A, as represented in the figure ; prevent- 
 ing its being attracted to M, as it otherwise would be under 
 the direct influence of the extra current produced by the 
 inductive action of the working current on itself. 
 
 Again, looking at the dotted arrows in the figure, it will 
 be observed that the resultant extra current from the shunt 
 i.e. what remains after neutralising the opposite extra cur- 
 rent of the relay flows from E to L in the opposite direction 
 to the working current, thus supplying the relay with a 
 momentary force of restitution, which tends to draw the 
 tongue immediately back to the rest stop A. 1 
 
 Thus, the E.M. shunt performs a second and important 
 function in counteracting the prolonging effects of discharge, 2 
 by exerting a contrary force on the tongue and drawing it back 
 to its rest stop A at the termination of each received signal. 
 
 It must be distinctly remembered, however, that the 
 E.M. shunt takes no part in discharging the line. This is 
 effected on a different principle altogether by discharging 
 instruments, which are explained in paras. 1 2 5-1 3 2. 3 
 
 From what has been said in the foregoing paragraphs 
 with regard to the action of the shunt at the termination of 
 each signal, its action at the commencement of each signal 
 will be readily understood. 
 
 The extra currents will be in the opposite direction to 
 that shown in the figure, and the superior extra current of 
 the shunt will tend to draw the tongue over to its working 
 contact M t aiding the working force, preventing retardation, 
 thus rendering the commencement of signals sharp, and 
 acting in the contrary direction to the ' charge ' current of 
 the line. 4 
 
 123. The shunt thus fulfils the following important 
 objects : 
 
 (1) At the commencement of each received signal, it pro- 
 duces an extra current in the same direction as, and adding 
 to the force of the initial working current through the coils 
 of the relay. 
 
 (2) At the termination of every received signal, it pro- 
 duces an extra current in the opposite direction to the work- 
 ing current, the effect of which is to hasten the return of the 
 tongue to A, thus remedying the defect of Siemens relay, to 
 which reference has been made. 5 
 
 61-63. 
 
 2 III, 112, 
 
 and 121 (a). 
 
 3 Observe that 
 shunts are 
 applied in the 
 Receiving 
 Circuit, and 
 discharging 
 instruments in 
 the Sending 
 Circuit. 
 
 4 109, no, 
 and 121 (a). 
 
 5 61-63. 
 
Efficiency. 
 
 Discharging 
 Arrangements. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 (3) As its extra current acts in the reverse direction to 
 the charge and discharge of the line, it sharpens received 
 signals by checking the retarding effects of ' charge ' at the 
 commencement and the prolonging effects of ' discharge ' at 
 the end of each signal. 1 
 
 The resistance of the shunt should be equal to that of 
 the relay with which it is joined. 
 
 On the principle of shunts or derived circuits, 2 it is 
 reasonable to conclude that half the working .current will be 
 diverted from the relay through the shunt circuit of equal 
 resistance ; and this is right so far as the permanent current 
 is concerned. It is necessary, however, to bear in mind the 
 action of the extra current of the shunt at the commence- 
 ment of each received signal, which, traversing the coils of 
 the relay in the same direction as the working current, adds 
 to its strength just at the moment when its force is required 
 to draw the tongue over from the rest stop to the working 
 contact ; this done, the strength of the working current in 
 the relay coils, although reduced by the derived circuit of 
 the shunt, is sufficient to hold the tongue over against the rest 
 stop, when it is once there, until the cessation of the signal. 
 
 124. As the object of the shunt is to produce an extra 
 current, its efficiency is tested by the extra current produced 
 in its coils by the minimum working current. The E.M. 
 shunt, when joined up with a relay of the same resistance 
 as its own, should give a flick with a current of three milli- 
 oerstedts. 3 
 
 125. In paras. 109-112 it was explained how telegraph 
 lines become electro-statically charged by the battery cur- 
 rent ; and how, after the release of the front contact (</) of 
 the key (fig. i8), 4 after each signal sent, a current of discharge 
 or return current rushes back to earth immediately contact 
 (2) is closed. 
 
 Now the result of this is that the return current causes a 
 momentary beat on the receiving instrument (which is in- 
 serted between 2 and earth) after each signal sent ; and it is 
 to prevent sent signals thus affecting the instrument of the 
 home station that discharging instruments are used, 
 the object of which is to provide a direct circuit to the earth 
 for the return current after each sent signal, and at the same 
 time to permit of the arriving working currents traversing 
 the coils of the receiving instrument. 
 
 77 
 
 i2i (a}. 
 
 2 Def. 22, and 
 Law 8, App. A. 
 
 289 
 
SECTION 
 G. 
 
 Discharging 
 Relay. 
 
 MANUAL OF TELEGRAPHY. 
 
 126. The discharging relay, which consists of an 
 ordinary Siemens relay of low resistance, fulfils this object 
 in the following manner : 
 
 It is inserted between the battery and the front contact 
 of the signalling key, as shown in fig. 24. The end of its 
 tongue, d, is permanently connected to earth. 
 
 line 
 
 FIG. 24. DISCHARGING ARRANGEMENT. 
 
 Suppose a current to be sent into the line by the depres- 
 sion of the key, contact 1 being thus closed 
 
 The tongue of the discharging relay (d) now works and 
 closes contact 3. 
 
 Now suppose the key to be released and contact 1 
 broken, by which 2 is closed ; then 3 is also broken ; and if 
 these actions are simultaneous, it is evident that all the dis- 
 charge of the line would go through the ordinary receiving 
 relay R, to earth, causing a beat of the tongue against the 
 working contact. 
 
 If, however, contact 3 were prolonged an instant beyond 
 -/, then it is equally apparent that the discharge would take 
 place through the short circuit to earth afforded by the 
 tongue of the discharging relay d. 
 
 This object is effected by the shunt s, which consists of 
 a coil of insulated wire equal in resistance to the relay coils; 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 the extra current produced in the shunt coil, on contact / 
 being broken, arrests the demagnetisation of the cores of the 
 discharging relay, by reason of which contact 3 is sufficiently 
 prolonged to exist for a moment simultaneously with contact 
 2, so that the whole discharge, or at any rate the greatest 
 part of it, has time to pass through contact 3 direct to earth, 
 instead of passing through the receiving relay R. 1 
 
 In the newest form of discharging relay the shunt has 
 been dispensed with, and the prolonging effect produced by 
 means of an ordinary translation spring, attached to the 
 tongue, which has been found to be more effectual than the 
 shunt. 
 
 127. The efficiency of the above arrangement to discharge 
 is tested by taking off the line from the key, and by substi- 
 tuting between the points L and E, as shown by the dotted 
 line (fig. 24), a condenser c, containing resistance or joined 
 parallel to a resistance box, and after charging the condenser 
 by closing contact / of the key, observing on its sudden 
 release whether the condenser is completely discharged 
 (as it should be) through the tongue of the discharging 
 relay. 
 
 If not, the receiving relay R, being in the derived circuit, 
 would of course work and tell the tale. 
 
 128. On the same principle the discharge current of the 
 line can be momentarily short-circuited to earth by means 
 of the discharging- key described in para. 108. 
 
 Referring to fig. 16 (para. 108), and supposing the re- 
 ceiving relay to be inserted between R and the earth, and 
 that the terminal z is joined direct to earth (i.e. the 
 battery e removed) ; then, from the order in which the con- 
 tacts R, z, and c, open and close, as explained in para. 108, 
 it is evident that at the cessation of each sent signal the line 
 is discharged through the contact z to earth during the 
 momentary interval when the contacts z and R are closed 
 together, before R is opened and the line connected with 
 earth through the receiving relay. 
 
 The time during which z and R are closed together can 
 be prolonged by means of springs, to ensure the complete 
 discharge of the line. 
 
 These same springs, however, are a cause of defect, in 
 that they get out of order ; and unless their play is very 
 limited they are a serious obstacle to speed. 
 
 79 
 
 1 The action of 
 the discharger 
 is accelerated 
 by the insertion 
 of a battery 
 between the 
 tongue of the 
 discharger and 
 earth, with its 
 copper pole to 
 earth, as in the 
 case of the 
 Zinc Sender 
 described in 
 196. 
 
8o 
 
 SECTION 
 C. 
 
 Discharging 
 Key used as a 
 Zinc Sender. 
 
 MANUAL OF TELEGRAPHY. 
 
 The discharging key is, therefore, inferior to the dis 
 charging relay described in para. 126. 
 
 I2p. While referring to this form of key, its use as a 
 zinc sender may be noticed. Fig. 25 represents it as 
 such ; and it will be observed that the circuit is identical 
 with that described in the foregoing paragraph, explaining 
 the use of the key as a discharging arrangement, with the 
 addition of the second battery e. 
 
 Discharge : 
 How effected 
 by d'Arlincourt 
 Relay. 
 
 The Receiver. 
 
 FIG. 25. CONSTANT RESISTANCE KEY USED AS A ZINC SENDER. 
 
 Knowing the operation of the key as a discharger, it will 
 be at once apparent, on reference to the above figure, that 
 at the cessation of each sent signal the line, instead of being 
 discharged direct to earth through the contact z, is now dis- 
 charged through the battery e ; and further, that this battery, 
 having its copper pole to earth and zinc to line (through z), 
 serves to send a momentary zinc or current into the line 
 after each + signal from the working battery E. 1 
 
 130. Another form of discharger is that adopted in the 
 d'Arlincourt relay, which effects the same object as the dis- found ex- 
 charging relay and key described in paras. 126 and 128 
 respectively, but on a different system to these instruments. 
 
 131. The d'Arlincourt arrangement is composed of two 
 distinct parts, the 'receiver' and 'discharger.' The 
 former consists of an electro-magnet containing a soft iron 
 core in the shape of a horse-shoe, with two projections 
 
 i The action 
 
 be 
 
TELEGRAPH INSTRUMENTS: SIGNALtt&3==Z> g r 
 
 opposite to one another, one on each arm, at the end nearest SECTION 
 the equator l (i.e. near the bend of the horse-shoe), between _ Ct . 
 
 which projections a polarised tongue plays ; 2 the electro- i Def. 39. 
 
 magnet is so wound that these projections, under the in- 2 The tongue 
 
 fluence of a current in the coils, form consecutive poles, each as?ong Sed ^ 
 
 being of opposite polarity to the outer extremity of its own permanent 
 
 arm of the horse-shoe ; and being so much nearer the 
 
 neutral line, the magnetism of these projections is much Siemens 
 weaker than that of the extremities. 6^ elay ' 59> 
 
 The process of demagnetisation which takes place at the 
 cessation of each signal, momentarily reverses the polarity 
 of the soft iron projections, by reason of the residual mag- 
 netism in the extremities, which is not only stronger and 
 opposite in kind, but, owing to the greater distance between 
 the outer poles, does not disappear so rapidly as the mag- 
 netism of the projections ; the result being that the tongue 
 is assisted back to its position of rest by this temporary 
 reversal of the magnetism of the projections ; speed of 
 signals is gained by the force of restitution thus supplied, 
 and the range of the instrument is made very large by the 
 prevention of the necessity for re-adjustment, the residual 
 magnetism of the core being in proportion to the strength 
 of the working current. 3 5 j n t hi s 
 
 This same property, which, in the case of the receiver, is ^eskJuai ma* 
 applied to the important purpose of accelerating the force of netism supplies 
 restitution, is made use of in the discharger in the following tut^instead 1 ' 
 manner, for the purpose of connecting the line momentarily of being a 
 to earth at the end of each sent signal. pediment to" 
 
 The circuit will be understood from the following figure, s P eed ( 6 s)- 
 which also explains the mode of connecting up the instru- 
 ment. 
 
 The 132. The discharging part of the arrangement, z>, is pre- 
 
 scharger. c i se ly the same in form as the receiver, /?, 4 but the position 4 in fact the 
 of the tongue between the soft iron projections is so adjusted anT^are D 
 that a current flowing through the coils of the discharger, in pivoted on the 
 the direction of the arrow, presses the tongue harder against 
 its rest stop, p. 
 
 When the current ceases, however, the momentary re- 
 versal of the polarity of the projections between which the 
 tongue lies, as described in the foregoing paragraph, causes 
 the tongue to fly across to the contact ^, but the magnetic 
 force which causes this action being only momentary, the 
 
 G 
 
 
82 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION tongue returns again immediately to/, by virtue of the bias 
 , ^ , it is originally given against that stop. 
 
 Siemens' 
 
 Lightning 
 
 Discharger. 
 
 > To line. 
 
 FIG. 26. THE D'ARLINCOURT ARRANGEMENT.! 
 
 1 In this 
 diagram the 
 Sounder and 
 Local Circuit 
 of the Receiv- 
 ing Relay R 
 are omitted for 
 clearness" sake 
 
 Examining the figure, it will be observed that every time 
 the key K is depressed, the tongue of s, which is an ordinary 
 Siemens relay of low resistance, joined in the circuit of the 
 line battery, is attracted to the working contact JF, and the 
 circuit of the local battery is closed through the coils of the 
 discharger, the tongue of which is thus kept pressed against/. 
 
 The moment contact 1 of the key is opened, the tongue 
 of the discharger makes an instantaneous contact with q, as 
 explained above, thus connecting the line for an instant 
 direct to earth through A", the discharger tongue, and contact 
 q ; by this means effecting the discharge of the line. 
 
 133. It is most essential that an efficient means be pro- 
 vided of discharging the line of lightning, which, owing to 
 its high potential, is a source of great danger to instruments; 
 demagnetising or reversing the polarity of magnets, fusing 
 the coils of electro-magnets, and rupturing their insulating 
 covering (or dielectric") 2 in rinding for itself a speedy path to 2 Def. 17. 
 the ground. 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 This property of high potential, causing it to leave a 
 metallic circuit for a more direct passage to earth through 
 air a property not exhibited by a galvanic current is 
 taken advantage of in the various forms of apparatus devised 
 for discharging lightning. 
 
 Siemens plate discharger is the departmental form, as 
 represented in fig. 27. 
 
 i: 
 
 FIG. 27. SIEMENS' LIGHTNING DISCHARGER (FOR A SINGLE LINE). 
 
 Efficiency. 
 
 Switches or 
 Commutators. 
 
 Fig. 47, 182. 
 
 It consists of two brass plates, one placed upon the other, 
 but kept from touching one another by very thin ebonite 
 washers, w^ w, w^ w. 
 
 The upper plate, L, is joined l in the circuit of the line 
 outside the instrument, and the lower, E, is connected to 
 earth, so that in the case of lightning it is discharged direct 
 to earth, passing in a spark from L to , instead of traversing 
 the metallic circuit of the instrument. 
 
 The inner surfaces of the plates are grooved, at right 
 angles to one another, on the principle that points facilitate 
 discharge. 
 
 134. The efficiency of the lightning discharger is proved 
 by the perfect conduction of each plate between its terminal 
 screws, s, s, and by absolute insulation between the plates 
 themselves. 2 
 
 135. In order to make such changes in circuits as may 
 
 be found necessary, without the inconvenience of taking off charger is 
 wires or otherwise interfering with fixed connections, 
 switches or commutators are used, whereby this opera- 
 tion is performed simply by the insertion or withdrawal of 
 metallic plugs or screws. 
 
 136. The form most commonly used is that known as 
 the S.T.D. switch, represented in fig. 28. 
 
 G 2 
 
 2 The faults to 
 
 Dis 
 
 bed h Sect 
 E, 233. 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 z, z>, s, T are brass blocks insulated from one another 
 by a circular wooden or ebonite base-board, but connected, 
 when necessary, by means of plugs inserted in the circular 
 apertures between them. 
 
 FIG. 28. S.T.D. SWITCH. 
 
 The chief use of this switch is for joining lines either 
 direct, or in translation, or for open circuit working. 
 
 The method of connecting it up for these purposes is 
 shown in figs. 47 and 48.* 
 L is joined to line. 
 s instrument. 
 
 D D of neighbouring switch. 
 
 T T instrument. 
 
 137. A more complicated form of switch, known as the 
 P. switch, is specially designed for connecting lines and 
 
 183. 
 
 182 and 
 
 "\2bT.efSutMJZanil Sounder 
 
 , --------- -jr ------ >- j- ------- _ ------- >. 
 
 Jfclay fiattery 
 
 ToKof JtigTit May^ \To. ofleftjlelay 
 FIG. 29. P. SWITCH. 
 
 instruments for parallel working, 2 admitting also of the 
 same changes of connections as are effected by the S.T.D. 
 switch described in the foregoing paragraph. 
 
 186, 
 
Lever Switch. 
 
 TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 The terminals are connected up as shown by the dotted 
 lines in fig. 29, 1 and plugs are inserted for working the 
 various circuits as follows : 
 
 In D for D working (paragraph 185) ; 
 
 In s, s, E s ( 180) ; 
 In r, T, E T ( 183) ; 
 In s, s, P, P P ( 186) ; 
 In s, s, z, z A ( 187) ; 
 In E, z connecting either line with earth direct. 
 
 By withdrawing plugs, either or both lines can be in- 
 sulated. 
 
 138. Another form of switch is that shown in the fol- 
 lowing figure. 
 
 See also 
 fig. 51, 187. 
 
 PLu.y 
 
 FIG. 30. CURRENT REVERSER. 
 
 This switch can be used for various purposes, 2 but is 
 generally employed for changing the poles of the battery. 
 
 Its connections, when used for this object, are repre- 
 sented in fig. 71, para. 293. 
 
 Supposing the terminals c and z to be joined to the 
 copper and zinc poles of a battery respectively, and L and E 
 to line and earth as shown by the dotted lines in fig. 30, 
 then it is evident that when plugs are inserted in 1 and 2 a 
 copper current is sent to line, and the zinc of the battery is 
 joined to earth. 
 
 But by changing the plugs to 3 and 4 the current is 
 reversed, zinc being to line, and copper to earth. 
 
 139. The same object is effected by the lever switch, 
 which is represented in fig. 31, this form of switch being 
 sometimes met with as a fixture on the base-board of the 
 
 2 In offices 
 into which only 
 two lines are 
 led, this instru- 
 ment is used as 
 a line commu- 
 tator ( 140). 
 
5 MANUAL OF TELEGRAPHY. 
 
 SECTION Wheatstone's bridge or other apparatus, nothing being 
 . c ' ^ visible of it except the handle H and the two stops s, s 
 between which it plays. 
 
 FIG. 31. LEVER SWITCH OR BATTERY REVERSEK. 
 
 It will be observed that the terminals correspond with 
 those of the current reverser described in the last paragraph, 
 and are similarly joined up. 
 
 With the handle turned to the right, as shown in the 
 above figure, copper is joined to line, and zinc to earth, by 
 means of the steel springs -7, 2, 3, 4, which press on two 
 semicircular pieces of metal, a and b, which are fixed to the 
 axis of the handle, being insulated by the ebonite ring R> 
 and separated from one another. 
 
 When the handle is turned to the left, the springs 2 and 
 3 press on ft, and 1 and 4 on a, so that copper is then 
 joined to earth, and zinc to line, and the current is thus 
 reversed. 
 
 The 140. Where the number of lines and instruments in an 
 
 Commutator. o ffi ce exceeds two, the bar switch or commutator is 
 used. 
 
 It consists of two sets of brass bars fixed at right angles 
 to one another, as shown in fig. 32, the upper set being 
 insulated from the lower. 
 
 There are, however, holes in all the bars, upper and 
 lower, corresponding with one another so that any one 
 upper bar can be connected to any one lower bar by means 
 of a brass screw. 
 
 Fig. 32 represents a commutator for four lines. 
 
 The L terminals of the upper series are all joined to the i Through 
 lines, 1 and the j terminals of the lower series to the instru- *? Lightning 
 
 . . . Dischargers 
 
 'ments : thus any line can be joined to any instrument, or ( 182). 
 
TELEGRAPH INSTRUMENTS: SIGNALLING. 
 
 to the spare bar T (for testing purposes), or direct to earth, 
 or looped together direct or through a galvanoscope. 
 
 SECTION 
 C. 
 
 The Battery 
 Commutator. 
 
 Connecting 
 Screw. 
 
 FIG. 32. THE COMMUTATOR. 
 
 141. On the same principle, and with a slight modifica- 
 tion in the number of upper and lower bars to meet the 
 requirements of the case (the lower bars s, T\, s 1 , E being 
 dispensed with), a similar instrument is used as a battery 
 commutator, enabling the number of cells in any battery 
 to be increased by the insertion of plugs in the commutator. 
 
 For suppose the terminal T (fig. 32) to be joined to 
 earth, and the zinc pole of a line battery to be joined to j l so 
 that it can be connected with earth by means of a screw in 
 the hole marked 1 ; then, suppose a spare battery to be 
 joined up with its zinc pole to J IH and its copper pole to L 19 
 it is evident that by taking out the screw from 1 and in- 
 serting it at 3, earth will be taken off the line battery and 
 put on to the zinc pole of the spare battery ; and by insert- 
 ing a second screw in 2, the line battery zinc will be con- 
 nected to the spare battery copper, and thus the strength 
 of the latter battery will be added to the former. 
 
38 MANUAL OF TELEGRAPHY. 
 
 In this way a commutator may be made to provide for 
 any number of line and spare batteries ; a lower bar being 
 reserved for the copper pole of each spare battery and one 
 for the earth ; and an upper bar for the zinc pole of each 
 battery both line and spare. l The faults to 
 
 _ J ... . , . . .. which Switches 
 
 It will be observed that the spare batteries can be joined and Commu- 
 up in series with one another in the same way that the spare ^dls^ribedtn 
 is added to the line battery as described above. 1 Sect. E, 234. 
 
8 9 
 
 PART II. TESTING INSTRUMENTS. 
 
 The 
 Galvanoscope. 
 
 142. Electric currents may be discerned in various ways ; 
 the most practically convenient of these being the effect 
 produced on a magnetised needle. 
 
 To indicate the presence of an electric current by this 
 means, instruments called galvanoscopes or galvano- 
 meters are used ; the principle of which has been already 
 explained under the head of electro-magnetism. 1 
 
 Fig. 33 represents the form of galvanoscope inserted in 
 the line circuit 2 to indicate whether a current is passing. 
 
 Galvano- 
 meters. 
 
 FIG. 33. GALVANOSCOPE. 
 
 It consists of a hollow coil of insulated wire containing 
 a small magnetic needle, rather more than half an inch in 
 length, supported horizontally on a small steel pivot. 
 
 A paper pointer or indicator is fixed on the centre of 
 the needle at right angles to its length, the movements of 
 which, under the influence of the current, are limited by the 
 copper stops s, s, so that, besides affording a means of 
 observing whether a current is passing, short and long beats 
 can be distinguished from one another, and passing signals 
 can thus be read. 
 
 If instead of limiting the deflections of this instrument 
 it were provided with a scale, whereby the extent of its 
 deflections could be measured, it would be called a gal- 
 vanometer. 
 
 143. There are many forms of galvanometers, varying in 
 
 Si-SS- 
 
 185. 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 Aids and 
 Obstacles to 
 Sensitiveness. 
 
 sensitiveness and in their mode of construction to suit 
 special requirements. 
 
 144. Sensitiveness, that is, the readiness with which 
 the needle moves under the influence of a current, is gained 
 in various ways. 
 
 First, by multiplying the number of convolutions or 
 coils of insulated wire around the needle as explained in 
 para. 54. 
 
 It must be remembered, however, that a point is arrived 
 at when an increase of the number of convolutions does not 
 increase the deflections in a corresponding ratio, because 
 each layer of coils has to be wound at a greater distance 
 from the needle than the layer next inside it, and thus the 
 effect of the current on the needle becomes less according as 
 the coils are further off, until it is so reduced as to render the 
 addition of convolutions an obstacle (owing to the increased 
 resistance they introduce) rather than a gain to sensitiveness. 
 
 Second^ by using as small a needle as possible so that it 
 may not be deflected out of the influence of the coils, 1 and 
 yet it should be so long that its poles are sufficiently far 
 apart to be unaffected by one another. 
 
 Third) by making the hollow space inside the coil, in 
 which the needle is placed, as small as possible, so that the 
 convolutions shall be as near to the needle as practicable, 
 and thus exert the greatest possible influence upon it. 2 
 
 Fourth) by causing the needle to move with as little 
 friction as possible about its support. 3 
 
 Fifth) by making the needle as light as possible with the 
 above object, and yet making it, or its indicator, so long 
 that the smallest deflection of the needle may be clearly 
 indicated on the scale. 4 
 
 Sixth) by rendering the needle as insensible as possible 
 to the effect of terrestrial magnetism ; either by the use of 
 directive magnets, 5 or by weakly magnetising the galvano- 
 meter needle. The magnetism of the needle must, however, 
 be sufficiently strong to return to the magnetic meridian on 
 the cessation of the current in the coils. 6 
 
 Seventh) by the use of an astatic pair of needles with the 
 same object. 7 
 
 Terrestrial magnetism must, however, be allowed to 
 exert sufficient force to bring the astatic pair back to zero 
 against the friction of the suspension of the needles. 
 
 1 Observed in 
 all delicate gal- 
 vanometers, 
 the Sine galva- 
 nometer ex- 
 cepted, in 
 which special 
 arrangements 
 are devised for 
 keeping the 
 needle under 
 the influence 
 of the coils 
 
 ( ISO). 
 
 2 Specially 
 observed in 
 Thomson's 
 Reflecting Gal- 
 vanometer and 
 all delicate 
 galvanometers 
 of high resist- 
 ance. 
 
 3 Generally 
 effected by the 
 use of a fine 
 silk or hair 
 fibre, free of 
 torsion. 
 
 4 Carried out 
 to perfection in 
 the Thomson 
 Galvanometer, 
 in which the 
 needle and 
 mirror together 
 weigh only 
 
 i grain, and 
 the indicator, 
 which is a 
 weightless ray 
 of light, is 
 6 feet in length 
 (i.e. twice the 
 distance be- 
 tween the mir- 
 ror and scale), 
 causing a large 
 traverse on the 
 scale for a fa ; nt 
 deflection of 
 the needle 
 ( 156). 
 
 5 See note * 
 next page. 
 
 6 See note f 
 next page. 
 
 7 See note J 
 next page. 
 
TELEGRAPH INSTRUMENTS: TESTING. g r 
 
 Simple form of 145. The foregoing remarks will explain how the in- SECTION 
 
 Galvanometer. strument described in para. 142 may be converted into a , _ ^ _ , 
 
 galvanometer of required sensitiveness by being furnished * Directive 
 
 with an accurately graduated scale (the movable stops s. s ma ^ ets . either 
 
 ,x , . . -I, fixed or sepa- 
 
 bemg removed) ; by containing a suitable number of con- rate, are gene- 
 
 volutions of wire in its coils, according to the measurements 
 
 to be taken ; by delicate suspension of the needle, whose nometers, the 
 
 weight and size are reduced to adequate limits ; and by JJg ^ced 
 
 suitable protection from the gravitating influence of terres- towards the 
 
 trial magnetism. SSfSfthua 
 
 Calibration. 146. With regard to the scale from which the angle opposing and 
 
 of deviation of the needle is read off, it is to be remarked influence of tha 
 
 that the same current does not necessarily cause the same ea rth's mag- 
 
 ,.,.*, netism on the 
 
 number of degrees deflection to be indicated on the scale, galvanometer. 
 
 even of galvanometers of the same description, owing to t And vice 
 
 the difference in sensitiveness which must exist in various netiic fidd muf t 
 
 instruments. be sufficiently 
 
 It is necessary, then, to find by actual experiment the 
 
 deflection each galvanometer indicates under the influence come to re st 
 
 _,,,.-,, , , . with a limited 
 
 of the same E.M.F. through the same resistance. number of 
 
 The Calibrated 147. On this principle the instrument, departmentally vibrations. 
 
 Galvanoscope. knQwn ^ the calibrated galvailOSCOpe, is Constructed. I ^ 32, and 
 
 It consists of a coil of insulated wire, less than i ohm in 
 resistance, so wound as to leave a hollow space inside the 
 coil slightly over 3 inches in length. 
 
 In this is pivoted a magnetised needle 3 inches long, the 
 ends of which support a light brass circular ring, marked in 
 divisions of 5 each. 
 
 It is observed by experiment through what resistance, 
 in each case, the needle deflects to these various divisions 
 respectively, under the influence of a current of a single 
 Minotti element, the E.M.F. of which has been shown to be 
 a constant quantity, equal in value to about i volt. l i Def. 7, and 
 
 The various resistances introduced to produce corre- 3 1 - 
 spending deflections are noted on a card, and afford a means 
 of determining the approximate value of any unknown re- 
 sistance (between the limits tabulated on the card) placed in 
 the circuit of the instrument and a Minotti cell, according 
 to the deflection produced ; a deflection of or near 15, for 
 example, representing the inserted resistance as being about 
 the same as that noted on the card opposite the deflection 
 
 r o 2 2 It is pre- 
 
 01 1 5 sumed that the 
 
Efficiency. 
 
 Sources of 
 Inaccuracy 
 in Galvano- 
 meters. 
 
 MANUAL OF TELEGRAPHY. 
 
 As in this instrument the scale moves with the needle, it 
 is necessary that some fixed object should indicate its de- 
 flections. This is effected by means of a line scratched on 
 a small circular piece of glass let into the cover of the instru- 
 ment, through which a portion of the scale is visible, so that 
 the division immediately under the scratch or near it can be 
 read off. 
 
 148. The performance of the instrument is best checked 
 by its agreement with that noted on the card referred to 
 above, which always accompanies the instrument. 
 
 On this is also noted the number of oscillations made by 
 the needle before coming to rest (never less than 60), also 
 the number made in two minutes (generally about 35). 
 
 149. Now in the simple form of galvanometer described 
 in principle, in para. 145, when the needle is deflected, 
 under the influence of a current, it is clear that the greater 
 the angle of deflection the further the poles of the needle 
 are removed from the influence of the coils. 
 
 On this account, the deflections observed in such gal- 
 vanometers are not to be relied upon as being proportional 
 to the current when they exceed 20. 
 
 FIG. 34 . 
 
 The disparity between the Sine and the Arc will be apparent from the 
 following : Supposing the circle A B C to represent the graduated 
 scale of a galvanometer, and D B the needle, pivoted at the centre 
 D (B indicating zero) ; the sine of 20 is represented by the portion 
 of the radius D E ; and the sine of any other angle would be found 
 by letting fall a perpendicular from the arc to the radius from the 
 number of degrees indicating the angle on the arc, B C, which can 
 be graduated into ninety equal divisions of i each. By letting fall 
 perpendiculars from any number of degrees up to 20, it will be 
 observed that the arc and sine agree nearly ; but when the deflec- 
 tion extends to multiples of 20, the values of the sines are not 
 multiplied in the same ratio, but diminish towards 90, E F being 
 less than D E, F G than E F, and GH than F G. (See Table of 
 Sines, App. B. ) 
 
 Again, the directive forcQ of the earth's magnetism, which 
 tends to bring the needle to rest in the magnetic meridian, 
 
 horizontal 
 magnetic in- 
 tensity of the 
 testing station 
 and that of the 
 station where 
 the instrument 
 was calibrated, 
 i.e. where the 
 table was 
 framed on the 
 result of ex- 
 periments, 
 sufficiently 
 agree (as they 
 do all over 
 India) to 
 render the 
 readings com- 
 parable. 
 
TELEGRAPH INSTRUMENTS: TESTING. 
 
 is proportional to the sine of the angle of deflection of the 
 needle, and not to the angle itself. l 
 
 Now between o and 20 the degrees on the arc of a i For proof 
 circle are fairly proportional to their sines. Above that of thi . s se ? 
 point, however, the sines differ considerably from the App. B. 
 degrees as they increase towards 90. 
 
 For this reason, also, deflections above 20 cannot be 
 taken to represent the relative strength of the currents which 
 produce the deflections. 
 The sine ISO. To meet these difficulties the sine galvanometer 
 
 Galvanometer, wag d ev i se d ? w hich consists of a circular coil of insulated 
 wire (of high or low resistance according to the purpose for 
 which it is required) in which a magnetised needle is deli- 
 cately suspended ; the coil is made movable in a horizontal 
 plane, so that it can be turned after the needle and kept 
 parallel with it, thus always exerting its full force upon it in 
 any position. 
 
 To effect this the needle is surrounded by a circular 
 graduated scale, the scale and coil revolving together ; out- 
 side this scale is a similar but fixed scale, forming part of the 
 stand of the instrument. 
 
 When the galvanometer is used the coil is placed in the 
 magnetic meridian so that the needle points to zero of the 
 inner scale, which is so placed with reference to the outer 
 scale that the zero points of both coincide. 
 
 When a current is sent through the coil, the needle is 
 deflected out of the magnetic meridian. The coil is then 
 turned after the needle until the zero point on the inner 
 scale and the needle again coincide. The number of degrees 
 through which the coil is turned, and which obviously re- 
 present the angle through which the needle was deviated out 
 of the magnetic meridian, is thus indicated on the outer 
 fixed scale. 
 
 When the needle is thus deflected, the directive force of 
 the earth's magnetism tending to draw it back to the mag- 
 netic meridian is proportional to the sine of this angle of 
 deflection as stated in para. i49; 2 and the force of the 2 Solution IX. 
 current on the needle being exerted in a direction perpen- App * B> 
 dicular to the plane of the coil, 3 it is evident that the 3 Solution VIII. 
 strengths of currents measured by the sine galva- App - a 
 nometer are proportional to the sines of the angles 
 through which the coil is moved after the needle. 
 
94 
 
 The Tangent 
 Galvanometer. 
 
 MANUAL OF TELEGRAPHY. 
 
 151. The tangent galvanometer also provides 
 against the inaccuracies referred to in para. 149. 
 
 The arrangement for rendering the effect of the current 
 on the needle constant in any position of the needle is 
 simpler than that of the sine galvanometer, this being ef- 
 fected in the tangent galvanometer by the use of a very 
 small needle as compared with the diameter of the coil, so 
 that the electro-magnetic effect of the current on the needle 
 is practically the same, whatever position the latter may 
 take. 
 
 The instrument consists of a coil of very thick insulated 
 wire, or sometimes a simple ring of copper, in the centre of 
 which the needle is suspended. The length of the needle 
 should not exceed ^ the diameter of the coil. 
 
 A long fine indicator of aluminium is fixed to the centre 
 of the needle at right angles to it, by which the deflections 
 are read on a circular graduated scale, the indicator point- 
 ing to zero of the scale when the coil is placed in the mag- 
 netic meridian, as it is when used. 
 
 The current passing through the coil is pro- 
 portional to the tangent of the angle of deflec- 
 tion. 1 
 
 This is the principle of the tangent galvanometer. 
 
 Owing to its accuracy this form of instrument is much 
 used for electrical measurements. The greater distance of 
 its coils from the needle renders it less delicate than the sine 
 galvanometer, but, on the other hand, admits of a gain in 
 sensitiveness by increasing the number of convolutions in 
 the coil, the increased distance of the outer convolutions 
 from the needle relatively to the inner being slight ; and the 
 wire being thick, a considerable number of convolutions 
 may be wound without sensibly increasing its resistance. 2 
 
 It was shown in treating of the sine galvanometer 3 that 
 the value of sines decreases as the number of degrees of the 
 angle increases. 
 
 With tangents, however, the reverse is the case, as may 
 be proved by drawing a tangent, A B, i.e. a line perpendicular 
 to any radius, DC, of a circle (fig. 35) : calling the point c 
 (at which the tangent touches the circle) zero ; and supposing 
 the arc to be graduated from zero (o) to 90, a line drawn 
 from the centre of the circle through any degree of the arc 
 till it touches the tangent represents the tangent of that 
 
 1 For proof see 
 Solution VIII. 
 App. B. 
 
 2 144- 
 5 i49~ 
 
TELEGRAPH INSTRUMENTS: TESTING. 
 
 angle : thus c E represents the tangent of 20, E F of 40, 
 FG of 60. 
 
 Now it will be observed that though c E and E F are 
 fairly equal, F G is considerably greater than E F ; and for 
 higher angles the tangent will be found to increase in an 
 
 95 
 
 FIG. 35. PRINCIPLE OF TANGENT GALVANOMETER. 
 
 Departmental 
 
 Tangent 
 
 Galvanometer. 
 
 enormous ratio, up to 90 (the tangent of which is infinity), 
 a line drawn through which from the centre would be parallel 
 to the tangent A B, and thus never meet it at all. 
 
 This great difference between the arc and its tangent is 
 observed in angles over 45, on which account observations 
 are likely to be more accurate when the deflections do not 
 exceed 45, for, as will be seen from the figure, the difference 
 of a degree or two on the scale is a small matter, but the 
 error is much exaggerated on the tangent of the higher 
 angles. 1 
 
 For the same reason, accuracy in reading off the deflec- 
 tions on the scale is of the greatest importance. 
 
 In comparing the strengths of currents with this instru- 
 ment, the deflections are noted, the value of the tangents of 
 the angles of deflection being taken from a table of natural 
 tangents and substituted for the angles themselves. 
 
 152. On the above principle the departmental tan- 
 gent galvanometer is constructed, which contains two 
 coils, one of thick wire, measuring about i ohm in resistance, 
 the other of thinner wire measuring about 100 ohms. 
 
 The length of the needle is ^ the diameter of the coils. 
 
 To render the instrument complete for the measurement 
 of electromotive forces and resistances it is furnished with 
 resistance coils ; two, measuring 20 and 200 ohms respec- 
 tively, being inserted in the circuit of the thick coil ; and 
 resistances of 1,000 and 2,000 ohms each are added to the 
 circuit of the thin coil. 2 
 
 1 The tangents, 
 of angles from 
 q to 45 only 
 vary between 
 o and i ; but 
 between i and 
 OO for angles 
 between 45 
 and 90. See 
 Table of Natu- 
 ral Tangents, 
 App. B. 
 
 260, 261. 
 
9 6 
 
 SECTION 
 C. 
 
 , , 
 
 Efficiency. 
 
 Astatic 
 Galvanometer. 
 
 MANUAL OF TELEGRAPHY. 
 
 By means of plugs, either coil can be cut off or placed in 
 circuit ; the same with the fixed resistances. 
 
 153- ( J ) Tne electrical efficiency of the instrument is 
 proved by the agreement of the resistance of the various 
 coils with the values marked on the instrument. 
 
 (2) The mechanical efficiency, by the number of oscilla- 
 tions the needle makes before coming to rest, and by its 
 accurate return to zero. 
 
 The number of complete oscillations or vibrations should 
 not be less than I5. 1 
 
 (3) The magnetic efficiency, by the number of oscilla- 
 tions of the needle in a given time, under the influence of 
 the earth's magnetism alone. 
 
 It should perform not less than 7 oscillations in 10 
 seconds. 
 
 (4) The sensitiveness of the galvanometer is proved by 
 the extent of the deflections of the needle under the in- 
 fluence of the E.M.F. of the standard cell through various 
 resistances. 
 
 The deflections should be not less than 
 
 5 through thin coil and 2,000 ohms. 
 10 1,000 
 
 55 C 
 
 thick 
 
 nil. 
 
 200 ohms. 2 
 
 154. From the descriptions of the various forms of gal- 
 vanometer already referred to, 3 it will have been observed 
 that one source of unsensitiveness, common to all, is the 
 directive action of the earth's magnetism, which in causing 
 the needle to take up its position in the magnetic meridian, 
 opposes its deflection under the influence of a current. 
 
 The astatic galvanometer provides a remedy for 
 this by the use of an 'astatic pair ' of needles. 4 
 
 Two magnetic needles almost** equal in strength are 
 connected, one above the other, by a light vertical shaft of 
 aluminium, with their poles reversed ; i.e. the north pole of 
 the upper is over the south pole of the lower, and vice versa, 
 so that the attractive force of the earth's magnetism on one 
 pole is counteracted by its repulsive force on the other. 6 
 
 The pair thus fixed is delicately suspended, so that the 
 lower needle is inside the coil and the upper one just above 
 it. 
 
 1 Dcf. 50. 
 
 2 Resistance of 
 cell presumed 
 to be 20 ohms. 
 In the three 
 prior cases the 
 resistance of 
 the cell is im- 
 material. 
 
 3 142-153- 
 
 4 Def. 32. 
 
 5 If the needles 
 were equally 
 magnetised, 
 then the earth's 
 magnetism 
 would have no 
 directive force 
 at all, and the 
 pair would not 
 return to zero, 
 but would re- 
 main in any 
 position. 
 
 6 Law 23, 
 App. A. 
 
Efficiency. 
 
 Thomson's 
 Reflecting 
 Galvanometer. 
 
 1 Law 13, 
 A pp. A, and 
 
 S3- 
 
 Def. 50. 
 
 TELEGRAPH INSTRUMENTS : TESTING. 
 
 Remembering Ampere's law, 1 it is evident that a current 
 passing through the coil causes both the upper and the 
 lower needle to deflect in the same direction, so that the 
 effect is double what it would be on a single needle ; and 
 further, the astatic principle on which the needles are joined 
 with reversed poles, almost nullifying the impeding effects 
 of terrestrial magnetism renders them free to move with 
 the weakest currents. 
 
 To add further to the sensitiveness of the instrument the 
 coils can be wound with as many convolutions as necessary, 
 and should be close to the needle. 
 
 The astatic principle can be applied to various forms of 
 galvanometers. 
 
 155. The astatic condition of the pair of needles is 
 measured by the time it occupies in making an oscillation 
 across the magnetic meridian. From five to ten seconds is 
 a fair time for one vibration. 2 
 
 156. In para. 144 it was explained how sensitiveness was 
 gained by the use of a long indicator, the extremity of which 
 would move through a considerable space on the scale for a 
 small deflection of the needle, and which would effect a 
 further advantage in admitting of larger divisions of the 
 scale, thus rendering readings more accurate. 
 
 There is, however, this disadvantage in increasing the 
 length of the indicators hitherto described, that their weight 
 is increased according to their length, and an obstacle to 
 sensitiveness is thus introduced. 
 
 In Thomson's reflecting galvanometer the advan- 
 tage is gained without the disadvantage attending it, by the 
 use of a ray of light as the indicator. 3 
 
 The needle itself, which is a piece of magnetised watch 
 spring | of an inch in length, fixed to a small circular con- 
 cave mirror, the whole weighing only i^ grain, is delicately 
 suspended in a coil of fine wire, the resistance of which is 
 generally about 2,000 units or more, the coil being very close 
 round the needle, which is so small as to be always under 
 the influence of the coil in any position. 4 
 
 The needle is generally made astatic, and the instrument and seventh 
 is provided with a directing magnet, a glass case for prevent- 
 ing any disturbing effect of the air on the needle, and adjust- 
 able feet for levelling the stand. 
 
 It is thus extremely sensitive and capable of responding 
 
 97 
 
 3 144, fifth 
 clause (note). 
 
 4 144, sixth 
 
 id seve 
 clauses. 
 
9 8 
 
 Shunts. 
 
 MANUAL OF TELEGRAPHY. 
 
 to the weakest currents, and is consequently adapted for the 
 measurement of very high resistances. 
 
 For observing the deflections of the needle, a horizontal 
 scale on a wooden stand is placed in front of the galvano- 
 meter at a convenient distance, found by experiment, gene- 
 rally about three feet from it. 
 
 Below the zero point of the scale, which is at its centre, 
 a vertical slit is cut containing a fine wire drawn down the 
 middle of the slit. A lamp is placed behind this so as to 
 shine through the slit on to the mirror of the galvanometer, 
 which reflects back on to the scale a spot of light containing 
 the image of the wire. The instrument is adjusted so that 
 when the needle is at rest this image is upon the zero point 
 of the scale. 
 
 A current through the coils deflects the needle and 
 mirror, causing a slight movement of the former to be indi- 
 cated by a readable deflection of the image on the scale. 
 
 As the ray of light moves through an angle double that 
 of the deflection of the needle, it is thus an indicator of 
 about six feet, i.e. twice the length between the mirror and 
 scale. 
 
 In taking measurements with this form of galvanometer, 
 it is customary to consider the strength of currents as pro- 
 portional to the angle of deflection of the needle ; which is 
 true when the deflections on the scale do not exceed 5, for 
 within this limit the values of tangents vary almost equally 
 with their angles ; and from the above description of the in- 
 strument, it will be seen that the principle of the tangent 
 galvanometer enters into the construction of the Thomson, 
 which contains a small needle in a large coil ; but as a de- 
 flection of i of the needle moves the image more than one 
 inch on the scale when the mirror and scale are three feet 
 apart (so that no reading of the scale can represent a deflec- 
 tion of more than 10 with this instrument), the readings 
 may be accepted as indicating the strength of the current 
 with sufficient accuracy. The deflections of this instrument 
 can be controlled within small limits by means of shunts 
 specially prepared for each instrument. 1 i The faults to 
 
 157. When a current passing through the coils of a gal- nomete^fa^e" 
 vanometer produces too large a deflection for observation or liable are 
 for accuracy, the terminals of the instrument are connected sec^ G 235. 
 by means of wire, which forms a shunt or derived circuit, 2 2 De f s . 2I> 22 . 
 
TELEGRAPH INSTRUMENTS: TESTING. 
 
 the resistance of which bears a known proportion to that 
 of the galvanometer, so that only a fraction of the current 
 passes through the coils to deflect the needle, the rest being 
 led through the derived path of the shunt. 1 ' 9 n . the same 
 
 Resistance of 158. The relative strength of the current in the galvano- derived circuits 
 
 meter and shunt circuits depends entirely upon their respec- the terminals of 
 tive resistance according to the law of derived circuits. 2 in the local 
 
 If, for example, their resistances were equal, half the 
 
 current would go through the galvanometer and half through shunted with a 
 
 the shunt wire of high re- 
 
 tne SHUnt. sistance, which 
 
 In practice it is found necessary to make the resistance forms a path 
 
 of the shunt a small proportion of that of the galvanometer, current o?de- 
 
 so as to admit of a large portion of the current being diverted magnetisation 
 
 by the shunt circuit of low resistance, the small portion tra- sec. c"and 
 
 versing the coils being sufficient (owing to the effect of con- l e spf"* at 
 
 the relay con- 
 
 volutions; 3 to deflect the needle. tact is pre- 
 
 It is customary to provide sensitive high resistance vented - 
 galvanometers with shunts containing three coils marked -J, AmT^'and 
 -^, and ^, implying that their respective resistances bear Solution I, 
 these proportions to that of the galvanometer ; or, in other * P J 
 words, that they reduce the deflections of the galvanometer I4 ^. s ' 
 by TO> TW and nroo- respectively ; so that in taking a 
 measurement with the galvanometer when it is shunted with 
 the shunt, the value of the deflection must be multiplied 
 by 10 ; with the ^ shunt by 100 ; and with the -gfa shunt 
 by 1,000. 
 
 Thus, the multiplying powers of these shunts are 10, 
 100, and 1,000 respectively. 
 
 In the first case 9 tenths of the current flow through the 
 shunt and i tenth through the galvanometer. In the second, 
 99 hundredths flow through the shunt and i hundredth 
 through the galvanometer : and in the third, 999 thousandths 
 flow through the shunt and i thousandth through the gal- 
 vanometer ; in each case the resistance of the shunt being 
 
 Resistance of galvanometer 
 
 Multiplying power of shunt i, 
 
 Or o = - * For proof of 
 
 n ~1 this formula 
 
 Where S = resistance of shunt : see Solution v. 
 
 A PP . B. 
 ,, G = galvanometer ; 
 
 n = multiplying power of shunt. 
 
 H 2 
 
100 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 "- r 
 
 Multiplying 
 Power of 
 Shunt. 
 
 Resistance 
 Coils. 
 
 159. By the following formula the multiplying power of 
 any shunt whose resistance and that of the galvanometer are 
 known, may be found : 
 
 1 60. It has been already shown 1 that electromotive 
 force is that property of the current which enables it to over- 
 come resistance ; and Ohm's law 2 establishes the fact that 
 the strength of the current, i.e. the property by which it de- 
 flects galvanometer needles, &c., 3 is directly proportional to 
 ts E.M.F., and inversely proportional to its resistance. 
 
 Thus, the property of resistance affords a convenient 
 means of uniformly controlling the visible effects of the cur- 
 rent ; for example, the current of one Minotti cell through a 
 Thomson's galvanometer with a resistance of 1,000 units in 
 circuit will cause a certain deflection ; by doubling the re- 
 sistance in circuit the deflection will be halved, and so on. 4 
 
 And conversely, if in the second case an unknown resist- 
 ance had been inserted, resulting in the deflection being 
 halved, its value would be at once known to be double that 
 which was in circuit in the first case. 
 
 For measurement and comparison various standards or 
 units of resistance have been decided upon, the unit most 
 generally adopted being the ohm or B.A. unit. 5 
 
 * For proof see 
 Solution IV. 
 App. B. 
 
 1 Def. 5. 
 
 2 Law i, 
 
 App. A. 
 
 3 Def. i. 
 
 4 A Thomson's 
 Galvanometer 
 is mentioned 
 for simplicity, 
 the deflections 
 being con- 
 sidered as pro- 
 portional to 
 the currents 
 flowing 
 through the 
 coils ( 156). 
 
 5 Defs. 12 and 
 34- 
 
 FIG. 36. RESISTANCE COILS. 
 
 Coils of wire varying in resistance from one ohm and up- 
 wards and connected so as to form a series, as represented 
 in fig. 36, are called resistance coils. 
 
 The resistance of the whole series is generally 10,000 
 units ; any coil or coils can be short-circuited by means of a 
 plug, as shown in the figure. 
 
 The value of each coil is marked opposite the plug hole. 
 
TELEGRAPH INSTRUMENTS: TESTING. 
 
 101 
 
 Heating Effect 
 of Currents on 
 Coils. 
 
 Bifilar Winding 
 of Coils and 
 its Object. 
 
 161. The wire of the coils is made of German silver, the 
 resistance of which does not vary much with change of 
 temperature. l 
 
 162. Further, instead of winding the coils in one con- 
 tinuous length, the wire of each coil is first doubled and then 
 wound on to the bobbin, as represented in fig. 37, the result 
 of which is that the convolutions of one half of the coil are 
 wound in the opposite direction to those of the other half. 
 
 SECTION 
 C. 
 
 1 The chief 
 cause of 
 change of tem- 
 perature is the 
 passage of the 
 current, which 
 flowing 
 through the 
 coils heats the 
 wire. It is 
 therefore ad- 
 visable not to 
 keep the cur- 
 rent flowing 
 longer than 
 necessary. 
 
 FIG. 37. SPECIMEN OF BIFILAR WINDING. 
 
 It has been already explained 2 that the inductive action 8and 
 
 r ........ .. r . , no, and Laws 
 
 of a current upon itself is manifested in a coil of wire by an 20-22, App. A. 
 * extra current,' the strength of which depends upon the force 
 of the primary current and the number of convolutions in 
 the coil. 
 
 The effect of these extra currents, especially in the case 
 of the high resistance coils, would seriously disturb the de- 
 flections of the galvanometer which is of necessity in their 
 proximity for testing purposes. 
 
 Now the above method of bifilar winding, although it 
 does not prevent the extra current being formed, yet, as will 
 be observed from the figure, it causes the extra current in 
 each convolution to be opposite in direction to that of the 
 adjacent convolution, so that the extra current in one half of 
 the coils is engaged in neutralising that of the other half, and 
 the galvanometer is undisturbed. 
 
 163. The influence of resistance in controlling the effec- 
 tive strength of the current on a galvanometer, as described 
 in the preceding paragraphs, is in direct agreement with 
 Ohm's law, 3 which states that current strength or potential 3 Law i, 
 is immediately proportional to the electromotive force and pp ' ' 
 
102 
 
 MANUAL OF TELEGRAPHY. 
 
 The Wheat- 
 stone Bridge. 
 
 inversely proportional to the resistance in circuit ; from which 
 it follows that with the same E.M.F. the greater the resist- 
 ance in circuit, the greater the fall of potential of the 
 current 1 l Defs. 3 and 4. 
 
 This law is just as true for a ' divided circuit ' 2 as for 2 Def. 21. 
 one composed of a single wire, so that if two precisely similar 
 galvanometers were inserted in each of the paths / and q of 
 the divided or derived circuit represented in fig. 38, in which 
 a uniform current is flowing in the direction shown by the 
 arrows, they would deflect equally when situated at points in 
 p and q at which the resistances from a were equal. 
 
 FIG. 38. DERIVED CIRCUIT. 
 
 Conversely, when the galvanometers in / and q were 
 situated at any points of equal resistance, their similar de- 
 flections would show the potential of the current at these 
 points to be the same. 
 
 Again, supposing these two points to be connected across 
 by a wire in which a galvanometer was inserted, it is evident 
 that as the potential of the current at each point is the same, 
 and enters the galvanometer in opposite directions, no de- 
 flection would be produced. 
 
 Thus it can be proved whether the potentials are equal 
 at any two points of a derived circuit by connecting them 
 through a galvanometer, and observing whether there is any 
 deflection manifesting the passage of a current. 3 
 
 164. Upon this principle the arrangement known as 
 Wheatstone's bridge or balance is based. 
 
 It consists of three sets of resistance coils, A, , and w, 
 with a sensitive galvanometer, G, joined across the points 
 p and q, as symbolically represented in fig. 39. 
 
 Any unknown resistance (x) which it may be required to 
 measure is joined up between the terminals L and L V 
 
 The battery current enters at /, divides between the 
 parallel circuits A, w and , x, and leaves at n. 
 
 From what has been said in para. 163, it is clear that 
 
 3 A deflection 
 would of 
 course show a 
 difference of 
 potential, re- 
 sulting in a 
 flow of current. 
 See Defs. 5 
 and 6. 
 
TELEGRAPH INSTRUMENTS: TESTING. 
 
 when the potentials at p and q are equal, no current will 
 pass through the galvanometer, and the needle will remain 
 unmoved. 
 
 This condition of equal potentials at / and q constitutes 
 * balance] which is obtained by adjusting the resistance of JF, 
 which consists of a set of resistance coils, adjustable from 
 i to 10,000. 
 
 w 
 
 103 
 
 FIG. 39. WHEATS-TONE BRIDGE. 
 
 The 
 
 Differential 
 
 Galvanometer. 
 
 A and B each contain three coils of 10, 100, and 1,000 
 units resistance respectively, and can thus be used for ad- 
 justing the ratio of one branch to the other. 
 
 When balance is obtained, as shown by the galvano- 
 meter needle standing at zero, it follows that the resistance 
 of the circuit A, w is equal to that of the parallel circuit 
 B) x and that if A is equal to *, x must be equal to w ; or 
 more generally 
 
 A : B . : w : x, 
 
 . X B 
 
 ' W ~ ~A 
 
 or x = -w* 
 
 A 
 
 This equation is called the condition of permanent 
 balance, and thus affords a means of determining the value 
 of any unknown resistance. 1 
 
 165. On the same principle, viz. the equalisation of 
 potentials, by means of resistance, the differential gal- 
 vanometer is used for the measurement of resistances. 
 
 * For proof see 
 Solution VI. 
 App. B. 
 1 The modes of 
 testing lines, 
 earths, and 
 various resist- 
 ances and 
 electromotive 
 forces with this 
 instrument are 
 explained in 
 Sec. F, on 
 'Testing.' 
 
104 
 
 SECTION 
 C. 
 
 'MANUAL OF TELEGRAPHY. 
 
 The instrument is symbolically represented in fig. 40. 
 It consists of two distinct coils of equal resistance, and equal 
 in their effect on the needle, the two wires being wound 
 together on to the same bobbin, but in such a way that a 
 current sent through the instrument shall traverse the coils 
 in opposite directions ; thus, if the copper pole of a battery 
 were connected to the terminal c, to which the inner ex- 
 tremities of both the coils are joined ; and the zinc pole to 
 A and , the outer terminals of the coils, the current would 
 divide at c, circulating from c to A, and from c to B in con- 
 trary directions and the needle would stand at zero. 
 
 FIG. 40. DIFFERENTIAL GALVANOMETER. 
 
 Further, if equal resistances were inserted between each 
 of the terminals and the battery, the needle would still 
 remain at zero. 
 
 Any difference, however, in the resistance of either 
 circuit would cause a difference of potential between the 
 currents in either coil, and thus the needle would be 
 deflected. 
 
 ^ and s' are shunts of equal resistance (generally -^ 
 that of the galvanometer ; thus having a multiplying power 
 of loo). 1 w is a box of resistance coils adjustable from i 
 
 See 158, 
 
 to 10,000 ; x, the unknown resistance to be measured, is I59> 
 inserted between L and L 
 
TELEGRAPH INSTRUMENTS: T$^& 105 
 
 When iv = x the needle stands at zero, and thus the SECTION 
 
 resistance of x is known. . - ' f - - 
 
 The condition of balance for this instrument is therefore 
 represented by the equation 
 
 X = W* * For proof of 
 
 this equation 
 
 If the shunt s is placed in circuit, then see Solution 
 
 VII. App. B. 
 X = TOO W. 
 
 Or if the shunt s' be placed in circuit, then 
 
 w t See Law 9, 
 
 X = --- T App. A, and 
 
 100 159. 
 
io6 
 
 MANUAL OF TELEGRAPHY. 
 
 PART III. 
 MAGNETO-ELECTRIC INSTRUMENTS. 
 
 SECTION 
 C. 
 
 Magneto- 
 electric 
 Induction. 
 
 Principle of 
 Magneto- 
 electric 
 Machines. 
 
 1 66. In para. nS 1 it was explained how, by indue- l See also 
 tion, a current flowing through a coil of wire produces a App A 
 momentary extra current, in a neighbouring wire, opposite 
 
 in direction to the primary current when the latter com- 
 mences, increases, or is brought nearer to the secondary 
 coil ; and that another momentary extra current is formed, 
 in the same direction as the original current, whenever the 
 latter ceases, decreases, or is moved away from the secondary 
 coil. 
 
 It was shown in para. 119 that the above effects were 
 also produced in a coil by the inductive action of a cur- 
 rent upon itself, the strength of the extra currents being 
 increased by the insertion of an iron core in the interior of 
 the coil ; and supposing a current to be sent through such 
 an electro -magnetic coil, the inverse extra current (caused 
 by commencing or increasing the direct current) would 
 retard the magnetisation of the core ; the direct extra cur- 
 rent tending to delay its demagnetisation. 
 
 Further, in para. 118 the converse also was shown to 
 be the case, viz. that if the pole of a permanent magnet, 
 instead of a battery current, were approached towards and 
 withdrawn from a coil of wire, it would produce extra cur- 
 rents in the coil in precisely the same way that the galvanic 
 current does ; that is to say, any alteration of the magnetic 
 field in the neighbourhood of the coil would create an extra 
 current therein, just as any alteration of the battery current 
 was shown to affect the magnetism of the core of an electro- 
 magnet. 
 
 167. Thus, if an ordinary electro-magnet, consisting of a 
 pair of coils containing soft iron cores, be so placed in the 
 field of a permanent magnet as to be capable of altering 
 
MAGNETO-ELECTRIC INSTRUMENTS. 
 
 their relative position, so that the coils may be under the 
 influence of either pole of the magnet successively, extra 
 currents in reverse directions will be formed in the coils for 
 each change of polarity produced in their cores. 
 
 The number of currents thus produced in a certain time 
 will of course depend upon the rapidity with which these 
 reversals are effected, and by means of a commutator, to be 
 explained hereafter, 1 the extra currents may be made to act } 169. 
 all in one direction, causing a continuous flow of current, 
 the force of this current depending also upon the strength of 
 the magnetic field. 2 2 Def. 40. 
 
 The Magneto- 168. Instruments made on this principle are called 
 
 Machine. magneto- or dynamo-electric machines. 
 
 FIG. 41. MAGNETO- OR DYNAMO-ELECTRIC MACHINE. 
 
 Fig. 41 explains the principle on which they act, in which, 
 for simplicity, the arrangement is confined to one permanent 
 magnet, in the form of a horse-shoe with its poles at N and s, 
 and a pair of coils whose soft iron cores A and B are con- 
 nected to a cross-bar of soft iron, capable of motion round 
 the axis o in the direction shown by the arrows, and so 
 pivoted (by means of a shaft running through o) that the 
 lower ends of the cores A and B rotate over the poles N and 
 s, close to, but not touching them. 
 
 The above figure represents the axis of the coils as lying 
 across that of the permanent magnet, the cores A and B 
 being equidistant from the poles N and s, so that the latter 
 have no influence on the former. 
 
 Now suppose the pair of coils to be turned to the right 
 till A is over N, when B must also be over s. 
 
 During this quarter-revolution magnetism is induced in 
 both the cores A and B, which goes on increasing until they 
 stand over the poles, when their magnetism is at its maximum. 
 
io8 
 
 The 
 Commutator. 
 
 MANUAL OF TELEGRAPHY. 
 
 This increase of magnetism in the cores forms an extra 
 current in the coils which surround them. 
 
 Again, continuing the motion in the direction of the 
 arrows, and observing the action of the core A (that of B is 
 precisely similar), as A recedes from N the magnetism of the 
 core decreases until it becomes nil again when midway be- 
 tween N and 5 ; continuing the motion towards s, however, 
 the magnetism of A begins to increase again till it stands 
 over s, but as the magnetism it now acquires is opposite in 
 kind to that with which it started from AT, its effect corre- 
 sponds to a decrease of its original magnetism from maxi- 
 mum + to maximum , thus acting in one direction ; its 
 motion, therefore, from N to 5 produces an extra current in 
 the coils without any reversal, but opposite in direction to 
 the extra current formed by the rotation of the coil in the 
 direction from s to TV, viz. from to +. 
 
 Thus, for every complete revolution of the pair of coils 
 two currents are formed, opposite in direction ; one while 
 the magnetism of the coils is increasing, i.e. while they are 
 approaching the poles of the permanent magnet, the other 
 while their magnetism is decreasing, i.e. while they are re- 
 ceding from the poles. 
 
 169. The commutator consists of two metal pieces 
 m and , fitted round the shaft on which the coils revolve, as 
 represented in section in fig. 42 ; these pieces being insu- 
 lated from one another by pieces of ebonite, e, e f , and from 
 the shaft by an ebonite ring between it and them. The 
 extremities of the coils are joined to m and n respectively. 
 
 FIG. 42. THE COMMUTATOR. 
 
 The above figure represents the position of the commu- 
 tator when the coils lie in the position of no current as they 
 are shown in fig. 41. 
 
 In this position it will be observed that the two steel 
 springs s s' which are connected with the fixed terminals 
 
Magneto- 
 electric 
 Currents of 
 High Potential. 
 
 Insulator and 
 Joint Detector. 
 
 MAGNETO-ELECTRIC INSTRUMENTS. 
 
 L and E of the instrument press against the ebonite pieces 
 e and e', the ends of the coils being thus insulated. 
 
 When the coils are turned from N to s in the direction 
 of the arrows (fig. 41), i.e. when the magnetism of the core 
 is increasing, m is connected to L, and n to E (fig. 42) ; but 
 by the other half-revolution, when the magnetism is decreas- 
 ing, n is connected to L, and m to E ; but as these two half- 
 revolutions produce two currents in opposite directions, it is 
 evident that by their alternate connection to the terminals 
 L and E their effect on the circuit outside these terminals is 
 that of a current in one direction. 
 
 170. By means of the magneto- electric machine currents 
 of very high potential 1 can be produced. 
 
 Its magnetic field can be made very powerful by the use 
 of a large number of strong compound magnets ; and the 
 rotatory speed of the coils can be rendered extremely rapid 
 by the use of steam power. 
 
 As such it is employed for generating powerful currents 
 for the production of the electric light and various purposes. 
 
 171. A small and portable form of magneto-electric 
 machine, turned by hand, is applied to great practical use in 
 Schwendler's insulator and joint detector. 
 
 The current, which is produced by even slow rotation of 
 the handle, is sufficient to produce severe shocks. 
 
 It is also such that when passed through a line insulator 
 offering some millions of ohms' resistance, it can be detected 
 by the shock the tester receives when placed himself in the 
 circuit A key, which is fitted up as part of the connections 
 of the instrument, and which when closed forms part of the 
 conducting circuit, affords him the means of doing this. 
 
 The key has two platinum-tipped knobs, one fixed, the 
 other movable. 
 
 By placing a finger on each knob, and pressing the 
 movable one, the key circuit is broken, and the current 
 passes through the fingers of the tester. 
 
 It is estimated that the resistance of an insulator which 
 would allow of a current being thus felt is less than a 
 megohm ; but that if the finger on the fixed knob be re- 
 placed by the tongue, the current can be detected, by taste, 
 through resistances up to 8 megohms. 2 
 
 To test the conductivity of a joint, the portion of the 
 circuit in which the insulator is placed is bridged over by a 
 
 109 
 
 Def. 3. 
 
 - The standard 
 of minimum 
 resistance for a 
 single insulator 
 is 2,000 meg- 
 ohms. 
 
no 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 Wheatstone's 
 ABC Instru- 
 ment. 
 
 direct circuit, by means of a plug, this direct circuit being 
 broken by the key as before. 
 
 The terminals of the coil are, however, also connected, 
 by insulated wires, to each end of the joint to be tested, 
 which thus forms a shunt or derived circuit ; l so that if the 
 joint is perfect, i.e. offers no resistance, no appreciable cur- 
 rent passes through the direct path. 
 
 If the resistance of the joint is over 5 ohms, the current 
 in the direct path is strong enough to be tasted on opening 
 the key contact ; and if over 200 ohms, the current can be 
 felt by the fingers. 
 
 172. In the electro-magnetic machine described in para. 
 170, the coils are represented as movable round the poles 
 of the permanent magnet ; it is evident, however, that if 
 each pair of coils with its cores be fixed to a pole of the 
 permanent magnet, an alteration of the magnetism of the 
 cores and the consequent production of extra currents in the 
 coils can be effected by causing a soft iron armature to 
 rotate instead of the coils themselves, with the gain of re- 
 ducing the friction and simplifying the connections between 
 the wires and the terminals of the instrument. 
 
 This plan is adopted in Wheatstone's ABC instru- 
 ment, in which the reversed currents thus produced are 
 applied to telegraph purposes, each reversal being made to 
 cause the armature of an electro-magnet at the distant end 
 of the line to vibrate, thus turning an escapement wheel 
 with an index needle attached. 
 
 It is found that the extra currents induced in a pair of 
 coils when the armature approaches the cores are not so 
 strong as those induced when it recedes from them ; this 
 inequality is remedied in Wheatstone's instrument by the 
 use of two pairs of coils, the ends of the armature thus being 
 made to approach one pole while it recedes from the one 
 opposite to it. 
 
 The breadth of the armature covers the space between 
 two neighbouring cores. Four reversals are effected by each 
 revolution of the armature. 
 
 The Wheatstone's ABC instrument consists of two 
 essential parts, the transmitter and the receiver. 2 
 
 The transmitter, which is inclosed in a wooden case, 
 consists of a magneto-electric machine, as described above, 
 but without a commutator, currents being formed by uni- 
 
 1 Def. 2 , and 
 
 157- 
 
 2 Fig. 57, 
 J 97- 
 
MAGNETO-ELECTRIC INSTRUMENTS. IIr 
 
 formly turning a handle which is in connection with the axis SECTION 
 of the armature. A . C ' _. 
 
 Connected to the same axis is a metal pointer, which 
 revolves outside the upper surface of the box, on which is 
 fixed a circular dial, divided into 30 parts, containing the 
 26 letters of the alphabet, 3 signs of punctuation, and an 
 asterisk ; one of which divisions is passed over by the metal 
 pointer for each current produced, i.e. at each quarter-revo- 
 lution of the armature. l i Fig. 57, 
 
 Round the dial are 30 metal keys, the depression of any J 97- 
 one of which stops the pointer at the letter or sign opposite 
 the depressed key, and at the same time cuts off the currents 
 immediately the pointer arrives at that spot, where it remains 
 until another key is depressed. 
 
 Underneath the circular row of keys is an endless chain, 
 which is forced out into a loop by a projection under the key 
 which is depressed ; the depression of any other key, how- 
 ever, at another point in the chain tightens it and pushes up 
 the key which was put down previously, and the pointer 
 moves on to the key last depressed. 
 
 The receiver also carries a dial marked to correspond with 
 that of the transmitter, and a fine pointer which rotates over 
 the dial ; its axis being that of a small escapement wheel, 
 moved by the backward and forward attractions of the 
 tongue of an electro-magnet under the influence of each 
 current sent by the transmitter. 
 
 Thus, if the pointers of both transmitter and receiver be 
 at the asterisk, which represents the zero of the dials, one 
 revolution of the handle which, as explained before, produces 
 four currents will turn the pointer through four letters, e.g. 
 to the letter ' d: 
 
 Suppose this to be the case, or, which is the same thing, 
 that the key opposite d is depressed, by which the currents 
 are cut off from the line terminal on the arrival of the pointer 
 at d\ and suppose the terminals of the electro-magnet of the 
 receiver to be at the same time joined to the terminals of 
 the transmitter, the four currents which caused the pointer 
 of the transmitter to traverse four divisions of the dial (up 
 to the letter d} will also produce four attractions of the arma- 
 ture of the receiver, causing its pointer also to move through 
 four spaces on the dial, stopping at the letter d. 
 
 Thus the depression of any key of the transmitter causes 
 
I T2 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 C. 
 
 The 
 Telephone. 
 
 the pointer of the receiver to stand at the corresponding 
 letter or sign, affording a means of telegraphing words and 
 sentences. 
 
 The receiver is provided with a switch, by means of 
 which the circuit of an alarum can be added to that of the 
 receiver when required for calling attention. 
 
 The mode of connecting up these instruments is shown 
 in para. 197, Sec. D, on 'circuits,' 
 
 172 (a). A description of the instruments in use in the 
 Indian Telegraph Department would be incomplete without 
 mention of the telephone and the apparatus connected there- 
 with. 
 
 The telephone itself is an instrument by which sounds 
 are reproduced, on the principle that sound is the result of 
 vibration ; also the converse, viz. that just as the pitch of 
 any note is dependent on the number of vibrations which 
 take place in a given time (the greater the number of vibra- 
 tions the higher the note, and vice versa), so also, by causing 
 the same frequency of vibration the identical note will be the 
 result. ! 
 
 The instrument consists of a small electro-magnet e (fig. 
 42 a), the soft iron core of which is connected with the end 
 of a steel permanent magnet jvs. 
 
 FIG. 42 (a). 
 
 A thin circular disc, z> t of soft iron, is placed opposite the 
 end of the core of the electro -magnet, being held in this 
 position by means of the shoulder of a mouthpiece, as re- 
 ferred to below, so as to be capable of vibrating under the 
 influence of any pressure of the air in the direction of the 
 arrow. 
 
 The instrument, as described in principle in the above 
 figure, is inclosed in a light cylindrical case of ebonite or 
 wood, as represented by dotted lines in fig. 42 (#), termi- 
 nating in a mouthpiece, the object of which is to concentrate 
 upon the disc D any sound directed against it. At the other 
 end of the case are two terminal screws, s, s, to which the 
 ends of the coil are led. 
 
 1 The intensity 
 or loudness of 
 the sound is 
 determined by 
 the amplitude 
 of the vibra- 
 tions ; the 
 timbre or cha- 
 racter by the 
 harmonics pro- 
 duced, these 
 being notes, 
 the frequency 
 of vibration of 
 which are exact 
 multiples of 
 the frequency 
 of vibration of 
 the funda- 
 mental note. 
 
MAGNETO-ELECTRIC INSTRUMENTS. I 
 
 To explain the principle of the instrument, its action SECTION 
 both as a transmitter and receiver of sound will be here de- ._ / 
 scribed, though, for reasons explained hereafter, its use is 
 almost entirely confined to receiving. 
 
 First, as a sending instrument. In its position of rest the 
 soft iron core of the electro-magnet is magnetised by induc- 
 tion from the permanent magnet w's, and itself induces 
 magnetism in the soft iron disc D. Every vibration of the 
 air caused by speaking in front of D is communicated to the 
 disc itself, each movement of which, in the direction of the 
 arrow (fig. 42, a\ increases the magnetism of the core, and 
 each return of the disc to its position of rest, by reason of 
 its own elasticity, causes a corresponding decrease in the 
 magnetism of the core. 
 
 Now it has been already explained } that every alteration l 166. 
 in the magnetism of the core of an electro-magnet is attended 
 by a current in the coil which surrounds it, the direction of 
 which is reversed according as the magnetism of the core 
 is increased or decreased, its E.M.F. being proportional to 
 the variations of magnetic force, and consequently to the 
 variations of pressure against the disc D. 
 
 Thus every vibration of the disc creates a current in the 
 coil, the strength of which is proportional to the intensity of 
 the vibration. Now suppose the two ends of the coil e to be 
 connected with the terminals of another similar telephone, 
 which in this case is made the receiving instrument, it is evi- 
 dent that every current produced in the coil e, corresponding 
 to the vibrations of the disc z>, is communicated to the coil 
 of the receiving telephone, the currents in which react upon 
 the magnetism of the core of the electro-magnet of the re- 
 ceiver, causing movements of the disc (corresponding to the 
 vibrations of the sending disc z>), which again vibrate the air 
 in front of the receiving disc, reproducing the sounds which 
 originally actuated D. 
 
 This instrument is extremely sensitive as a receiver, re- 
 vealing sounds which the ear, unaided, is not capable of 
 detecting ; the electromotive forces produced in the coil, 
 however, under the influence of the small vibrating disc, are 
 so small that it lacks the power required for perfect sending. 
 
 172 (b). This desideratum in the telephone as a trans- 
 mitter led to the employment of special sending instruments 
 termed microphones, in which increase of E.M.F. is gained 
 
 i 
 
MANUAL OF TELEGRAPHY. 
 
 by applying a permanent galvanic current to the circuit, the 
 resistance of which, being made to vary with each vibration 
 of the sending disc, a corresponding variation in the strength 
 of the battery current in the coils of the receiver is effected, 
 which thus causes the variations of the receiving telephone 
 to correspond with those of the transmitter. 
 
 Fig. 42 (b} represents in its simplest form the general 
 principle on which a microphone, M, is used as a transmitter 
 with a telephone, r, as the receiver. 
 
 Line 
 
 \Receiver\ 
 
 M 
 
 (Transmittn 
 
 FIG. 42 (6). 
 
 B is the microphone battery, which sends a permanent 
 current through the circuit including the line and the coils 
 of the receiving telephone T, causing the disc D to be 
 attracted to the core with a certain force, which is constant 
 so long as the strength of the current remains the same. 
 
 That part of the microphone circuit, however, between 
 the points L' and E' is so constituted that with every vibra- 
 tion of the disc D' the strength of the battery current is 
 altered. This is effected by the insertion of one or more 
 carbon pencils, the pointed ends of which, /, /, rest loosely 
 in sockets fixed to the back of the thin disc D', which is 
 made of some light resonant wood, 1 against which the voice 
 or sound to be transmitted is directed, this being concen- 
 trated upon the disc by means of a mouthpiece. 
 
 Every vibration of the air in front of the disc D' is thus 
 communicated to the carbon conductors, the resistance of 
 which varies in proportion to the intensity of each vibration, 
 which again, in accordance with Ohm's law, produces a cor- 
 responding change in the strength of current in the coils of 
 
 1 Deal is the 
 best, this wood 
 being chosen 
 for the sound- 
 ing boards of 
 pianos &c. on 
 account of the 
 readiness with 
 which it re- 
 sponds to 
 vibrations. 
 
MAGNETO-ELECTRIC INSTRUMENTS. 
 
 the receiving telephone, reacting upon the disc of that in- 
 strument and causing it to vibrate the air in front of the disc 
 D, and to reproduce the sounds which originally vibrated the 
 transmitting disc D' . 
 
 In some forms of transmitter, such as the ' Gower ' and 
 'Crossley,' an induction coil is inserted in the microphone 
 circuit ; the battery poles being closed through the primary 
 coil and carbon conductor the secondary coil being joined 
 up between the line and earth, the telephone being included 
 in the latter circuit. 
 
 Any change in the strength of the current which, in this 
 case, flows through the primary coil instead of through the 
 line, produces an extra current in the secondary coil, 1 tra- 
 versing the line and coils of the receiver which thus responds 
 to the vibrations of the transmitter as explained above. 
 
 In Johnston's form of transmitter a direct current is 
 used ; but the microphone M (fig. 42, c\ instead of forming 
 
 Line 
 
 SECTION 
 C. 
 
 FIG. 42 (4 
 
 Outgoing Microphone Circuit 
 
 Contacts 2 and 3 open ; i and 4 closed 
 
 and s, s connected through c. 
 Incoming Telephone Ditto. 
 
 Incoming Bell ,, i, 3 and s, s open ; 2 and 4 closed. 
 
 Outgoing Bell i. 4 and s, s open ; 3 and 2 closed. 
 
 [Bell Circuits dotted.] 
 
 part of the line circuit, is inserted as a shunt to the trans- 
 mitting battery z?, the poles of which are respectively con- 
 
 i 2 
 
 1 118, and 
 Law 20, 
 App. A. 
 
Il6 MANUAL OF TELEGRAPHY. 
 
 nected to either end of the carbons ; so that when the 
 instrument is at rest nearly the whole of the battery current 
 is short-circuited through the low resistance of the carbon 
 transmitter ; but when the latter, under the influence of the 
 voice, vibrates with the disc (to which it is attached), corre- 
 sponding variations occur in the strength of the current 
 flowing out through the line, these being greater by reason 
 of the microphone being in a derived circuit, than they 
 would be if it were in the direct circuit of the line. 
 
 An alarum A or other form of electric bell is generally 
 used in connection with the telephone T for calling atten- 
 tion when the talking apparatus is not being used. By 
 means of a switch x v either the telephone or the bell is 
 placed in circuit, the outer arm of the switch Y being 
 arranged as a receptacle for the telephone, which, when not 
 in use, is always placed in it. The weight of the telephone 
 draws down the arm, thereby closing contact 2, which places 
 the bell in circuit, in a position to call attention when re- 
 quired. When the telephone is removed from the hook, 
 contact 2 is opened and -/ is closed, and at the same time 
 the two springs s, s are connected to one another by the 
 metallic contact c, which is insulated from the lever of the 
 switch 
 
 A separate line battery (either galvanic or magneto- 
 electric) for ringing the bell can be applied to the line circuit 
 by means of a key, usually in the form of a push button p 
 fitted up as part of the transmitting apparatus. 
 
 In Johnston's arrangement the same battery (generally 
 six Minotti cells in series for short lines) is used for the 
 microphone and the alarum, the latter being included in a 
 local circuit l and worked by a relay ;?, as shown by the i 181. 
 dotted line (fig. 42, c). In most arrangements Bell's tele- 
 phone is used as the receiving instrument. 
 
 For exchanging communication between two stations 
 connected by a telegraph line, each station is fitted with the 
 complete apparatus referred to above. 
 
SECTION D. 
 
 TELEGRAPHIC CIRCUITS 
 
 173-197. VARIOUS CIRCUITS OF SINGLE TELEGRAPHY 
 198-210. VARIOUS CIRCUITS OF DUPLEX TELEGRAPHY 
 211-213. CIRCUITS CONTAINING COVERED WIRES 
 
TELEGRAPHIC CIRCUITS. 
 
 Battery Circuit. 173. A circuit has been described 1 as consisting of a 
 battery and everything between its two poles which takes a 
 part in conducting the current, as represented in the follow- 
 ing figure, in which the direction of the current is shown by 
 arrows : 
 
 Zinc 
 FIG. 43. ELECTRICAL CIRCUIT. 
 
 Def. 13. 
 
 Internal and 
 External 
 Circuit and 
 Resistance. 
 
 Conventional 
 Symbols for 
 the various 
 portions of 
 Telegraphic 
 Circuits. 
 
 It will be observed that the current starts from the gene- 
 rating or + plate, 2 whence it is conducted by the liquid to 2 i (note), 
 the negative plate ; thence to the -f pole, from which it is 
 conducted by a wire to the pole, which is the outer 
 extremity of the + plate from which the current started, 
 thus traversing a complete round or circuit as it is called. 
 
 174. That portion of the circuit which is between the 
 plates is called the internal circuit, and its resistance the 
 internal resistance of the battery ; the portion between 
 the poles is called the external circuit, and its resistance 
 the external resistance of the circuit. 
 
 175. The following paragraphs describe various forms of 
 telegraphic circuits, the component parts of which are, for 
 the sake of simplicity, represented symbolically. 
 
 As conventional symbols are universally adopted, their 
 
I2O 
 
 MANUAL OF TELEGRAPHY. 
 
 Wire Connec- 
 tion. 
 
 Branch Con- 
 nections. 
 
 use is recommended in taking down diagrams of any circuits 
 whatever. 
 
 The following explanations will render them intelligible : 
 (i) A straight (or curved) line repre- 
 
 " sents a wire connecting two points of a 
 
 circuit. 
 
 (2) The jpoint at which any branch 
 connection is made is indicated by a dot. 
 
 Cross Wires. 
 
 (3) In order to trace wires which cross 
 one another, one of them is looped at the 
 point where they cross. 
 
 Direction of 
 Current. 
 
 Resistance 
 Coils. 
 
 Battery Cell. 
 
 Earth Plate. 
 
 (4) The direction of currents is shown 
 by arrows. 
 
 (5) A sinuous line denotes resistance 
 coils l inserted in the circuit. 
 
 (6) Represents a battery cell, its + 
 
 and poles 2 being indicated. Any 2 i (note), 
 number of such cells can be drawn in 
 series or parallel 3 to represent batteries 
 so connected. 
 
 (7) Represents an earth plate. 
 
 160. 
 
 Fig. 46, 
 181. 
 
 Polarised 
 Relay. 
 
 Sounder (Non- 
 polarised). 
 
 Galvanoscope 
 or Galvano- 
 meter. 
 
 -o 
 
 (8) A polarised relay. 4 In rough dia- * 59. 
 grams the nuts and shoes can be omitted. 
 
 (9) Represents a sounder. The spring 
 shown in the figure explains that the 
 sounder indicated must be a non-polarised 
 
 one/ 
 
 (10) Represents any form of galvano- 
 scope or galvanometer. 6 
 
 5 73-84- 
 
 142-156. 
 
 Differential 
 Galvanometer. 
 
 (n) A differential galvanometer. 7 
 
 165. 
 
Coils repre- 
 senting Shunts, 
 &c. 
 
 Electric Bell 
 or Alarum. 
 
 TELEGRAPHIC CIRCUITS. 
 
 121 
 
 (12) Represents any coil of wire ; its 
 use, whether as a shunt, a magneto-induc- 
 tion coil, or any other purpose, being ex- 
 pressed by a referring letter. 1 l 157 and 
 
 (13) Represents an electric bell. 2 
 
 98-103. 
 
 Signalling 
 Key. 
 
 Constant 
 
 Resistance 
 
 Key. 
 
 (14) Signifies an ordinary single cur- 
 rent signalling key. 3 5 J0 ^ 
 
 (15) A constant resistance key. 4 4 108. 
 
 Condenser. 
 
 (16) A condenser. 
 
 5 "6. 
 
 K.M. Shunt. 
 
 Lightning 
 Discharger. 
 
 Switch. 
 
 Commutator. 
 
 Current 
 Reverser. 
 
 Current 
 Strength the 
 same at all 
 points of a 
 Direct Circuit. 
 
 (17) An electro-magnetic shunt. 6 6 122. 
 
 (18) A Siemens' lightning discharger, 
 the upper plate forming part of the line 
 circuit, the lower being joined to earth. 7 7 133. 
 
 (19) Represents the s. T. D. switch, the 
 insertion of a plug being indicated by a 
 cross, as in the figure. 8 
 
 (20) Represents the bar commutator, 
 in which the cross signifies the point at 
 which the upper and lower bars are joined 
 by the connecting screw. Only so many 
 bars as are necessary for the circuit under 
 description need be shown. 9 
 
 /*^r (21) Represents a current reverser, 
 
 ~T+~> crosses indicating the insertion of plugs. 10 
 
 176. The external part of the circuit represented in 
 fig. 43 n may be supposed to be composed of a telegraph u 
 line, one end of which is connected to the -f pole of the 
 
 Z 3 6 - 
 
 140. 
 
 138. 
 
 173. 
 
122 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 D. 
 
 Earth Circuit. 
 
 battery, the other end being brought back to the pole, 
 the flow of current through the line in the direction of the 
 arrow being caused by the completion of a conducting 
 circuit. 
 
 If, however, this circuit were broken at any point, the 
 current would cease to flow, and the + and electricities 
 of the battery, being deprived of a conducting medium 
 through which to unite, would accumulate at their respective 
 poles. 
 
 When the poles are joined by a conductor these electri- 
 cities reunite, resulting in the flow of a current from the + 
 to the pole, the strength of which is the same in every 
 point of such a circuit, independently of its resistance, pro- 
 vided the insulation of the circuit is uniform. 
 
 177. The earth, being a conductor of electricity, may 
 form part of the circuit between the battery poles, as repre- 
 sented in fig. 44. 
 
 FIG. 44. EARTH CIRCUIT. 
 
 This fact is of the greatest importance in telegraphy, as 
 it renders it unnecessary to connect both ends of a telegraph 
 wire to the battery poles to form a circuit, as in fig. 43, the 
 current being afforded a means of returning to the pole 
 of the battery through the ground, in the direction shown 
 by the arrows in fig. 44. 
 
 Earth Plates. Ij8. Connection with the ground is made by means of 
 
 earth plates, which consist of sheets of copper J of an 
 inch in thickness, and measuring about 4x3 feet, which are 
 buried vertically in moist ground. 
 
 Although the specific resistance of earth is much greater 
 than that of iron, its actual resistance can be reduced to 
 almost nothing by the use of large plates, thus making the 
 sectional area of the earth circuit very great ; * for it may be 
 considered as a column of earth, of which the plates at each 
 end represent the section. 
 
 i $ee Law 
 ZI (*) A PP- A - 
 
TELEGRAPHIC CIRCUITS. 
 
 12 
 
 Open and 
 Closed Circuit. 
 
 Open Circuit 
 or S Working. 
 
 The action of the earth in thus taking the place of a 
 return wire effects the important result of reducing the re- 
 sistance of the complete circuit to about one-half what it 
 would be if it were all of wire. 
 
 It is. of course, necessary that the plates should make 
 as good connection with the ground as possible, which is 
 effected by using a metal whose surface keeps clean. 
 
 Conducting wires are soldered to the plate ; they are of 
 insulated wire, to prevent electrolytic action at points of 
 leakage and consequent corrosion of the wire. 1 
 
 SECTION 
 D. 
 
 The conducting nature of the soil itself affects the readi- 2 39 
 ness with which the current is diffused ; and when it is 
 necessary to lay an earth plate in badly conducting soil, it 
 is advisable to lead the conducting wires underground a 
 few yards from the building, and there to sink the earth 
 plate vertically in a pit filled with charcoal and old scrap 
 iron up to the edge of the plate, measures being taken 
 to keep the soil above it always moist, and to lead, if pos- 
 sible, all surface drainage and discharge from water-pipes 
 to it. 
 
 179. The circuit represented in fig. 44 is called a closed 
 circuit, as the poles of the battery are closed through a 
 conductor, the result of which is that a continuous current 
 is caused to flow in the direction shown by the arrows. 
 
 A circuit in which the poles of a battery are not thus 
 permanently connected, but closed by means of a key to 
 form signals, is called an open circuit. 
 
 Telegraph circuits are worked on both the open and 
 closed principles, as explained in the following paragraphs. 
 
 180. The open circuit system is that generally adopted 
 in departmental offices. 
 
 Fig. 45 represents the essential parts of the circuit neces- 
 sary to explain its working. 
 
 The figure represents two stations, A and B, with their 
 keys in the position of rest, the circuit thus being open. 
 
 Suppose A to send a signal to B by closing contact 1 of 
 his key. Contact 2 is opened, and /i's battery circuit is 
 closed through the line contact 2 of #'s key relay, and 
 earth ; and the current flows in the direction shown by the 
 arrows, forming a signal on &'s relay at each depression of 
 -4's key. 
 
 Thus signals are exchanged, and stations as far apart as 
 
 1 Def. 26, and 
 
124 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 800 miles and upwards are able to communicate with one 
 _. another. 
 
 Local Circuit. 
 
 Lint 
 
 ,1 
 
 
 
 FIG. 45. MORSE OPEN CIRCUIT WORKING WITH RELAYS. 
 
 l8l. These signals, however, are not read off the relays, 
 for reasons already explained ; l the sounder, which performs 
 this office, being joined up with the relay and a local battery 
 in what is called the local circuit, as separately shown for 
 clearness' sake in fig. 46, which represents two relays and 
 sounders joined up in the circuit of one local battery, con- 
 sisting of two series of four cells each, joined parallel. 2 
 
 42. 
 
 FIG. 46. LOCAL CIRCUIT. 
 
 It will be seen from the above that every attraction of 
 the relay tongue against its metal working-contact, under 
 the influence of a current from the line, closes the circuit of 
 the local battery, which can be adjusted to any required 
 strength in order to work the sounder, which the current, 
 arriving through a long line, reduced in strength by the 
 resistance and leakage thereof, would fail to do. 
 
 The sounder terminals can be shunted by a coil of wire 
 to prevent a spark being formed at the opening of the relay 
 contacts. 3 
 
 3 157, and 
 marginal note. 
 
Office Circuit 
 between Line 
 and Signalling 
 Instruments. 
 
 TELEGRAPHIC CIRCUITS. 
 
 182. The apparatus included in that portion of the office 
 circuit which is between the line and the signalling key, not 
 directly affecting the principle of open circuit or s working 
 and the mode of connecting it up, is, for the sake of clear- 
 ness, separately represented in fig. 47, and it is important 
 that the same order with regard to the relative position of 
 the various instruments be followed in all cases in joining 
 them up. 
 
 Line 
 
 125 
 
 FIG. 47. S OFFICE CONNECTIONS. 
 
 The lightning discharger is placed next the line in order 
 to protect all the apparatus from the effects of lightning ; * * 133- 
 and it will be observed that the lower plate of the discharger 
 is connected to a special earth plate of its own, distinct from 
 the battery and instrument earth. 
 
 183. It was shown in para. 180 that there is a limit 
 to the length of line through which two stations can com- 
 municate with each other direct, owing to the reduction 
 of the strength of the original current by the resistance 
 opposed to it, as well as through leakage at all the points 
 of support, for which the addition of battery power fails to 
 compensate. 
 
 It then becomes necessary that intermediate stations 
 should either repeat i.e. receive the distant signals in open 
 circuit, 2 which is called ' opening out,' and re-transmit them 2 g ee fj g> 43> 
 by hand towards their destination or translate them by an 
 
I2 6 MANUAL OF TELEGRAPHY. 
 
 arrangement which shall cause received signals to be auto- 
 matically repeated by the instruments themselves. 
 
 The latter object is effected by means of switches in the 
 intermediate office, plugs being inserted between the L and 
 T terminals of the switches attached to the lines to be 
 joined. 
 
 Lines thus connected are said to be joined in T. 
 
 Fig. 48 illustrates the circuit of the intermediate T office 
 and the course of the incoming and outgoing currents ; and 
 it will be observed that the incoming current from station A, 
 in working the relay, causes each attraction of its tongue to 
 be the means of picking up a fresh battery current and 
 automatically sending it on to B by means of the sounder in 
 its local circuit, which acts as a key. 
 
 Current frvmA 
 
 ToJB 
 
 FIG. 48. TRANSLATION CIRCUIT (of an Intermediate Station 
 joined in T). 
 
 To render the Translation Circuit as clear as possible, only those Connections 
 are shown which are traversed by the Line Currents. 
 
 The relay and sounder on the A side are represented 
 above as actually working under the influence of a current 
 from A, which enters through T of the left-hand switch into 
 the T pillar of the right-hand sounder (at rest), thence 
 through the lever and pillar 2 to L of the left-hand relay, the 
 coils of which it traverses and goes to earth. 
 
 In doing so, however, it works the left-hand sounder, 
 the lower pillar (1) of which is connected to the copper pole 
 of the right-hand battery, whose current therefore passes 
 through 1 into the attracted lever, thence into the pillar T, 
 
Circuit of T 
 working with 
 Discharging 
 Arrangements. 
 
 TELEGRAPHIC CIRCUITS. 
 
 which is joined to T of the right-hand switch, and thus pro- 
 ceeds through the line to station B. 
 
 To secure a good contact between the lever and battery 
 pillar (/) of a translating sounder, both are tipped with 
 platinum, and the former is provided with a spring which 
 prolongs the contact. 
 
 184. Fig. 49 explains how discharging arrangements may 
 be connected up in the circuit of translation working. 
 
 127 
 
 FIG. 49. TRANSLATION WITH DISCHARGING ARRANGEMENTS. 
 
 R and R' are Receiving Relays. 
 D ,, D' ,, Discharging Relays. 
 E ,, E' ,, Line Batteries. 
 
 In this figure, as in fig. 48, the Local Batteries &c. are not shown, for the 
 sake of clearness. 
 
 D and G 
 Working 
 Circuits. 
 
 The line current from A is shown in the above (as in 
 fig. 48) to be actually working the left-hand relay and 
 sounder, causing contact 1 of the latter to be closed, and 
 thus placing the line to B in connection with its own battery. 
 
 It will now be observed that the coils of the discharging 
 relay D' are inserted in this battery circuit, the current of 
 which causes the tongue of discharger D' to be drawn against 
 its working contact, thus connecting the line (on the B side) 
 to earth for a moment through contact 2, after each sent 
 signal, and discharging it direct instead of through the coils 
 of the receiving relay x^ 1 
 
 185. The intermediate office, whose translation circuit is 
 represented in figs. 48 and 49, may be joined over direct, or 
 in z>, as it is called, by connecting the lines to A and B 
 directly with one another. 
 
 126. 
 
128 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 D. 
 
 P Working 
 Circuit. 
 
 This is done by means of the switches shown in the 
 diagrams, the D terminals of which may be joined to each 
 other by a short length of covered wire. 
 
 By removing the plugs from T and inserting them be- 
 tween L and D of both switches, the office is joined in D. 
 
 A galvanoscope 1 may be inserted in this wire for the l 142- 
 purpose of observing passing signals, in which case the office 
 is said to \>t joined in G. 
 
 G galvanoscopes are always provided with a short circuit 
 plug, so that their resistance is only introduced into the 
 circuit when they are being used. 
 
 186. To obviate the necessity for the continuous and 
 close watching required to observe and attend to passing 
 signals when indicated only by a galvanoscope needle, the 
 system of parallel relay (or P) working, as it is called, 
 has been introduced, by which such signals can be read by 
 sound. . 
 
 Fig. 50 represents the circuit of an intermed ate station 
 joined in /, from which it will be observed that the E 
 
 FIG. 50. PARALLEL POLARISED RELAY OR P WORKING. 
 
 The P Switch described in 'para. 137 affords the means of joining from 
 S to P. It is omitted from, this diagram in order that the actual P circuit 
 may be more clearly understood. 
 
 terminal of the left-hand relay R is connected to the L 
 terminal of the right x l , and vice versa, so that a current 
 entering from either line passes through a divided circuit 2 
 between the middle contacts of the two keys ; but from the 
 mode of joining up the relays, a current from A works R 
 only, pressing the tongue of /e 1 harder against its rest stop. 
 Similarly a current from B works y? 1 only ; thus each relay 
 responds to the current of its own line. 
 
 According to the law of derived circuits 3 the strength of 3 Law 
 
 Def. 21. 
 
 current in each path is inversely proportional to the resist- 
 
 App. A. 
 
TELEGRAPHIC 
 
 ance of each branch ; hence the two relay^da^^^be^f SECTION 
 equal resistance, in order that one may not divert more of . ^ . 
 the current than the other. 
 
 Another important result of using relays of equal resist- 
 ance is that the extra currents formed in their coils will be 
 equal, and thus counteract one another. 1 i n 9 , and 
 
 To obviate the necessity for readjustment on changing ^ 2 a ^ p 21 ^ d 
 from s to P work, by which the resistance of the circuit is 
 considerably altered (the working current being manifestly 
 stronger in the former than in the latter circuit), equating 
 batteries e and e' are inserted in the derived circuits, so as 
 to add the necessary strength for p working to the received 
 current from whichever side it comes, these batteries being 
 so placed that their current is in the same direction as the 
 working current in either branch ; but, being equal in 
 strength (generally one cell each), and opposed to one 
 another in direction, each cancels the other's influence 
 on the line. 
 
 To facilitate adjustment a horse-shoe directing magnet 
 is placed on each relay cover, by turning which the tongue 
 can be delicately adjusted as near to the neutral line be- 
 tween the shoes as possible, so that the force required to 
 drive the tongue back to its rest stop may be so small as 
 to be produced by the cessation even of the weakest line 
 current ; so that ' sticking,' a common fault in p working, 
 resulting from weakness of the line current and inefficient 
 adjustment, may be avoided. It is obvious, therefore, that 
 the strength of the received current should never be less 
 than three milli-oerstedts. 2 - 289. 
 
 187. In fig. 51 the same circuit as that shown in fig. 50 
 is reproduced with the addition of a P switch, illustrating 
 how the lines are changed from open circuit to p circuit 
 working and vice versa ; the local circuit (in dotted lines) is 
 also added, in which it will be observed an alarum is intro- 
 duced. 
 
 By inserting plugs in s s E both relays are placed in open 
 circuit, as already explained, 3 the left-hand instrument com- 3 137. 
 municating with station A and the right with B. 
 
 By removing the plug from , and having plugs in 
 s, s, P, P, the instruments are joined for parallel or P 
 working. 
 
 An alarum a is shown in the local circuit of the left-hand 
 
 K 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 D. 
 
 instrument, by means of which the intermediate office thus 
 joined over can be called in when closed for traffic. 
 
 During working hours the alarum is short-circuited by 
 means of a plug, signals passing in p being audible and suffi- 
 cient of themselves to arrest attention. 
 
 FIG. 51. CIRCUIT OF A WORKING. 
 
 T and D and Line Battery Connections are not shown in figs. 50 and 51, 
 in order that the A and P Circuits may be more distinctly traced. 
 
 When, however, the P office closes, the short circuit 
 plug is removed from a, so that the bell rings every time 
 a current from station A causes the local circuit to be 
 closed. 
 
 In order that ordinary signals passing through the p 
 circuit may not do this, the intermediate office adjusts the 
 parallel relays just so unsensitively that the ordinary working 
 currents between A and B do not affect them. 
 
 The equating batteries are therefore thrown out of circuit 
 by removing the plugs from p, p and inserting them in z, z 
 the office is then said to be joined in A (plugs in s, s, z, z) ; 
 and whenever it is necessary that it should be called in, this 
 is done by means of an increased current sufficiently strong 
 to work the relay on the A side. 
 
 A effects this by causing B to reverse his battery 1 and 
 himself giving a beat ; the left-hand relay of the inter- 
 mediate office works, closes the local circuit, and rings the 
 alarum. 
 
 1 This is best 
 done by a Cur- 
 rent Reverser 
 (see 138) 
 inserted 
 between the 
 Battery and 
 Key. 
 
Looped 
 Open Circuit 
 (Morse). 
 
 TELEGRAPHIC CIRCUITS. 
 
 In practice any terminal office, observing that an inter- 
 mediate ' A office ' is being called, immediately reverses his 
 battery and gives a beat. 
 
 188. Railway stations close to one another are often con- 
 nected in loops by the ' train ' or * block wire ' i.e. the 
 special wire used for blocking trains from station to station, 
 and worked in open circuit. 
 
 Fig. 5 2 represents such an arrangement .4 and B are the 
 terminal stations of a train wire connected by a loop con- 
 taining the intermediate station c. 
 
 l/nic 
 
 Closed Circuit 
 (Morse). 
 
 E3 
 
 FIG. 52. LOOPED OPEN CIRCUIT (MORSE). 
 
 It will be observed that when the key of any one of these 
 stations is depressed, the instruments of all the stations in 
 the loop are worked, each station containing one battery of 
 which the copper pole is to line. 
 
 The number of stations which may be joined together 
 in such a loop is limited to five, each of the intermediate 
 stations being joined up similarly to c. 
 
 They may, however, be provided with a means of joining 
 the zinc battery pole to earth in case of interruption on either 
 side. 
 
 189. The same three stations are represented in fig. 53 
 as joined up in closed circuit. 1 
 
 It will be observed here that only the terminal stations 
 are provided with batteries, that of A being joined with its 
 copper pole to line, and that of B with zinc to line and 
 copper to earth ; thus a continuous current flows from A to JB, 
 attracting the sounder armatures in each station when all 
 the keys are at rest, as they are shown in the figure. The 
 interruption of this current, by depressing any one of the 
 keys in the circuit, causes all the armatures to be released, 
 
 K2 
 
 179- 
 
132 
 
 SECTION 
 D. 
 
 MANUAL OF TELEGRAPHY. 
 
 signals being read from the upward or reversed beats of the 
 levers. 
 
 The ' main ' or ' talking wire ' that is, the wire by which 
 paid messages are sent on State railway lines is usually 
 worked in closed circuit, as many as five stations being 
 joined in one loop. 
 
 Line 
 
 Line 
 
 FIG. 53. CLOSED CIRCUIT (MORSE). 
 
 The State Railway Sounder used for both the Circuits described in paras. 
 188 and 189 will be found illustrated in fig. 7, para. 80, which explains 
 its connections. 
 
 The advantages this system has over the looped open 
 circuit, described in the foregoing paragraph, are as follows : 
 
 (1) The instrument being worked with no current, the 
 necessity for readjustment to suit the strength of various 
 received currents is avoided. 
 
 (2) By dispensing with batteries at the intermediate 
 stations, the supervision and conservancy of batteries is cen- 
 tralised at the terminal stations. 
 
 (3) The number of battery cells required is reduced to 
 \ of what is necessary for open circuit working in a loop 
 containing three instruments, \ for four instruments, and i 
 for five. 
 
 On the other hand, the disadvantages of closed circuit 
 working are : 
 
 (1) That the batteries are always at work, thus becoming 
 more rapidly exhausted than if used intermittently. 
 
 (2) The continuous action of the current tends to injure 
 the covered wires in circuit. 
 
 (3) Translation in closed circuit is complicated. 
 
 To the practical reader cases will doubtless suggest 
 themselves in which the advantages will outweigh the dis- 
 
Looped Open 
 
 TELEGRAPHIC CIRCUITS. 
 
 advantages, rendering the adoption of the closed circuit 
 system desirable. 
 
 ipo. Another form of circuit, which, on account of its 
 simplicity and inexpensiveness, has been extensively adopted 
 on railways for working ' station to station ' or ' train ' wires, 
 is the needle circuit, in which a large number of stations 
 can be looped together in open circuit ; there being, how- 
 ever, a limit to this number, as the battery of each station 
 must be strong enough to work the instruments of all the 
 other stations in the loop. 
 
 Fig. 54 represents three stations JT, F, and z so joined, 
 with needle instruments, the action of which is described in 
 paras. 96 and 97. 
 
 Each station is provided with one battery, which, in the 
 case of intermediate stations such as F, serves to work in 
 both directions. 
 
 SECTION 
 D. 
 
 Line 
 
 Zine 
 
 FIG. 54. LOOPED OPEN CIRCUIT (NEEDLE INSTRUMENTS). 
 
 The line circuit is completed through the coils and 
 handles of all the instruments in the loop, as shown by 
 fig. 54, signalling currents being switched on to the lines by 
 means of the handles, as explained in para. 97. 
 
 Intermediate stations are provided with a separate instru- 
 ment for each station with which they are in communication, 
 and further, with a means of joining to earth (as shown by 
 the dotted line, fig. 54). which, in the case of interruption 
 between v and z, enables Y still to communicate with x 
 (disconnecting the z line meanwhile). 
 
134 
 
 Reverse Cur- 
 rent Working. 
 
 MANUAL OF TELEGRAPHY. 
 
 A single-stroke bell, <V is placed in the circuit of each 
 office for calling attention. 
 
 As the needle instrument works by reversals of the battery 
 current, a copper current deflecting the needle in one direc- 
 tion, and a zinc current in the other, it can only be used for 
 open circuit working. 
 
 191. The system of reverse current or double cur- 
 rent working can be adopted on short lines in which 
 Siemens' relays or other polarised instruments are joined up 
 in open circuit, with the effect of increasing the speed of 
 signals, by rendering the receiving instruments more sensi- 
 tive than when influenced by a single current only. 
 
 It has been already explained l why it is necessary that 
 the tongue of the Siemens' relay should be given a bias to- 
 wards the rest stop, to give it a ' force of restitution ' after 
 each attraction by the current, thus introducing an obstacle 
 to the sensitiveness of the instrument. 
 
 It has been also shown 2 how a current, opposite in 
 direction to the working current, may be employed to draw 
 the tongue back to its position of rest after each signal, thus 
 providing an external force of restitution, and rendering it 
 possible to adjust the tongue in the neutral line between the 
 shoes that is, in its most sensitive position. 
 
 Further, as the strength of the working current may and 
 does undergo changes in consequence of the variation of the 
 state of the line, it is desirable that the force of restitution 
 should vary accordingly. 
 
 By using an opposite current as the force of restitution, 
 this is to a certain extent the case ; 3 whereas, in the case of 
 single current working, however much the current from the 
 line may vary, the force of restitution, which depends upon 
 the bias of the tongue, remains the same, rendering read- 
 justment necessary another defect, against which double 
 current working provides a remedy. 
 
 Fig. 55 describes the essential parts of the circuit of an 
 office connected up for reverse or double current sending 
 and receiving. 4 
 
 It will be observed that when the plug is inserted at 3 
 for sending, the zinc pole of the battery is connected directly 
 with the line ; but when signals are transmitted by the de- 
 pression of the key, copper currents are sent through the 
 
 IOT. 
 
 61-64- 
 
 65- 
 
 3 This may be 
 considered the 
 case for short 
 well-insulated 
 lines, but not 
 for long and 
 defective ones. 
 See 194. 
 
 4 The figure 
 represents the 
 operation as 
 performed with 
 an ordinary 
 key and two 
 batteries. 
 
 One battery, 
 however, may 
 be made to 
 suffice, by the 
 use of a 
 ' Double Cur- 
 rent Key,' as 
 described in 
 106 and 107. 
 
TELEGRAPHIC CIRCUITS. 
 
 line, followed by a zinc current each time the key is re- 
 leased. 
 
 These zinc currents, acting on the relay of the distant 
 station in the reverse direction to the working currents, tend 
 to draw back the relay tongue against its rest contact, thus 
 terminating each signal sharply. 
 
 135 
 
 SECTION 
 D. 
 
 Advantages of 
 Reverse Cur- 
 rent Working. 
 
 Disadvantages. 
 
 55- 
 
 For Sending 
 ,, Receiving . 
 ,, Putting Line to Earth 
 
 Plug 3. 
 
 192. Reverse current working thus presents the following 
 advantages : 
 
 (1) It remedies a defect in Siemens' relay by supply- 
 ing a force of restitution varying with the working force, 
 which increases the sensitiveness of the instrument and 
 reduces the necessity for readjustments. 
 
 (2) It improves communication by admitting of finer 
 adjustment than in single current working, and thus renders 
 receiving instruments susceptible to weaker currents. 
 
 (3) It counteracts the effects of residual magnetism in 
 the cores of receiving relays. 
 
 (4) During the pauses between signals the reverse current 
 renders the relay less sensitive to the disturbing effects of 
 earth currents, contact beats, &c. 
 
 193. On the other hand, the system is attended with the 
 following disadvantages : 
 
MANUAL OF TELEGRAPHY. 
 
 Circuits 
 worked with 
 Positive Cur- 
 rents. 
 
 Circuits 
 worked with 
 Negative 
 Currents. 
 
 (1) It is necessary to alter the circuit by means of a 
 switch, as shown in fig. 55, so that either the battery may be 
 joined to the line for sending (plug in 3), or the instrument 
 to the line for receiving (plug in i) ; but as this necessitates 
 the sending signaller's instrument being cut out of circuit, 
 and the receiver's battery cut off, the latter is deprived of 
 the means of stopping the former or of sending acknowledg- 
 ing signals, and no corrections can be called for until the 
 receiver switches his instrument off and his battery on, to 
 send, and the sender alters his circuit to receive. 
 
 (2) Translation with double currents is difficult, and 
 requires somewhat complicated apparatus : nevertheless the 
 principal duplex circuits in India are now worked on the 
 double current differential system with an intermediate 
 station in translation. 
 
 Though its advantages are great, the system has not 
 hitherto been adopted on the Indian single circuits ; the 
 E.M. shunt l having been found to effect (by its extra 
 current) the principal object of double current working, viz. 
 the acquisition of a force of restitution for the tongue of 
 the polarised receiving instrument, admitting of its being 
 adjusted in its most sensitive position. 
 
 (3) Further, on long and imperfectly insulated lines the 
 reverse currents do not exert equal force on the receiving 
 instrument, which is one of the theoretical advantages 
 claimed in favour of double current working [para. 192 (i)]. 
 
 194. This is to be accounted for by the fact that electro- 
 lytic action 2 takes place at any and every point where the 
 insulation is so imperfect as to admit of the passage of a 
 current to the ground ; a copper current causing a deposit 
 of chloride of copper and oxygen to take place at the point 
 of leakage, a zinc current causing a deposit of salt and 
 hydrogen ; the former deposit is of an insulating nature, the 
 latter the reverse ; hence the zinc current makes for itself a 
 way of escape. 
 
 The above fact explains the reason why it is customary 
 to work the departmental circuits with copper currents, i.e. 
 copper to line and zinc to earth. 
 
 195. When the telegraph circuit, however, consists of 
 covered wire, as in the case of a submarine cable, the effects 
 of this electrolytic action are manifested in a way which 
 renders it desirable that zinc currents should be used for 
 
 122. 
 
 Def. 26. 
 
TELEGRAPHIC CIRCUITS. 
 
 signalling (i.e. zinc to line and copper to earth), for the SECTION 
 
 copper deposit, alluded to above, which is formed at any . _ D ' 
 
 point where the dielectric l is not perfect, although it has i Def. 26. 
 the effect of sealing up the fault by its insulating nature, 
 nevertheless corrodes the copper conductor, gradually eating 
 
 it away until it breaks altogether. 2 2 The same 
 
 The zinc current, on the other hand, though it decreases ^served in the 
 
 the insulation of the cable, has the effect of breaking down covered wires 
 
 a fault of insulation, rendering its discovery practicable while Sections, 001 
 
 the conductor is intact and communication is maintained. especially the 
 
 . r leading wires 
 
 Circuit of Zinc igo. It is necessary that the system of double current connected to 
 
 working should not be confounded with that of the zinc ^oj^ron 
 
 sender, whereby a momentary reverse current is sent after batteries. 
 
 each signal with the object of counteracting the prolonging 
 
 effects caused by the charge of the line, or in other words, 
 
 its incomplete discharge through the receiving instrument 
 
 of the distant station. 3 s m, n 2 , 
 
 It has been shown already (paras. 125-128) how the and I21 - 
 effects of this prolongation are reduced by means of dis- 
 charging arrangements at the sending end, connecting the 
 line instantaneously to earth with the object of reducing its 
 potential to zero after each sent signal. 4 4 > e f t 3. 
 
 It is evident, however, that the potential of the line 
 would be lowered still more (viz. below zero) if it were 
 connected to the zinc pole of the sending battery instead 
 of direct to earth ; and that thus its discharge would be 
 accelerated. 
 
 This is effected in the following manner by the 'zinc 
 sender' depicted in fig. 56 : 
 
 The zinc sender, d, is a Siemens' relay of low resist- 
 ance, placed between the line battery E and the front 
 contact of the signalling key. 
 
 The tongue of d plays between two metal contacts 3 and 
 4, the former of which is joined to the L terminal of the 
 receiving relay R, the other to the zinc pole of a reverse 
 battery *, whose copper pole is joined to earth. 
 
 When signals are sent by the depression of the key, the 
 battery circuit is closed through contact / of the key and the 
 coils of the zinc sender d, causing its tongue to be attracted 
 to contact 4> 
 
 At the same time contact 2 of the key is opened. 
 
 The shunt b, attached to the terminals of the zinc sender, 
 
138 
 
 SECTION 
 D. 
 
 Circuit of 
 ABC In- 
 struments. 
 
 MANUAL OF TELEGRAPHY. 
 
 causes contact 4 to remain closed a moment after the current 
 ceases in the coils of the relay d. 1 
 
 line. 
 
 1 The action 
 of the Shunt is 
 fully explained 
 in S 126. 
 
 FIG. 56. ZINC SENDER. 
 
 Thus, at the cessation of each signal, when the key 
 is released and contact 1 broken, 2 being simultaneously 
 closed, the line is discharged through contact .2, the tongue 
 d, contact 4> and the zinc battery , before the tongue (d) 
 falls back against its rest contact 3, connecting the line to 
 the receiving relay R. 
 
 197. Fig. 5 7 represents the circuit of a line worked by 
 Wheatstone's ABC instruments. 2 
 
 e t e' represent the coils of the electro-magnets in the re- 
 ceivers, ;;/, m' the magneto-electric coils of the transmitters. 
 
 Supposing x to be sending to y, currents are generated 
 in the coils of m by turning the handle (h) of that trans- 
 mitter ; these currents passing out through e to line, and 
 thence through e' to earth at y, cause the indicators of these 
 three instruments to work together, stopping when they 
 arrive at that letter on the dial at which the key of m is 
 depressed. 
 
 When the pointer of the transmitter reaches this de- 
 pressed key the connection between the coil m and the line 
 is broken, as explained in para. 172, and no further current 
 can flow out to the line until the key is raised by the de- 
 pression of another key, when all three needles travel on to 
 the letter on the dial corresponding to the last depression, 
 and so on. 
 
 172. 
 
TELEGRAPHIC CIRCUITS. 
 
 s, s' are the ends of brass levers, by turning either of 
 which to the left a clockwork alarum is switched on to the 
 circuit of the receiver for calling attention. 
 
 When the instrument is in use for telegraphing, the 
 switches are turned to the right, as shown in the figure, in 
 which position they cut the alarum out of circuit. 
 
 Lightning dischargers, d and d' ', are inserted in the line 
 circuit, as it is most important that the instruments should 
 be protected from the effects of lightning, which would 
 readily demagnetise the delicate polarised electro-magnets 
 of the receivers. 
 
 139 
 
 ac 
 
 FIG. 57. ABC CIRCUIT. 
 
 If the needles of all the instruments in circuit do not 
 work in unison, the line and earth wires should be tempo- 
 rarily replaced by a short piece of wire connecting the 
 T and E terminals of the instrument. 
 
 If, on turning the handle of the transmitter, it be found 
 that the pointers of the transmitter and receiver thus joined 
 together in short circuit still fail to correspond, reverse the 
 connecting wires, joining the terminal A to E and L to T, 
 
 If this remedies the failure the line should be joined up 
 accordingly (i.e. A joined to E instead of to z., and z joined 
 to earth) ; but if not, and the needle of the receiver loses 
 letters, i.e. falls behind the pointer of the transmitter, the 
 springs between which the escapement wheel plays require 
 tightening ; if the reverse be the case, i.e. that the receiver 
 gains letters, the springs require loosening. 
 
 If, however, the receiver agrees with the transmitter 
 when they are joined on short circuit as above, but fails 
 
140 
 
 SECTION 
 D. 
 
 Duplex 
 Telegraphy. 
 
 Duplex Cir- 
 cuit. Bridge 
 Method. 
 
 MANUAL OF TELEGRAPHY. 
 
 when connected through a line with another instrument, as 
 represented in fig. 57, it is probable that the line is in fault. 
 
 It is important that lines worked with magneto-electric 
 currents should be well insulated, especially in the case of 
 circuits in which Wheatstone's A B G instruments are used, 
 as the failure of a single reversal would be liable to destroy 
 the sense of a whole word. 
 
 In order to bring the needle of the receiver to the same 
 letter as the pointer of the transmitter, the former is provided 
 with a small handle, by which the tongue of the electro- 
 magnet is moved backwards and forwards by the hand in 
 the same way as it is actuated by the magneto-electric 
 currents. 
 
 198. The circuits described in the foregoing paragraphs 
 have been confined to those which admit of single working, 
 that is, of transmission in one direction only on the same 
 wire. 
 
 Duplex telegraphy admits of transmission in opposite 
 directions at the same time on the same wire. 
 
 To effect this, it is evidently necessary that the circuit be 
 so arranged that signals formed by the depression of the key 
 at one station shall work the receiving instrument of another 
 station without affecting that of the sending station ; and 
 further, that the transmitting and receiving apparatus at both 
 ends be always in circuit, so that either or both stations be 
 in a position to send and receive. 
 
 Various modes of accomplishing this object have been 
 devised, the most practical of which may be divided into two 
 classes : the one including those methods which are based 
 on the principle of the Wheatstone bridge, 1 in which outgoing 
 currents have no effect on the receiving instrument of the 
 same station by placing the latter in a null branch, similarly 
 to the bridge galvanometer ; the other on the principle of 
 the differential galvanometer, 2 in which the effect of the out- 
 going currents on the home receiving instruments are nullified 
 by dividing the coils of the latter into two separate circuits 
 of equal resistance, so that the outgoing current dividing 
 between them exercises an equal and opposite magnetic ./ 
 effect on the cores. 
 
 199. The first of these systems is known as the bridge 
 method, the circuit of which is illustrated by fig. 58. 
 
 A and B are two stations so connected that A can both 
 
TELEGRAPHIC CIRCUITS. 
 
 transmit to and receive from B at the same time, or vice 
 versa. 
 
 Examining the arrangements at station A (those of B are 
 precisely similar), the circuit of the Wheatstone's bridge 
 (fig. 39) l will be at once recognised ; the line circuit 2 taking 
 the place of the x branch, and the relay R that of the gal- 
 vanometer, the branch resistances a and b, and the adjustable 
 resistance ;r, corresponding with those similarly lettered in 
 fig- 39- 
 
 ' B 
 
 141 
 
 SECTION 
 D. 
 
 2 I.e. the re- 
 sistance of the 
 line together 
 with that 
 portion of the 
 derived circuit 
 comprising the 
 receiving 
 apparatus at 
 the distant end. 
 
 FIG. 58. BRIDGE DUPLEX. 
 
 Thus, when the condition of balance is fulfilled, i.e. 
 when 
 
 a : b : : w : x 
 
 or, in other words, 
 
 a x = b w, 
 
 a current made to enter the system at the point ;;/, by the 
 depression of the key A', will have no effect on the relay x, 
 the potentials at the points p and q being equal. 
 
 Thus (in the ist case) the signalling current does not affect 
 the relay of the home station, when the resistances are adjusted 
 according to the above condition, in which the system is said 
 to be in a state of permanent balance. 
 
 Tracing A'S current from the point m, where it divides 
 between two circuits of equal resistance, viz. through I' and 
 w to earth, and through the line and receiving apparatus of 
 station B to earth, the latter portion, which may be called 
 the signalling current, enters B at the point p', at which it 
 again divides, one portion traversing and working the relay 
 K', and completing the circuit to earth through w' , the 
 
1 4 2 
 
 SECTION 
 D. 
 
 Adjustable 
 Resistance. 
 
 MANUAL OF TELEGRAPHY. 
 
 other portion passing through the branch resistances a' and 
 b'j which form a shunt circuit to the relay. 
 
 Thus (in the 2nd case} A'S signals work B'S relay. 
 
 In precisely the same manner ris signals would not affect 
 the home relay A'', but would work ^'s relay R. 
 
 Now, suppose that while ^'s key is depressed, B also 
 sends a signal by the depression of K' ; g's current divides 
 at the point ;//', one portion flowing to earth through V w 1 , 
 the other through a' to line ; but, as explained before, with- 
 out altering the potentials at p' and q', so that the effect of 
 ^'s current on tfs relay R' is not disturbed. 
 
 B^ current, however, on arriving at A alters the potential 
 at the point /, making it greater than that at </, so that a 
 current flows from/ to $, causing ^'s relay R to work. 
 
 Thus (in the $rd case), when both keys are depressed, the 
 relays respond to the signals from the distant end, being un- 
 affected by those of the home station, and duplex work is main- 
 tained. 
 
 Signals formed by the depression of both keys simul- 
 taneously are called duplex signals ; those formed by the 
 depression of one key only being called single signals ; and, 
 as in the course of communication words or even letters 
 may be made up of both duplex and single signals, it is evi- 
 dent that tbe receiving relay should be equally influenced by 
 both. 
 
 Now this is the case when balance is maintained at the 
 time when both keys are depressed, as will be understood 
 from the above remarks explaining the course of the duplex 
 current in ' the $rd case ' for, under these circumstances, 
 the receiving relay is worked by the difference of current in 
 the opposite branches viz. the line and the w branch 
 which balance one another ; the incoming current from 
 station A (for example) reducing the strength of the outgoing 
 current from B at the point/' by an amount equal to its own 
 strength, i.e. the strength of the current which forms single 
 signals. 
 
 The same, of course, holds good for signals received at A. 
 
 200. It has been stated above that w and /F', the resist- 
 ances which balance that of the line, are adjustable. If the 
 resistance of the line never altered, this would not be neces- 
 sary, as, balance once obtained, the condition 
 a x b w 
 
TELEGRAPHIC CIRCUITS. 
 
 would remain a fixed quantity ; but, as the effects of climate 
 on the insulation of lines cause great changes in their resist- 
 ance, it is necessary that the artificial resistance JF, used to 
 balance that of the line x, should be made to vary with it ; 
 w is therefore made to possess a range sufficiently great that 
 it may be increased or decreased to the limits between which 
 the resistance of the line varies, and usually consists of a 
 fixed coil approximating the minimum resistance of the line, 
 joined in series with a circular set of resistance coils, which 
 are added to or removed from the circuit as required by 
 means of a switch turned by a handle. 
 
 When the resistance of the vv branch exactly equals that 
 of the line branch i.e. when ax = b w the system is said 
 to be in permanent balance. 
 
 201. Now, it has been shown that the results of perma- 
 nent balance are : 
 
 (1) That the currents forming duplex and single signals 
 are equal in strength. 
 
 (2) That outgoing currents have no effect on the receiving 
 instrument of the home station. 
 
 Permanent balance is therefore tested prior to com- 
 mencing duplex working, in the following way : 
 
 Station A, desiring to obtain balance, gives the signal 
 ' balance ' to B. B gives attacks, and A adjusts his relay to 
 receive JB>S single signals. A then depresses his key, con- 
 tinuing to receive attacks from B, whose signals are now 
 duplex signals, as both keys are down. If A'S relay works 
 equally well with duplex as with single signals, balance is 
 perfect ; but if not, the resistance of w is adjusted till it 
 does. If received signals miss, it is evident that too much 
 current passes to earth through /F, and too little through the 
 relay ; the resistance of iv is therefore increased ; if, how- 
 ever, signals stick, then the reverse is the case, and the 
 resistance of w is decreased. 
 
 After thus altering the resistance of JF, A releases his key 
 again, observing whether his relay still responds perfectly to 
 .B'S single signals. 
 
 Minor adjustments are made by means of the micro- 
 meter screw. A then gives ' RtJ and B repeats the opera- 
 tion. 
 
 Either station tests the effect of his outgoing current by 
 observing the magnetic indicator on his relay, which, of 
 
144 
 
 Duplex 
 Condensers. 
 
 MANUAL OF TELEGRAPHY. 
 
 course, remains unaffected when permanent balance is 
 perfect. 
 
 202. Permanent balance, however, is not the only ad- 
 justment necessary in the case of long and well-insulated 
 lines ; for, although the w branch may compensate for the 
 resistance of the line, it does not balance the effects of its 
 capacity. 
 
 It has been explained 1 how, by virtue of the latter pro- 
 perty, momentary currents of charge and discharge are 
 formed when the battery contact is closed and opened ; and, 
 as the receiving relays are always in circuit, they would be 
 affected by these induced currents, which would thus inter- 
 fere with ordinary signals. 
 
 It is, therefore, necessary that the artificial line w, which 
 balances the resistance of the real line, should also be made 
 to balance its capacity. 
 
 FIG. 59. DUPLEX CONDENSERS. 
 
 This is done by means of condensers joined parallel to 
 the resistance coils of w, as shown in fig. 59, the action of 
 which may be explained as follows : 
 
 Considering the resistance w to occupy its proper 
 position in the duplex circuit as represented in fig. 58, it is 
 evident that every time A depresses his key to send a signal 
 to B, the current which goes to charge the line splits at the 
 point ;, dividing between the line circuit (through a) and 
 the w circuit (through />). 
 
 - 116 and 
 
 Now if the capacity of the condenser c, 2 the outer and Def. 15. 
 
TELEGRAPHIC CIRCUITS. 
 
 inner plates of which are respectively joined to the line and 
 earth terminals of iv t be equal to the capacity of the line, 
 the amount of charge held by the condenser will be exactly 
 equal to that of the line, so that when ^'s key is released, 
 the current of discharge rushing out from the line in the 
 direction from p to q will be met by the equal and opposite 
 current of discharge from the condenser, in the direction q 
 to /, thus counteracting the effects of the charge of the line 
 on the relay R. 
 
 The capacity of the condenser is made adjustable, like 
 the resistance of the branch w, to correspond with variations 
 in the capacity of the line. 
 
 There is an important point, however, in which the action 
 of the line and a single condenser of equal capacity differ, 
 and that is in the time they take to discharge. 
 
 A long well-insulated line takes a considerable time to 
 discharge, as explained in para, in, owing to the resistance 
 it offers at every point to the passage of the current. In 
 order, therefore, to produce a corresponding retardation in 
 the discharge of condensers, resistance is inserted between 
 each pair of plates. 
 
 When the condenser capacity of the branch w is equal 
 to the capacity of the line the system is said to be in a state 
 of transient balance. 
 
 TO obtain 203. As the effects of inductive capacity are manifested 
 Balance^ by extra currents of momentary duration, the absence of 
 induction beats on the relay indicator of the sending station 
 is a proof that transient balance is perfect. 
 
 After permanent balance is obtained, as described in 
 para. 201, the capacity of the condenser is adjusted for 
 transient balance, as follows : 
 
 A depresses his key and releases it sharply ; any move- 
 ment of the relay indicator, manifesting a return current 
 from the line, shows that its capacity is not balanced. 
 
 If the depression of A'S key causes a sudden deflection of 
 the indicator needle the condenser capacity is too small. 
 
 If, on the other hand, A'S condenser capacity be too 
 large, it can be discovered by causing B to give a continuous 
 beat, during which the depression of A'S key will cause a 
 momentary interruption of x's signal ; A therefore reduces 
 capacity until the depression of his key has no effect on #'s 
 signal. 
 
146 
 
 Respective 
 Advantages 
 and Disadvan- 
 tages of the 
 Bridge and 
 Differential 
 Methods of 
 Duplex. 
 
 Differential 
 
 Duplex 
 
 Working. 
 
 MANUAL OF TELEGRAPHY. 
 
 204. The bridge method of duplex working presents 
 this advantage, that as the receiving instruments are placed 
 in a null circuit, between the points p and q (fig. 58), no 
 special form of receiving apparatus is necessary, as is the 
 case in the differential system described in the following 
 paragraph. 
 
 It has, on the other hand, a great disadvantage, detract- 
 ing from its efficacy on long lines, viz. loss of working force, 
 owing to the battery current being split up by means of the 
 various derived circuits offered to its course by the bridge 
 apparatus. 
 
 The result of this is that the bridge system of duplex 
 telegraphy requires four times the battery power used for 
 working the same line single or simplex. 
 
 Differential duplex is accomplished with 2\ times the 
 battery power used for single working. 
 
 205. In the differential system of duplex working 
 the effect of outgoing currents on the home instrument is 
 nullified, on a similar principle to that of the differential 
 galvanometer, 1 viz. that equal currents passing through equal 
 coils produce equal magnetisation in the cores thereof. 
 
 Line 
 
 FIG. 60. DIFFERENTIAL DUPLEX. 
 
 Fig. 60 represents two differential (Siemens') relays, R and 
 R', each with two equal coils wound on the above principle; 
 the permanent magnets of the instruments are shown in the 
 figure, in order that their action may be more clearly under- 
 stood. 
 
 With no current, the tongue 5 (see station A) is a south 
 pole by induction, and the two shoes jv, , north poles, as 
 explained in para. 60. 
 
 When A sends a signal by the depression of his key, the 
 current splits at the point K, dividing itself equally between 
 
Constant 
 Resistance 
 Keys for 
 Differential 
 Duplex. 
 
 Permanent 
 and Transient 
 Balance. 
 
 TELEGRAPHIC CIRCUITS. 
 
 the two coils which, besides being equal themselves, are 
 placed in circuits of equal resistance (the resistance of w 
 being made equal to that of the line, the front coil and the 
 derived circuit of the back coil, artificial line and battery of 
 the distant station) ; the effect of the home current in both 
 coils is to decrease equally the north polarity of the shoes 
 N, #, which thus exert an equal magnetic force on the tongue 
 s, from opposite directions, so that the relay does not work 
 under the influence of outgoing currents. 
 
 An incoming current, however, acts differently. Entering 
 by the right coil (in the opposite direction to the outgoing 
 current), it increases the north polarity of the shoe N t and 
 thence proceeding through the left coil and w to earth, it 
 decreases or reverses the polarity of n, so that both coils act 
 in the same direction, their tendency being to cause the 
 tongue s to be attracted towards N. 
 
 206. In the case of long and well-insulated lines, when 
 condensers are not available, constant resistance keys may 
 be used, as described in para. 128, with the object of dis- 
 charging the line of return currents 
 
 207. The condition of permanent balance in differential 
 duplex working is that the resistance of w be equal to that 
 of the line circuit, which comprises the line wire and the 
 circuit of the distant station ; transient balance is obtained 
 by making the condenser capacity c l equal to that of the 
 line. 
 
 The mode of adjusting permanent and transient balance 
 is precisely similar to that described for obtaining balance 
 for working bridge duplex. 2 
 
 The above conditions of balance being fulfilled, it will 
 be seen that the strength of single and duplex signals is the 
 same, and that the depression of either key is responded to 
 by the distant relay, the instrument of the home station 
 remaining unaffected by outgoing currents. 
 
 For, suppose a signal to be sent from A to B by the 
 depression of the key K ; the battery current divides at the 
 point K, half traversing the line circuit, the other half the 
 derived circuit to earth through w ; but as these equal cur- 
 rents traverse the relay R in opposite directions, the tongue 
 s does not move. Thus (in the first case) the signalling 
 current does not affect the relay of the home station. 
 
 The portion of A'S current traversing the line circuit, 
 
 L 2 
 
 147 
 
 202. 
 
 201 and 
 
148 
 
 Double 
 Current 
 Differential 
 Duplex. 
 
 MANUAL OF TELEGRAPHY. 
 
 however, flows to earth at station B through the coils of the 
 relay R' and w (fig. 60), causing R' to work, as described in 
 para. 205. Thus (in the second case) A'S signals work tis relay. 
 
 Similarly, ^'s signals do not affect the home relay R', but 
 are responded to by relay R at station A. 
 
 Next, suppose that while A'S current is working B'S relay, 
 B depresses his key, K' . As explained before, R' is un- 
 affected by the home current of JB, but is responded to by /?, 
 the distant relay at A ; for the front portion of A'S current 
 which flows out to the line is opposed by the received 
 current from B so long as .s's key is depressed, the result 
 being that the opposite current from B neutralises the current 
 flowing out to line through ^'s front coil, 1 so that R works 
 under the influence of the current in its back coil, which 
 renders n a south pole, so that the tongue s is drawn towards 
 N by each depression of ^'s key (K*). 
 
 Similarly, tfs relay would respond to ^'s signals when 
 both were sending, the magnetic effects of the currents, in 
 each case, on the distant relay being the same as that pro- 
 duced by the currents which form single signals. Thus (in 
 the third case), when both keys are depressed, the relays respond 
 to distant signals, but are unaffected by those of the home 
 station. 
 
 208. Differential duplex is sometimes worked by double 
 currents, 2 with the object of accelerating the action of the 
 relays and thus increasing the speed of signals. 
 
 Fig. 6 1 represents such an arrangement, in which Gerrit 
 Smith's battery reverser is used to transmit reverse 
 currents. 
 
 The battery reverser consists of an electro-magnet m, the 
 coils of which are joined in circuit with a local battery e and 
 an ordinary key k. Each depression of the key k causes 
 the armature a to be attracted. 
 
 The armature a is permanently connected to earth, and 
 its two continuations c, d are so arranged that one or other 
 of them is in contact with one of the steel springs s, s f , both 
 of which have a tendency to press against p, which is a fixed 
 pillar permanently connected with the line and artificial line 
 through the differential relay K. 
 
 One of the steel springs, s, is permanently connected to 
 the copper pole of the line battery E, the other s' to the zinc 
 pole. 
 
 1 The front 
 coil in each 
 case is that 
 which is joined 
 to the line ; 
 and the back 
 that which is 
 joined to the 
 artificial line 
 w QT: w'. 
 
TELEGRAPHIC CIRCUITS. 
 
 When the armature a is at rest, as it is shown in the 
 figure, the zinc pole of the line battery is connected to the 
 line through the spring s' and pillar /, the other spring s 
 being pressed away from p by the lever c, which further 
 connects the copper pole of the line battery with earth. 
 
 line. 
 
 149 
 
 FIG. 61. DOUBLE CURRENT DIFFERENTIAL DUPLEX. 
 
 When, however, the key k is depressed, the armature a 
 is attracted towards m, drawing down the end c of the lever, 
 which permits the spring s to press against /, connecting 
 copper to line, zinc being joined to earth through s' by 
 means of the end d of the lever, which at the same time 
 presses s' away from /. 
 
 Thus each depression of the ordinary key k causes the 
 electro- magnet to send a copper current into the line, a 
 continuous zinc current flowing to line during the pauses 
 between the signals. 
 
 The play of the springs s, s' should be so carefully 
 adjusted that contact with the pillar p is made by one just 
 before it is broken by the other, the point K being thus 
 always in connection either with the line or battery. 1 
 
 209. Another system of duplex working is that known 
 as split battery duplex, which has the advantage possessed 
 by the bridge system, in that a special form of receiving 
 instrument is not required, provided, however, that the 
 instrument be a polarised one ; 2 and there is no necessity 
 for any special form of transmitting key ; further, the ' split 
 battery *' is superior to the ' bridge ' method in that the 
 arriving current in the former system is not reduced to the 
 same extent by derived circuits, as in the bridge method. 
 
 1 By using two 
 batteries, one 
 with its , the 
 other with its + 
 pole to earth, 
 this special 
 apparatus can 
 be dispensed 
 with, the 
 simple key k' 
 being used, its 
 middle point k 
 joined direct to 
 K, its front and 
 back contacts 
 being joined to 
 the poles of the 
 + and - 
 batteries 
 respectively. 
 This plan is 
 found the 
 simpler, and 
 is generally 
 adopted. 
 
 and 60. 
 
 59. 
 
MANUAL OF TELEGRAPHY. 
 
 SECTION 
 D. 
 
 The same amount of battery power is required for split 
 battery duplex as for the system described in para. 205 as 
 differential duplex, there being a similarity between these 
 two methods in that they are both differential ; l the batteries 
 being in the former case differentiated, and in the latter the 
 instruments a difference in favour of the latter. 
 
 Fig. 62 represents the circuit of two stations, A and B, 
 connected for split battery duplex working. 
 
 Def. 30. 
 
 Line 
 
 FIG. 62. SPLIT BATTERY DUPLEX. 
 
 Examining the circuit of station A (that of B is precisely 
 similar), w represents the middle point of the signalling 
 battery, the back and front halves , e of which are com- 
 posed of an equal number of cells of equal resistance. 
 
 K is the signalling key, the depression of which short- 
 circuits the whole of the line battery, the copper pole of 
 which is joined to the line and the zinc pole to earth through 
 the adjustable resistance w^ which is made equal to that of 
 the line and the derived circuit of the distant station ; the 
 condenser c being adjusted to balance the capacity of the 
 line, as explained in para. 202. 
 
 The middle point m of the battery is joined to earth 
 through the receiving relay R, which is an ordinary Siemens' 
 polarised relay connected up in reverse direction, i.e. with 
 its L terminal to earth, so that a copper current entering the 
 relay from the line will cause the tongue to be attracted 
 towards its insulated stud, as shown in the figure, instead of 
 towards the metal working contact, as would be the case if 
 joined up in the usual way. 
 
 When employed in the split battery duplex circuit, how- 
 ever, the tongue is adjusted with a bias 2 against the metal 
 contact, so that when no current flows through the relay 
 
 61. 
 
TELEGRAPHIC CIRCUITS. 
 
 the local battery circuit is closed, causing the sounder to 
 work. 
 
 Now referring to fig. 62, it will be observed that when 
 no signals are being sent, i.e. both keys AT, K 1 at rest with 
 their contacts open, the battery currents flow in the direc- 
 tions shown by the arrows, the front half e of ^'s battery 
 being opposed by the front half e' of B'S battery, which 
 thus counteract and neutralise one another ; a continuous 
 current from the back half E of either battery thus flows 
 through the relay of its own station, causing the tongue 
 to be held against its insulated stop, as represented in the 
 figure. 
 
 The principle of closed circuit working 1 will be here 1 189, and 
 recognised, signals being formed by the interruption of the g ' 53 * 
 permanent current flowing through the coils of the receiving 
 instrument. 
 
 Single and duplex signals 2 are both formed by the * 199 (last 
 absence of current in the receiving relay ; in the former case 
 by nullifying the continuous current by an equal current 
 opposite in direction, and in the latter by cutting off the 
 current altogether. 
 
 How this is effected by the transmitting station without 
 influencing the home station will be understood from the 
 following paragraph. 
 Permanent 210. The condition of permanent balance in this system 
 
 (with batteries split at their middle point, as described above) 
 is that w, the resistance of the artificial line, and x, that of 
 the real line and the derived circuit of the distant station, 
 be equal. v 
 
 Transient balance is obtained by making the capacity of 
 the condenser c equal to that of the line. 3 3 202. 
 
 The mode of adjusting permanent and transient balance 
 is precisely similar to that described for obtaining balance 
 for working both the bridge and differential systems of 
 duplex. 4 4 201 and 
 
 Supposing balance to be thus adjusted, and that A de- 
 presses his key to send a signal to B (K 1 open), ^'s battery is 
 then short-circuited ; its front half e is no longer free to 
 oppose the front half e' of x's battery, the current from 
 which therefore flows to A, dividing between the derived 
 circuit of /? and w to earth, thus keeping the tongue of R 
 still held against its insulated stop. 
 
MANUAL OF TELEGRAPHY. 
 
 Thus (in the first case} the signalling current does not affect 
 the relay of the home station. 
 
 It does, however, affect that of the distant station in the 
 following manner : 
 
 Tracing the current from e' (the. front half of ^'s battery), 
 which, as described above, on the depression of ^'s key, 
 flows through R and w, thence through the point n to earth 
 and back to the zinc pole of its own battery via n', R', and 
 the point m', the course of this current through #'s relay 
 R' is opposed by the copper current of E' (the back half of 
 's battery) ; and these two opposite currents being equal 
 (because w 1 = x), they counteract one another, causing no 
 current to flow through the coils of R', the tongue of which 
 consequently falls back against its metal contact, producing 
 a signal on the sounder so long as ^'s key is depressed. 
 
 Thus (in the second case) A'S signals work B'S relay. 
 
 Similarly, #'s signals do not affect the home relay /?', but 
 work A'S relay R. 
 
 Next, considering the case in which both A and B depress 
 their keys together. 
 
 The act of doing so short-circuits both their batteries so 
 that the currents are entirely cut off both from the line and 
 instruments ; the tongue of R' therefore falls back, as ex- 
 plained in case 2, producing a signal so long as the key K is 
 depressed, and vice versa ; relay R responds to the signals of 
 K', the strength of duplex and single signals being obviously 
 the same, since both are produced by ' no current.' 
 
 Thus (in the third case), when both keys are simultaneously 
 depressed, the distant relays work, those of the home station 
 being unaffected. 
 Circuits 211. The circuits described in the foregoing paragraphs 
 
 containing o f this section embrace such systems of working as are 
 Covered Wires. , . . T .. 
 
 adopted in India. 
 
 Those portions of the circuit included between the line 
 and earth, termed 'office connections,' are invariably 
 composed of covered copper wires, the insulating material 
 employed being a specially prepared form of indiarubber, 
 known as Hooper's core, which bears exposure to high tem- 
 perature better than guttapercha or any other kind of 
 insulating covering. 
 
 Guttapercha not only cracks by exposure to heat as well 
 as light, but becomes plastic, allowing the position of the 
 
TELEGRAPHIC CIRCUITS. ! q -, 
 
 conductor inside to shift ; and its resistance decreases with 
 rise of temperature. At a given temperature, the resistance 
 
 of guttapercha is far below that of indiarubber. l i At 75 Fahr. 
 
 The superiority of indiarubber for office, or earth, or l ^^ s ^ e 
 
 aerial connections is thus obvious ; and, besides possessing Core is sixteen 
 
 the advantage of higher insulation, its specific inductive S^uoP 8 ** 
 
 capacity is lower than that of guttapercha, 2 an important guttapercha. 
 
 point in considering its merits as regards use for circuits 2 The specific 
 
 including long submarine wires, by virtue of which property capacity of air 
 
 retardation is reduced and speed enhanced. 3 being =i ; 
 
 The following, however, can be said in favour of gutta- pe rcha=4-2 ; 
 
 percha : Hooper's 
 
 Core = 3 ' i ; 
 
 (1) Guttapercha never perishes under water, and insu- pure india- 
 
 lates sufficiently well at low temperatures, its resistance rubber =2-8. 
 
 increasing with pressure. 4 I2: 
 
 (2) Indiarubber, on the contrary, suffers decomposition rature ofthe 
 under water, or under any circumstances whereby air is sea below 1,200 
 
 J fathoms is 
 
 excluded. supposed to be 
 
 (3) Guttapercha is more easily jointed than indiarubber. 
 
 (4) Faults in indiarubber (accidents excepted) are gene- Pressure, of 
 rally extended over a considerable length, occurring through 
 
 a general deterioration of material ; those in guttapercha depth. 
 
 are usually confined to a single spot : hence the greater ease 
 
 with which the latter may be localised. 
 
 joints in 212. In making joints in the insulating covering, 
 
 Covered Wires. or core ' of wires used for circuits laid under water as, for 
 
 example, submarine cables special apparatus is used, and 
 
 great care and experience are necessary in order to insure 
 
 the insulation of the joint being not less perfect than that of 
 
 the cable at any other point. 
 
 For office connections, however, the following will be 
 
 found a simple and effectual form of joint : 
 
 Unwind the felt tape which forms the outer surface of 
 
 the core for about two inches from the ends of the wires to 
 
 be jointed, allowing the tape to hang down, as shown in 
 
 fig- 63. 
 
 Next, pare off the indiarubber from the copper wire for 
 the same distance, tapering the ends as shown in the figure. 
 
 Thoroughly clean the ends of the copper wires, thus 
 stripped, with sandpaper ; for the presence of the least por- 
 tion of dirt will endanger the continuity of the joint and 
 render soldering difficult. 
 
-MANUAL OF TELEGRAPHY. 
 
 Then firmly twist the wires round one another, as shown 
 below. 
 
 FIG. 63. JOINT IN OFFICE CONNECTIONS. 
 
 Connections 
 between Wires 
 and Terminal 
 Screws. 
 
 Bind a piece of damp rag round each end of the core to 
 prevent its being melted by the heat of the soldering bolt. 
 Apply the solder, removing the bolt immediately the solder 
 is seen to run through the joint, so that the wire may not be 
 unnecessarily heated. 
 
 After wiping the joint with a dry cloth it should be 
 covered with Chatterton's compound (a prepared mixture of 
 Stockholm tar, resin, and guttapercha) until the covering is 
 rather thicker than the ordinary core ; the tape ends should 
 then be tightly wound round this and neatly fastened ; the 
 joint is then complete, but should be allowed to become 
 perfectly cool before any strain is put upon the wire. 
 
 213. Every joint connecting office wires should be sol- 
 dered, except, of course, in cases where such connections 
 are made by means of terminal or binding screws, when 
 perfect continuity should be secured by keeping the contacts 
 scrupulously clean. 
 
 Further, the wire should be looped closely round the 
 terminal screw in the same direction as the screw turns 
 that is, almost invariably to the right so that the tendency 
 of the loop is to close round the screw when the latter com- 
 mences to 'bite,' instead of opening out from it, as the 
 tendency would be if the wire were looped in the contrary 
 direction. 
 

 SECTION E. 
 
 FAULTS IN TELEGRAPHIC 
 APPARATUS 
 
 AND THEIR REMEDY 
 
 214-215. GENERAL FAULTS 
 216-224. BATTERY FAULTS 
 225-237. INSTRUMENT FAULTS 
 
 238. GENERAL REMEDIES FOR INSTRUMENT FAULTS 
 
 239. FAULTS IN OFFICE CONNECTIONS 
 
 240. EARTH FAULTS 
 241-242. LINE FAULTS 
 
FAULTS IN TELEGRAPHIC CIRCUITS 
 AND THEIR REMEDY. 
 
 First Causes 214. The telegraphic circuit may be considered as com- SECTION 
 
 of Faults. prising four different parts, viz. the battery, the instruments, . ^ 
 
 the 0$^ connections (including that with the eartJi), and the 
 line. 
 
 The failure of any of these parts to perform their re- 
 quired functions results in the interruption of the circuit. 
 
 Each class of apparatus is subject to faults peculiar to 
 itself. The line is most exposed to the weather and to 
 accidents, whereby it may be broken or displaced from the 
 insulators which support it. 
 
 In offices, faults generally result from less obvious causes, 
 being often so gradual in their development as to lead to 
 waste of battery power or reduction of sensitiveness in the 
 instruments, or complete eating away of the connecting wires 
 for a long-continued period, before making their presence 
 felt by actual interruption or even imperfect communication. 
 At the same time evidences of the existence of such faults 
 are not wanting to those who know where to look for them ; 
 it therefore becomes a matter of great importance that the 
 earliest signs indicating incipient failure of any part of the 
 circuit should be understood, and further, that they should 
 be acted upon immediately they are observed, so that pre- 
 vention may obviate the necessity for cure. 
 
 injurious 2I 5- Before proceeding to describe the various kinds of 
 
 Effects of Dust faults peculiar to the different parts of the circuit it may be 
 well to draw attention to the great importance attaching to 
 cleanliness in all matters relating to telegraphy. Dirt is 
 a semi-conductor ; its presence, therefore, in a conduct- 
 ing circuit, e.g. between contact points or between terminal 
 screws, must introduce unnecessary resistance into the circuit. 
 
Faults in 
 Working 
 Batteries, and 
 their Indica- 
 tions. 
 
 Mixing of 
 Liquids. 
 
 158 MANUAL OF TELEGRAPHY. 
 
 Being a semi-conductor, it must also be a semi-insulator; 
 its effect, therefore, on the surface of an insulating body 
 must be to reduce its insulation, and to cause an unneces- 
 sary loss of current. 
 
 Its exclusion, therefore, from instruments, keys, switches, 
 lightning dischargers, and all metallic contacts cannot be 
 too scrupulously attended to. 
 
 In the line circuit its presence in joints is detrimental 
 to conduction ; on the surface of insulators it impairs the 
 insulation of the line. 
 
 In the battery circuit its adherence to the surface of the 
 jars tends to leakage, and its presence on the battery plates 
 or screws adds to the resistance of the circuit. 
 
 216. The faults most likely to occur in telegraph 
 batteries are detailed in the following paragraphs, it being 
 understood that the battery cells have been carefully pre- 
 pared and set up in the first instance. 
 
 217. Mixing of the copper and zinc solutions in 
 one or more of the cells, whereby a copper salt is deposited 
 on the zinc disc and the E.M.F. of the cell is destroyed. 
 
 Indication. The zinc disc becomes discoloured. 
 
 Cause and remedy. This fault is generally the result of 
 the cell having been suddenly shaken, in which case the dis- 
 coloured disc should be replaced by a clean one. 
 
 It may be due to the imperfect preparation .of the cell, 
 in which case the disc will soon become discoloured again ; 
 the cell should then be removed altogether and dismantled. 
 
 218. Local action within the cell. 1 
 
 Indication. Loss of effective current, frequently attended 
 with frothiness of the surface. 
 
 Cause and remedy. The presence of dirt or other im- 
 purities in the cell is the general cause of this fault ; the 
 froth and all impurities therein should be removed from the 
 surface of the water immediately on its appearance. In view 
 of prevention^ the sand or sawdust is well washed previous 
 to use. 
 
 Leakage. 2IQ. Leakage, by imperfect insulation from the ground, 
 
 resulting in continuous loss of current. 
 
 Indication. Dampness of the battery stand or of the 
 outer surface of the battery jars, or the deposition of crystals 
 thereon. 
 
 Cause and remedy. The first two indications are likely 
 
 Local Action. 
 
 33- 
 
FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 to be due either to moisture of the air or carelessness in 
 replenishing the cells with water. The jars and the battery 
 stand should be wiped dry, and care taken that the latter is 
 not resting against a damp wall, nor in fact against any 
 object whatever. 
 
 The crystals formed on the sides of the jars also act like 
 
 1a syphon, and draw off the liquid over the edges of the 
 jar. 
 They should be wiped off immediately they appear, but 
 as their formation is due to saturation of the zinc solution, 
 it should be prevented by keeping the zinc disc always 
 covered with clean water. 1 
 
 Cross Leakage. 220. Cells short-circuited or shunted, resulting in 
 the whole or part of the current of one or more cells being 
 
 I lost, by the completion of a circuit within the poles of the 
 
 battery. 
 Indications. In the case of a complete short circuit, it 
 will probably be found that the leading wires between the 
 copper and zinc poles of two or more cells are in metallic 
 contact with one another. 
 
 A partial short circuit may be indicated by the insulated 
 covering of the leading wire between two cells being either 
 cracked, or loose, or covered with crystals of sulphate of 
 zinc, or by the surface being damp, or by a drop of liquid 
 being visible at the end of the wire which is fastened to the 
 zinc terminal, or by discoloration of the zinc disc. 
 
 Cause and remedy. The cause in both cases is to be 
 found in the insulating covering of the leading wire. If it 
 is loose and allows of the liquid percolating between the 
 core and the wire, the end should be sealed up with Chat- 
 terton's compound. 
 
 The conduction of the liquid over the outer surface of 
 the wires is prevented by soaking the covered wires, before 
 use, in melted wax. 
 
 Disconnection. 221. Cells disconnected wholly or partially, by 
 which the resistance of the battery is increased and the 
 current weakened. 
 
 Indication. Deposition of white crystals of zinc sulphate 
 on the terminal screws of the zinc plate or discoloration of 
 the disc ; or the appearance of dirty froth thereon. 
 
 Causes and remedy. The action of the cell causes the 
 formation of zinc sulphate at the zinc plate, which crystallises 
 
 159 
 
 1 The Zinc 
 Solution is 
 removed by 
 means of a 
 sponge. See 
 34- 
 
i6o 
 
 MANUAL OF TELEGRAPHY. 
 
 Rise in 
 Resistance. 
 
 Fall in 
 Resistance. 
 
 when the water becomes saturated with the salt ; hence the 
 necessity for frequently changing the water, as this sulphate 
 when dry offers a high resistance. 1 
 
 If the zinc is discoloured, it should be removed. If the 
 liquid is only frothy, the froth should ,be removed. 
 
 The appearance of the above indications together in the 
 cells of a battery would indicate neglect. 
 
 If disconnection in a battery be manifested, without 
 any of the above appearances, and the cells are clean, it 
 is probable that one of the terminal screws has become 
 loosened by accident. If this is not the case, then probably 
 the remedy will be found by piercing the diaphragm. 2 
 
 Such a fault occurring through the looseness of a ter- 
 minal screw may be intermittent in its nature, causing the 
 cell to emit a current of varying strength. 
 
 222. Increase of internal resistance of a cell, 
 whereby the current is weakened. 
 
 Indication. The only sure indication in this case is that 
 afforded by actual measurement ; the cell often presents a 
 clean and passive appearance, but this is not an infallible 
 criterion. 
 
 Cause and remedy. If there are no external evidences 
 of corrosion, or dirt or neglect, this fault is probably due to 
 polarisation of the copper plate, 3 which even in a constant 
 battery may take place when strong currents are generated 
 through low resistance. 
 
 The free hydrogen, collecting in the form of bubbles at 
 the copper plate, may, however, be liberated by piercing the 
 diaphragm with a pointed piece of wood or bone or non- 
 metallic substance. 
 
 If the above remedy fails to reduce the resistance and 
 increase the current, and the terminals are clean and firmly 
 connected, the fault is probably due to a dirty plate, and the 
 zinc should be changed. 
 
 223. Decrease of internal resistance of a cell 
 signifies consumption of material, and is therefore to be 
 expected to attend the working out of the battery. 
 
 Indication and cause. The zinc disc sinks in the jar as 
 the sulphate of copper becomes consumed by the action of 
 the cell. 
 
 When it has sunk to a depth corresponding to the 
 original depth of the sulphate of copper, there is no more 
 
 1 To prevent 
 the formation 
 of crystals 
 at the zinc 
 terminal a 
 composition is 
 applied to the 
 neck of the 
 disc ( 35). 
 
 222. 
 
FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 161 
 
 Battery Faults 
 indicated by 
 Regular Tests. 
 
 Instrument 
 Faults. 
 
 to consume, the action of the cell may be expected to 
 cease. 
 
 The resistance of an exhausted Minotti is about five ohms. 
 
 224. It will be observed that the faults enumerated 
 above all affect more or less the internal resistance of the 
 battery ; regular testing, therefore, as will be explained here- 
 after, 1 affords an accurate and valuable means of detecting 
 the first traces of falling off, the symptoms of such being 
 directly indicated by the deflections of the testing galvano- 
 meter. 
 
 225. The efficiency of telegraph instruments is 
 practically tested by their sensitiveness under the influence 
 of working currents. 
 
 The principal defects to which instruments are liable 
 may be stated in general terms as follows : 
 
 (i.) Mechanical faults, due to friction, imperfect 
 springs, contacts working loose, &c. 
 
 (ii.) Electrical faults, due to defective insulation of 
 covered wires in coils or connections, causing contacts or 
 derived circuits or leakage, through uninsulated parts of the 
 instrument ; also to imperfect contacts through dust, or dirt, 
 or oxidation ; also to discontinuity, caused by the fusion of 
 a coil by lightning, or by corrosion, or accidental breakage of 
 any part of the instrument connections. The end of a coil is 
 frequently found to be the spot at which such a defect occurs. 
 
 (iii.) Faults of magnetisation, whereby permanent 
 magnets lose their magnetism through exposure to heat ; or 
 their polarity is reversed from the same cause or by the 
 effects of lightning ; also when the iron cores of electro- 
 magnets retain their magnetism after the cessation of the 
 current. 
 
 (iv.) Faults of adjustment, whereby the current is 
 employed in overcoming unnecessary mechanical inertia ; 
 as, for instance, when the play of tongues or levers is too 
 great, 2 or antagonistic springs are too strong ; or where 
 extra currents produce unnecessarily large sparks in conse- 
 quence of too great play, impairing the points thereby ; or 
 by want of judicious adaptation of battery power to the 
 resistance in circuit. 
 
 226. The relay is subject to the following defects : 3 
 (a) Loose carriage. In those relays in which the micro- 
 meter screw effects adjustment by moving a traverse, con- see 
 
 M 
 
 SECTION 
 E. 
 
 267-288. 
 
 2 64 and 
 121 id). 
 
 3 For descrip- 
 tion of the 
 instrument, 
 59- 
 
MANUAL OF TELEGRAPHY. 
 
 t 
 
 SECTION taining the limiting points between which the tongue plays, 
 ^_ t * this traverse or * carriage,' after long use, or through rough 
 
 handling of the micrometer screw, becomes loose. This is 
 a serious fault, for the instrument is thrown out of adjust- 
 ment by the working of the tongue under the influence of 
 the current, which alters the position of the carriage, and 
 consequently the play of the tongue. 
 
 Immediately such a fault shows itself, the instrument 
 should be sent for repair. 
 
 (b) Unequal coils. This fault occurs when, from acci- 
 dent or other cause, the insulating covering of the wire in 
 either coil becomes imperfect, admitting of contact between 
 several convolutions of wire. 
 
 Such a defect shows itself by the unequal magnetic 
 effect exerted by the two cores on the tongue, and is further 
 proved by testing the resistance of each coil separately. 
 
 Coils in which such a defect is manifested must be re- 
 wound. 
 
 (c) Imperfect insulation of working contact. 
 
 The metal working contact is itself insulated from the 
 body of the relay, and is further connected by insulated 
 wire, through the sounder coils, to the copper pole of the 
 local battery. The zinc pole is joined to the relay tongue ; 
 and, as this is not insulated from the rest of the instrument, 
 any defect in the insulation of the working contact or of the 
 wire connected with it would short-circuit the local battery 
 through the tongue, a fault which would make itself known 
 by causing a permanent beat on the sounder. 
 
 (d) Demagnetisation of permanent magnet. 
 
 This may be the result of exposure to heat or the effects 
 of lightning. 
 
 The failure of the relay to act, under the influence of 
 a current, as a polarised instrument, as displayed by the 
 attraction of the tongue by either core indifferently, would 
 at once indicate such a fault. 
 
 Remagnetisation, by means of a powerful permanent or 
 electro- magnet, would be necessary. 
 
 (e) Residual magnetism in cores or shoes. This fault 
 arises from impurity of the iron, which may have become 
 hard in course of use. It is rendered apparent by the 
 sluggish movement of the tongue, the result of magnetic 
 inertia, 1 and is remedied by annealing the iron ; the more i 121 (b\ 
 
FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 gradually the iron is allowed to cool the more effectually will 
 the process of annealing be accomplished. 1 
 
 (/) Oxidation of platinum contacts. The platinum points i 238 (/). 
 between the tongue and the working contact become fused 
 by the sparks caused by the extra currents, which are formed 
 on the making and breaking of the local circuit, 2 a black 2 119. 
 oxide being deposited upon the points, which introduces 
 considerable resistance into the local circuit, thus affecting 
 the strength of signals on the sounder. 
 
 The oxide is removed by the application of chalk and 
 oil, and the spark is prevented or reduced by connecting 
 the terminals of the sounder coils through a comparatively 
 high resistance, and thus forming a derived circuit or shunt 
 through which the extra currents pass instead of discharging 
 in a spark through the air. 
 
 The strength of these extra currents is dependent on the 
 strength of the current of the local battery ; hence, again, 
 the necessity for regulating the battery power according to 
 the resistance of the local circuit. 3 3 42 and 
 
 (g) Mechanical inertia of the tongue. l8l> 
 
 The delicacy of suspension of the tongue between its 
 bearings is sometimes impaired by their becoming dirty or 
 rusty, in which case they should be cleaned with chalk and 
 oil. The smallest drop of best watchmaker's oil will serve 
 to connect the tongue with the bearings as well as to keep 
 them clean. 
 
 The friction between the tongue and its bearings should 
 be so small that when the shoes are removed, and the tongue 
 is made to move under the influence of a small magnet held 
 above the middle of the iron part of it, it should swing 
 rapidly, and should only come to rest after a great many 
 oscillations. 
 
 (/i) Imperfect adjustment. 
 
 The sensitiveness of the instrument is greatly dependent 
 upon its accurate adjustment, which should be carried out 
 in accordance with the processes already described. 4 4 66-70. 
 
 (/) A broken coil of course interrupts the circuit alto- 
 gether, being indicated by the failure of the tongue to work, 
 or the magnetic indicator to deflect, under the influence of 
 a received current. 
 
 Faults in the 22 7- () The various forms of sounder described 
 
 Sounder. i n Section C 5 are, in common with the relay, subject to the 5 73-87. 
 
 M 2 
 
164 
 
 Faults in the 
 Ink-writer. 
 
 MANUAL OF TELEGRAPHY. 
 
 defects described in clauses (b\ (e\ (ti), and (/) of the fore- 
 going paragraph, and the remedies therein prescribed apply 
 equally to the case of the sounder. 
 
 Dirt and dust must be prevented from collecting on 
 contact points, this precaution being specially necessary 
 when the instruments are used for translation. 1 
 
 (b) Imperfect insulation of the pillars from the base plate 
 and from one another, or of the coils or connecting wires 
 from the base plate, would give rise to serious faults in 
 translation working, creating metallic contacts between these 
 parts of the instrument, which would cause a permanent beat 
 on the sounder with which it is joined up in translation. 
 
 (c] The spiral antagonistic spring is liable to lose its 
 power by long-continued use, thus failing to exert the re- 
 quired force of restitution, the result of which is that the 
 armature ' sticks / in spite of careful adjustment. 
 
 When this is the case a new spiral spring should be 
 fitted. 2 
 
 228. (a) The ink-writer 3 is liable to the same faults 
 as the sounder, and besides these, is subject to defects in its 
 clockwork machinery and inking arrangements, the result of 
 accident or long-continued use. 
 
 (b) Broken spring. The steel spring which sets the 
 machinery in motion may be broken by over-winding, an 
 accident which should be prevented by care ; and as a 
 further precaution against which the clockwork should never 
 be allowed to run down completely. 
 
 (c) Dust and rust seriously affect the springs and all the 
 steel and iron part of the machinery, and are excluded as 
 much as possible by inclosing the clockwork in an air-tight 
 case. 
 
 The wheels and bearings become clogged from the same 
 cause, in which case the latter should be lubricated with 
 pure watchmaker's oil. 
 
 The bearings of the wheels rest in countersunk holes in 
 the sides of the brass case, and are covered by thin, closely 
 fitting brass plates, which should only be opened for the 
 purpose of oiling the bearings. 
 
 (d) Friction of the paper wheel^ i.e. the wheel which holds 
 the tape, may impede the action of the clockwork, in which 
 case the axle should be oiled ; or the sides of the wheel 
 may be screwed up too tightly against the roll of tape ; or 
 
 1 183, and 
 
 fig. 48. 
 
 * The rules 
 for adjusting 
 various forms 
 of Sounders 
 will be found 
 in 76, 81, 
 83,' and 86. 
 
 5 93- 
 
FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 the tape itself may offer friction by the adhesion of the 
 different layers to one another. 
 
 In this case, without taking off the wheel, unroll the tape 
 by hand and roll it up again, so that when unwound by the 
 clockwork it may be relieved of the strain necessary to 
 separate the layers. 
 
 (<?) Failure of the inking-wheel to mark. 
 
 The adjustment of the ink-writer is effected in a precisely 
 similar manner to the ordinary sounder; it sometimes occurs, 
 however, that when the lever is correctly adjusted for in- 
 dicating audible signals clearly, the inking-wheel fails to 
 press against the tape and thus record the signals. This is 
 remedied by means of a small adjusting screw near the point , 
 where the lever of the armature is joined to the arm which 
 carries the inking-wheel. 
 
 This arm acts as a spring, the tendency of which is to 
 press the wheel upwards towards the tape ; the tightening 
 of the screw resists the pressure of the spring ; by loosening 
 the screw the spring or arm is enabled to press the wheel 
 as close to the tape as may be desired. 
 
 The inking-wheel may sometimes fail to mark anything 
 but dots, in consequence of its having worked loose on its 
 axle through use. 
 
 It is tightened by turning it round the axle. 
 
 The bearings of this wheel sometimes become clogged 
 with ink, which can be remedied by the application of a few 
 drops of olive oil ; the wheel can be slipped off the axle for 
 cleaning. 
 
 To prevent the ink from being spilt and entering the 
 bearings of the instrument and clogging the wheels, the 
 vessel which holds the ink should be carefully filled, but not 
 too full, and its cover should always be kept closed, not only 
 with the object of preventing the ink being spilt on the 
 apparatus, but in order to exclude dust, which would render 
 the ink thick and blotchy. 
 
 The inking-wheel should not dip deeper than one six- 
 teenth of an inch into the ink. 
 
 Faults in the 22 9> ( a ) The needle instrument * is liable to the i 9 6. 
 
 Needle instru- rupture or imperfect insulation of its coils or connections, or 
 to the demagnetisation of the needle, defects which, with 
 their respective remedies, are described in para. 226 (clauses 
 b and z) and para. 238 (d). 
 
1 66 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 E. 
 
 Faults in 
 Electric Bells, 
 Electro-mag- 
 netic Shunts, 
 and Discharg- 
 ing Relays. 
 
 Faults in -the 
 Condenser. 
 
 (b) Imperfect continuity of the sending or receiving circuit 
 of the instrument, or both, may be caused by the accumula- 
 tion of rust or dirt on the steel springs which press against 
 the contact T (fig. n), and between which the handle makes 
 battery contact. 
 
 This is indicated by a falling off in the strength of the 
 deflections of the needle under the influence of the sent or 
 received current, or both. 
 
 If the application of chalk and oil does not remove the 
 dust or rust, the springs should be cleaned with kerosine oil, 
 rubbed on with brown paper ; if this fails, fine emery powder 
 may be used. 
 
 The same fault would result from weak springs, in which 
 case they would require to be re-tempered or renewed. 
 
 Such a fault in the sending circuit can be at once dis- 
 covered by short-circuiting the instrument with a piece of 
 wire connecting the terminals A and B (fig. n). 1 On turning l 96. 
 the handle the needle should deflect violently. 
 
 The receiving portion of the circuit is tested by connect- 
 ing the battery poles direct with the terminals A and , 
 leaving the handle at rest, when the same deflections should 
 be observed. 
 
 (c) The permanent deflection of the needle out of the per- 
 pendicular is sometimes the result of terrestrial magnetism 
 or of earth currents, in which case the disc is turned round 
 until the needle hangs midway between its limiting studs. 
 
 The roughness of these studs sometimes causes the 
 needle to cling to them. Rubbing them with a black-lead 
 pencil is a remedy against this. 
 
 230. Electric bells, 2 electro-magnetic shunts, 3 2 98-103. 
 and discharging relays 4 are subject to the faults general 3 122. 
 
 to electro-magnets, as described in paras. 226 and 227, the 4 126. 
 only special source of defect to which attention is necessary 
 being in the steel spring of the alarum, which is likely to lose 
 its power. 
 
 Cleaning the spring with oil, and thus keeping it free 
 from rust, is the best prevention against this fault. . 
 
 231. The condenser 5 only suffers from two defects, 5 n6. 
 viz. : 
 
 (1) Deterioration of the dielectric. 
 
 (2) Surface leakage between the terminals. 
 
 Both these faults produce the same effect, viz. that ot 
 
FAULTS IN CIRCUITS AND THEIR REMEDY. T 
 
 impairing the inductive capacity of the condenser by creating SECTION 
 a conductive circuit between its terminals. E - 
 
 In the first case, which is the most serious, a path of 
 more or less resistance is opened to the current between two 
 adjacent plates through the injured dielectric ; this fault is 
 generally the result of exposure to heat or light, and when 
 once developed there is no remedy for it short of re- 
 insulating the plates ; hence it becomes most important to 
 take measures to prevent the occurrence of this fault. With 
 this object, the boxes of condensers should be inclosed in 
 air-tight cases or cupboards, always kept closed, these cases 
 being a further protection against the settlement of dust or 
 damp on the surface of the condenser boxes. 
 
 The terminals are usually, for the sake of insulation, 
 fitted into sockets of ebonite, the surface of which should 
 be kept dry and clean. 
 
 Faults in Keys. 232. The key l is so simple in its mechanism that it is i 105. 
 not liable to many faults. 
 
 (a) Dirty contacts are the chief cause of defect, which 
 should be carefully guarded against by exercising the pre- 
 cautions recommended in para. 226 (/). 
 
 The unnecessary resistance introduced by dirt or dust 
 between the front contacts would reduce the strength of 
 sent currents ; and received signals would be weakened by 
 the presence of dirt between the back contacts. 
 
 (b) Weak antagonistic spring. Imperfect continuity of 
 the received circuit is sometimes caused by the steel spiral 
 antagonistic spring becoming weak and failing to effect a 
 firm back contact when the handle is released by the 
 hand, 
 
 (<r) Large play is to be avoided, as besides being an 
 impediment to speed, and causing unnecessary noise, it is 
 likely to cause the platinum facings of the contact points to be 
 worn away unevenly, resulting in their failing to make perfect 
 contact. 
 
 (d) Electrolytic action 2 is liable to take place at the * Def. 26. 
 copper terminal of the key, especially when connected with 
 a strong battery, the result being that the terminal becomes 
 oxidised, and the connection between it and the battery wire 
 is rendered imperfect. 
 
 In view to preventing this action the insulating covering 
 of the wire should reach close up to the key terminal, which 
 
1 68 
 
 MANUAL OF TELEGRAPHY. 
 
 Faults in the 
 
 Lightning 
 
 Discharger. 
 
 Faults in 
 Switches or 
 Commutators. 
 
 should itself be well insulated. The base-boards of keys 
 should therefore be kept dry and clean. 
 
 (e) Constant resistance keys 1 are liable to the same i I0 s. 
 faults as ordinary keys, and in addition are open to the 
 defects arising from the use of the steel springs which pro- 
 long the contacts (see para. 230). 
 
 233. (a) The faults peculiar to the lightning dis- 
 charger 2 are contacts either between the upper and lower 2 133. 
 plates or between the upper plates themselves. 
 
 Both are usually the result of dust collecting between 
 the plates or on the surface of the ebonite washers which 
 separate them, 3 causing either loss of current by leakage to 3 Fig. 27, 133. 
 the ground, or forming a partial contact between two or 
 more lines, by which the currents sent through one affect 
 the receiving instruments of the others. 
 
 Lightning dischargers are provided with glass cases; and 
 as a further protection from dust, the holes through which 
 connecting wires are passed should be closed up with wax, 
 and any spare holes should be plugged up with wooden pegs. 
 
 The dischargers should be frequently examined and the 
 slightest appearance of dust on the ebonite washers removed ; 
 the plates themselves being cleaned when necessary with a 
 dry rough cloth or with the interior of the husk of the cocoa- 
 nut when dry. 
 
 (6) Imperfect connections through dirty or loose terminals 
 are, of course, to be avoided ; and to prevent the liability to 
 this fault in the case of the thick stranded wire from the 
 earth plate, which is joined to the lower discharger plate, a 
 circular copper washer may be soldered to the end of the 
 wire, the washer being fitted on to the terminal screw of the 
 discharger plate. 
 
 234. (a] Switches or commutators 4 are liable to 4 136-141. 
 the same faults as the lightning discharger, resulting from 
 
 the effects of dust, or dirt, or imperfect connections. 
 
 (b) The plugs used for connecting the various parts of the 
 switch or commutator are liable to corrosion, which is assisted 
 by the action of the current. They become coated with a 
 greasy green oxide, which introduces considerable resistance 
 into the circuit. 
 
 It is important, therefore, that the formation of this oxide 
 in any quantity should be prevented by wiping all plugs daily 
 with a dry cloth or chamois leather. 
 
Faults in 
 Galvanoscopes 
 or Galvano- 
 meters. 
 
 FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 If corrosion has taken place to such an extent as not 
 to be removed by rubbing, boiling water is the only 
 remedy short of scraping, which latter is to be avoided if 
 possible. 
 
 (c) The conductivity of the bars of the commutator or 
 switch is sometimes rendered imperfect by a portion of the 
 lacquer with which their surface is varnished having entered 
 the plug holes. 
 
 It is necessary to carefully remove this with a penknife. 
 
 The bars themselves may occasionally work loose, and 
 open out on the insertion of a plug between them, thus 
 failing to make perfect contact, in which case the screws 
 which fix them to the base-board require tightening. 
 
 2 35. ( a ) Galvanoscopes or galvanometers * are, in i 142-156. 
 common with all electro- magnets, subject to the disorders 
 resulting from a broken coil or the imperfect insulation 
 thereof, as described in para. 226 (b) and (/). 
 
 (b) A more common fault, however, is demagnetisation 
 of the needle by being subjected to strong currents, or 
 through exposure to heat, which defect is discovered by its 
 failing to perform the usual number of oscillations in a given 
 time. 
 
 This fault is remedied by remagnetising the needle as 
 described in para. 238 (d) or (e). 
 
 (c) Defective suspension is another fault seriously affecting 
 the sensitiveness of a galvanometer, reducing the deflections 
 thereof, and further showing itself by the number of times 
 the needle swings before coming to rest. The greater the 
 friction caused by its suspension, the less, of course, will be 
 the number of oscillations. 
 
 If the needle be supported on a pivot, the defect will be 
 probably found to be due to the pivot having become dirty 
 or rusty, in which case it should be cleaned with chalk and 
 oil, and rubbed with leather on no account sandpaper ; if 
 the pivot be at all bent, it should be carefully straightened : 
 or, if blunt, it may be sharpened with a very fine file. 
 
 The socket in which the pivot is placed may be dirty, in 
 which case it can be cleaned with a sharply pointed piece of 
 soft wood, great care being taken not to scratch the inside 
 of the socket. 
 
 In those galvanometers which admit of it, the needle 
 should always be raised off its pivot ; or, in the case of a 
 
Faults in 
 
 Resistance 
 
 Coils. 
 
 Faults in the 
 ABC Instru- 
 ment. 
 
 General 
 Remedies for 
 Instrument 
 Faults. 
 
 To remedy 
 Dirty or Fused 
 Contacts. 
 
 MANUAL OF TELEGRAPHY. 
 
 fibre suspension, the fibre should be relieved of the weight 
 of the needle when the instrument is not in use. 
 
 When the latter form of suspension becomes defective, 
 it is necessary to renew it altogether. 
 
 236. The only fault peculiar to resistance coils, 1 to i 
 which attention need here be drawn, is that which arises 
 from the fact that the passage of a strong current through a 
 fine wire heats it considerably, and the resistance of wire is 
 affected by temperature thus the standard by which electrical 
 measurements are made is itself subject to variation under 
 the influence of the current. 
 
 To reduce the effect of this source of inaccuracy to a 
 minimum, the coils of resistance boxes are usually made of 
 German silver wire, which is less affected by temperature 
 than copper wire. 
 
 As a further precaution, the currents should be kept on 
 as short a time as possible. 
 
 237. The ABC instrument 2 suffers chiefly from 
 mechanical faults, due to continued or rough usage, resulting See 
 in the handle of the transmitter being jammed so that it will 
 not turn. This may be due to broken or loose bearings, or 
 
 to wheels becoming clogged with oil. 
 
 To prevent this, only the best watchmaker's oil should 
 be used. 3 
 
 Sometimes, on depressing one of the keys round the dial 
 of the transmitter, the key previously depressed fails to rise ; 
 when this is the case, the chain which encircles the keys has 
 become too loose, and requires tightening by means of the 
 screw which effects that purpose, and which also admits of 
 the chain being loosened in the event of its being so tight 
 that any one key cannot be properly depressed. 
 
 The receiver is liable to the defects which are common 
 to electro-magnets generally, as described in para. 226 (b\ 
 (d\ (/), and (i). 
 
 238. The following are general remedies for the 
 faults most frequently met with in telegraph in- 
 struments. 
 
 (a) Imperfect contacts resulting from the settlement of 
 dirt or dust, or from oxidation at points of contact, such as 
 the platinum points of relays, sounders, or keys, resulting 
 from the action of strong currents at these points, are best 
 remedied by daily or more frequently passing a piece of 
 
 2 172- 
 197. 
 
 228 (<:). 
 
To preserve 
 Steel Springs 
 from Rust. 
 
 To secure 
 Pivots from 
 unnecessary 
 Friction. 
 
 To Magnetise 
 Needles. 
 
 To Magnetise 
 an Astatic Pair. 
 
 FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 smooth paper between such contacts until no trace of dirt is 
 observed on the paper. 
 
 Sandpaper should never be used, as, besides wearing 
 away the facing of platinum, it renders the surfaces rough 
 and uneven, and the efficacy of the points to secure good 
 contact when closed is impaired. 
 
 If paper fails to remove the oxide and to produce a 
 bright surface, the points should be cleaned with chalk and 
 oil and rubbed with chamois leather. 
 
 If it is found that the surface is at all eaten away, it 
 should be smoothed with a very fine file, and then burnished 
 and cleaned as above. 
 
 (b) Failure of springs. If this is the result of loss of 
 temper, the springs must of course be re-tempered. 
 
 The prime cause, however, is often rust, from the action 
 of the climate. This is effectually prevented by the applica- 
 tion of a little kerosine oil on all exposed steel springs. 
 
 (c) Friction of pivots , the result of dust or rust. Clean 
 with chalk and oil, and rub with chamois leather, avoiding 
 the use of sandpaper, and subsequently apply the smallest 
 quantity of pure watchmaker's oil. 
 
 Sufficient oil for application will be obtained by dipping 
 the point of a needle into the oil. 
 
 The sockets in which pivots are balanced are best cleaned 
 with a sharp-pointed piece of soft wood. 
 
 (d) Demagnetisation of needles, under the influence of 
 strong currents or lightning ; or the result of exposure to 
 heat. 
 
 Galvanometer needles, and those used as magnetic in- 
 dicators on the covers of relays, can be re-magnetised either 
 by placing them across the poles of a permanent horse-shoe 
 magnet, or across the cores of an electro-magnet, such as a 
 sounder (the armature of which is removed), the needle 
 being at the same time rubbed two or three times across 
 the poles of the permanent or electro-magnet, as the case 
 may be. 1 
 
 A needle may be considered sufficiently magnetised 
 when it can support its own weight of soft iron on either 
 pole. 
 
 (e) Demagnetisation of an astatic pair? due to strong 
 currents passing through the coils, to heat, or lightning, 
 resulting in loss of sensitiveness. 
 
 171 
 
 1 The end of 
 the needle 
 which is to be 
 a North Pole 
 is, of course, 
 placed on the 
 South Pole of 
 the permanent 
 magnet or 
 electro-magnet 
 and vice versa. 
 
 2 Def. 32, and 
 154- 
 
Ij 2 MANUAL OF TELEGRAPHY. 
 
 Each of the needles is separately magnetised as above, 
 with reverse poles facing one another. Then suspend the 
 pair by a thread without torsion; if the system will remain in 
 a position at right angles to the magnetic meridian, it is 
 perfectly astatic ; if not, the magnetism of one needle must 
 be stronger than that of the other. First try to add mag- 
 netism to the weaker magnet ; if, however, this be magnetised 
 to saturation, 1 then magnetism must be abstracted from the l Def. 47 . 
 stronger till both become nearly equal. 
 
 This must be effected by stroking the needle in the 
 reverse direction to that by which it would be magnetised, 
 but with a very feeble magnet, such as a magnetised sewing 
 needle or penknife. 
 
 To render Iron (f) Residual magnetism owing to the impurity of the. iron 
 soft - used for electro-magnetic cores. 
 
 Anneal the iron by heating it in a charcoal fire and 
 allowing it to cool gradually. 
 
 To protect (g) Lightning faults, such as the fusion of coils, the 
 
 fronitbeE^cts ru P ture ^ their insulating covering, or the demagnetisa- 
 
 of Lightning, tion of permanent magnets, are prevented or mitigated by 
 
 protecting them by means of the lightning discharger, as 
 
 described in para. 133. 
 
 The discharger is inserted in the circuit at the point 
 where the line enters the office, so that the whole apparatus 
 may be protected. 
 
 The inductive effects of lightning, commonly called 
 'lightning beats,' are often indicated by receiving instru- 
 ments in spite of the discharger, owing to the potential of 
 these currents not being sufficiently great to effect discharge 
 between the plates of the discharger. It is therefore im- 
 portant that the instrument itself should not present a better 
 means of discharge than the discharger. The iron cores of 
 electro-magnets should, for this reason, never be connected 
 with the earth, as discharge might take place to the ground 
 through the covering of the coils. 
 
 To secure firm (//) Imperfect connections between wires and terminal 
 Terminal ni '* screws mav be the result of oxidation or dirt, as explained in 
 Screws. the case of contact points in clause (a) of this paragraph, 
 
 where the remedy is also given. 
 
 Loose connections are, however, a not unfrequent cause 
 of imperfection, which, though sometimes the result of care- 
 less dusting or wiping, are more often originated by neglect 
 
FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 of the precautions given in para. 213 for forming the loops 
 of connecting wires. 
 
 The wire loop should always close round the terminal in 
 the same direction as the screw. 
 
 239. (a) The covered wires used for office connec- 
 tions invariably deteriorate, not only from the effects of 
 exposure to heat and light, but from the action of the 
 currents which pass through them, which causes them to 
 crack and perish. 
 
 (<) Leakage is the result of this, by which the effective 
 strength of the current is reduced. 
 
 (c) Electrolytic action taking place at these points causes 
 the copper conductor to corrode, under the influence of 
 copper currents, resulting in its being eaten completely 
 through, when sudden interruption ensues. 
 
 The practice of fastening office wires with metal staples 
 is thus a most dangerous one. Leather straps or wooden 
 battens are to be preferred. 
 
 When office wires show signs of perishing, the only 
 remedy is to renew them throughout 
 
 The leading wire joined to the copper pole of the 
 signalling battery is likely to be the first to become defec- 
 tive. 
 
 240. (a) Earth connections, 1 besides being liable to 
 the defects described in the foregoing paragraph, may be- 
 come defective from the following causes : 
 
 (/>) Corrosion of plate. The resistance of copper earth 
 plates is often found to increase after long use, in conse- 
 quence of the formation of a green greasy oxide on their 
 surface similar to that described in the case of commutator 
 plugs. 2 On the first indication of this fault the plate should 
 be thoroughly cleaned, as the longer oxidation goes on, the 
 more difficult its removal becomes. 
 
 (c) Polarisation may result from such a fault as the 
 above, or from any cause whereby electrical contact between 
 the plate and the ground is rendered imperfect, the conse- 
 quence of which is that the plate fails to diffuse the current 
 into the earth and becomes charged or polarised, as it is 
 called, discharging its current back through the line circuit 
 in the opposite direction to the working current. 
 
 241. Faults in the line circuit may be due to either 
 of the following causes : 
 
 173 
 
 178. 
 
 234 (a). 
 
174 
 
 Natural Causes 
 obstructing 
 Communica- 
 tion. 
 
 Atmospheric 
 Electricity. 
 
 MANUAL OF TELEGRAPHY. 
 
 (a) Total disconnection, caused by breakage of the line 
 through accident, a bad joint, or rusty wire. 
 
 (b) Partial disconnection is almost invariably the result 
 of an imperfectly soldered joint which has not actually given 
 way. This fault is often intermittent in its nature. 
 
 (c) Total or ' dead" 1 earth is caused by the line wire 
 forming connection with damp ground, either directly or 
 by touching an iron post or stay, or a wet tree, whereby a 
 perfect contact is formed with the ground, the resistance of 
 the fault in this case being nil. 
 
 (d) Partial earth<\s due to similar causes, which do not, 
 however, cause perfect connection with the ground. 1 
 
 (e) Contacts are caused by two or more wires being 
 twisted together or connected by a piece of wire thrown 
 upon them, by which they are placed in metallic contact, the 
 resistance of such a fault being generally nil, when the 
 contact is called perfect ; or by kite-strings, dead snakes, 
 leaves of trees, or other imperfect conductors, whereby the 
 contact itself offers resistance, in which case it is called a 
 partial contact? 
 
 Disconnections , earths, and contacts may occur together ; 
 for example, in the case where a line is broken and the ends 
 on the ground and touching other wires. 
 
 Sec. F, on * Testing,' includes a description of the mode 
 of localising the various faults which occur in telegraph lines. 
 
 242. Communication through telegraph lines is subject 
 to disturbances due to what are termed natural currents, 
 such being attributable to any of the following causes : 
 
 (1) Atmospheric electricity. 
 
 (2) Earth currents. 
 
 (3) Thermo-electric currents. 
 
 (4) Moisture. 
 
 (a) Atmospheric currents are generated by the electricity 
 of the air or clouds, either by induction or by direct dis- 
 charge ; any change of state in the air or clouds producing 
 a current in the line. 
 
 Lightning dischargers 3 are the only protection against 
 disturbances of this kind ; and when they fail, it becomes 
 necessary to stop work and put the line direct to earth, until 
 the atmospheric electricity is sufficiently discharged to allow 
 of communication being resumed. 
 
 1 A trouble- 
 some and not 
 uncommon 
 cause of earth 
 faults is the 
 persistent 
 building of 
 crows' nests on 
 the telegraph 
 posts. These 
 nests contain 
 bits of wire, 
 such as ends of 
 binding or 
 jointing wire, 
 which may 
 have been left 
 on the ground, 
 and form a 
 conducting 
 circuit between 
 the line and 
 the iron post. 
 Hence the 
 necessity for 
 requiring the 
 workmen to 
 gather up and 
 bury all useless 
 scraps of wire. 
 
 2 Snakes are 
 not unfre- 
 quently 
 
 dropped across 
 the wires by 
 birds. 
 
 133- 
 
Earth 
 Currents. 
 
 Thermo- 
 electric 
 Currents. 
 
 Moisture. 
 
 FAULTS IN CIRCUITS AND THEIR REMEDY. 
 
 These currents must be clearly distinguished from 'earth 
 currents,' the causes of which are as follows : 
 
 (ft) Earth currents simply appear to be caused by 
 currents flowing from one part of the earth to another (owing 
 to a difference of potential between these points) through 
 the line, which is placed in metallic contact with the ground 
 by means of the earth plates. These currents must not, 
 however, be confused with the currents produced by polarisa- 
 tion of earth plates, 1 though they exhibit the same features. 
 
 Earth currents are intimately connected with magnetic 
 storms. They are observed to be strongest in lines running 
 N.E. and S.W., and are variable in strength and direction. 
 When they are so strong as to interfere with communication 
 their effect can be nullified by throwing off earth and using 
 a spare line, when there is one, as a return wire. 
 
 (c) Thermo-electric currents are generated between dif- 
 ferent points of the line, varying in temperature. These 
 currents, however, are rarely, if ever, sufficiently strong to 
 interfere with the working current. 
 
 (d) Moisture, such as rain, or dew, or sea air, has the 
 effect of producing polarisation currents at points of defective 
 insulation. 
 
 Such faults are felt least when working with a copper 
 current, 2 but when the strength of the polarisation currents, 
 due to electrolytic action, is such as to obstruct communica- 
 tion, their direction may be reversed by sending a zinc 
 current through the line for a short time. 
 
 240 (< 
 
 Def. 26, and 
 239 (c). 
 
SECTION F. 
 
 TESTING 
 
 243-266. MEASUREMENT OF ORDINARY RESISTANCES BY 
 VARIOUS METHODS, WITH RULES FOR THE 
 SELECTION OF SUITABLE TESTING BATTERIES 
 AND INSTRUMENTS 
 
 267-278. MEASUREMENT OF BATTERY RESISTANCES (OR ANY 
 RESISTANCES CONTAINING E.M.F.), BY VARIOUS 
 METHODS 
 
 279-287. MEASUREMENT OF ELECTROMOTIVE FORCE 
 (VARIOUS METHODS) 
 
 288. REGULAR TESTS OF BATTERIES 
 289-291. MEASUREMENT OF ARRIVING CURRENTS 
 292-318. TESTING OF LINES (REGULAR TESTS) 
 319-333. DITTO (FAULT TESTS) 
 
 334. MEASUREMENT OF CAPACITIES 
 
 335-343. 'EARTH' AND 'LIGHTNING CONDUCTOR' TESTING 
 (VARIOUS METHODS) 
 
 344. TESTS OF INSTRUMENTS AND CONNECTIONS 
 
TESTING. 
 
 Object of 
 Testing. 
 
 To measure 
 
 ordinary 
 
 Resistances. 
 
 Resistances 
 measured 
 by the 
 Wheatstone 
 Bridge. 
 
 243. Electrical testing has two objects : first, that of 
 examining the normal conditions of batteries, lines, and 
 earths, which is the purpose of regular testing \ any sudden 
 or unaccountable change from the regular readings recorded 
 thus affording a means of detecting and remedying faults 
 before they affect communication or otherwise show them- 
 selves ; the second object, which is accomplished by fault 
 testing (and which is confined to the lines\ being that of 
 ascertaining the locality of a fault immediately its presence 
 is manifested. 
 
 244. The simplest electrical measurement which can be 
 made is that of a resistance not containing an electromotive 
 force : for example, a piece of wire or a coil, such being 
 called a ' dead resistance.' 
 
 A resistance of this kind may be measured in various 
 ways. 
 
 245. By the Wheatstone bridge, described in para. 
 164 (see fig. 39). The unknown resistance, which we may 
 call x, is joined up between the terminals L and L' of the 
 bridge, and, as explained in the above-mentioned paragraph, 
 
 X W. * * For proof 
 
 A of this 
 
 formula, see 
 
 By this means resistances from o'oi unit up to one million Solution vi. 
 
 J App. B. 
 
 units can be measured. 
 
 As the measurement of x depends upon the * balance ' 
 of the bridge, which is adjusted by the resistance of the 
 various branches, it is necessary that this should be tested, 
 which is done as follows. 
 
i8o 
 
 To test the 
 Branches of 
 the Bridge. 
 
 To test the 
 Insulation of 
 the Bridge. 
 
 Resistances 
 measured 
 by the 
 Differential 
 Galvano- 
 meter. 
 
 MANUAL OF TELEGRAPHY. 
 
 246. To test the bridge for conduction. 
 
 Opposite branches being equal, the 
 
 First, make 
 
 x == o 
 W o 
 A =10 
 
 potentials at p and q should be 
 equal, and therefore there should 
 be no deflection of the galvano- 
 meter, indicating balance to be 
 perfect. 
 
 A strong deflection of the needle 
 should result, indicating that W 
 is too large. 
 
 A stronger deflection of the needle, 
 in the opposite direction, should 
 show x too large. 
 
 Fourth, make W also ( O P p P osite branches bein S a S ain 
 infinity, A and B ( 
 being each =1000 i 
 
 Second, make W i, 
 x, A, and B remain- 
 ing as before 
 
 Third, now make x J 
 infinity 
 
 equal, balance should be re- 
 stored, as indicated by no deflec- 
 tion of the needle. 
 In testing with equal branches A and B their resistance 
 should be determined by that under measurement. 
 
 If x be between i and 100 units, A and B should 
 
 be = 10. 
 
 100 1000 A and B should 
 
 be = 100. 
 
 1000 i oooo A and B should 
 
 be = 1000. 
 
 247. To test the bridge for insulation. 
 
 As the limit of measurement (by balance) with the bridge 
 is one million units, it is only possible to test whether its 
 insulation resistance is above that standard. This is done 
 by making 
 
 A = 10 
 
 B = 1000 
 
 W= 10000 
 
 x = infinity. 
 
 The deflection of the needle should show x greater than 
 
 T> 
 
 W, i.e. greater than one million. 
 A 
 
 248. The differential galvanometer, described in 
 para. I65, 1 affords another means of measuring x. 
 
 When neither shunt is used, 
 
 1 See fig. 40. 
 
TESTING. 
 
 With the shunt 5 in circuit, 
 
 Rules for 
 testing the 
 Accuracy 
 of the 
 Differential 
 Galvano- 
 meter. 
 
 Resistances 
 measured by 
 the Deflec- 
 tion of a 
 Galvano- 
 meter. 
 
 = 100 w:- 
 
 And with the shunt s', 
 
 TW vy ' 
 
 The differential galvanometer, like the bridge, has a 
 range of measurement from o - oi to 1,000,000 units ; the 
 bridge, however, is the more accurate measurer of the two, 
 in consequence of the difficulty experienced in winding both 
 coils of the differential galvanometer alike. 
 
 249. The accuracy of the differential galvanometer is 
 tested as follows : 
 
 * For proof, see 
 Solution VII. 
 App. B. 
 
 For conduction. 
 
 X = O 
 
 or make 
 Second, make 
 then make 
 
 Third, make 
 then make 
 
 vV \J 1 
 
 W= o wlth or 
 ( without \ 
 
 X =00 
 
 rr^ 1 shunts | 
 
 X =00 \ 
 
 W= o { 
 
 x = o i 
 
 x = o I 
 
 w= i! 
 
 r 1 
 x = i 
 
 W= o \ 
 
 For insulation. 
 
 Make 
 
 X =00 
 W =10000 
 
 with the 
 shunt s 
 
 I Balance should be indi- 
 cated by no deflection. 
 
 The needle should in 
 both cases be deflected 
 strongly, but in oppo- 
 site directions. 
 
 The needle should be 
 deflected in opposite 
 directions, but less 
 strongly than in the 
 second case. 
 
 The deflection should 
 show x greater than 
 100 W, i.e. greater 
 than one million. 
 
 250. By deflections, on the principle that the angle of 
 deflection of a galvanometer needle (or, in the case of sine 
 and tangent galvanometers, 1 the sine or tangent respectively * 150 and 
 of the angle of deflection) is proportional to the strength of * 51 ' 
 the current, which is itself inversely proportional to the re- 
 sistance in circuit ; 2 that is to say, if a certain resistance, w, * Law i, 
 be joined up in the circuit of a battery and galvanometer, a A PP- A - 
 certain deflection of the needle will be the result. Suppose, 
 now, that w is replaced by some other resistance, x. Then, 
 if x be equal to JF, the deflection of the needle will be the 
 
T g 2 MANUAL OF TELEGRAPHY. 
 
 SECTION same as before ; or, if x be one half oi w^ then the deflection 
 , R .. will be double what it was before ; or, if x be twice as great 
 as w, the deflection will be half what it was before. In 
 other words, with the same electromotive force, the strength 
 of current, as shown by the deflection of the galvanometer, 
 is inversely proportional to the resistance in circuit i.e. 
 
 * This follows 
 directly from 
 Ohm's Law 
 
 R being the resistance in circuit ; c= E 
 
 E the electromotive force, as represented by the number of R 
 
 cells in series ; from which 
 
 Cthe strength of current, as denoted by the angle of de- 5~~ 
 
 flection of the galvanometer. 
 
 Now if, in the circuit above described, R (the known re- '*' R ^~c' 
 sistance) be replaced by x, an unknown resistance which it 
 is desired to measure E remaining the same it follows 
 that 
 
 E 
 
 x =c> 
 
 C' representing the deflection of the galvanometer when the 
 unknown resistance x is in circuit. 
 
 Now if C= C' that is, if the deflection is the same with 
 x in circuit as it was when R was in circuit it follows that 
 
 xR. 
 
 If not, however, then 
 
 And, E being the same in both cases, 
 
 ~ r * It is pre- 
 
 _ = _ sumed here, for 
 
 R C' ' the sake of 
 
 simplicity, that 
 
 _ C R the Galvano- 
 
 .*. x -- - meter used is 
 
 one in which 
 the deflections 
 
 that is, the unknown resistance x is equal to the known re- a re propor- 
 sistance R, multiplied by the ratio between the angles of ^jonai to th e 
 deflection in the first and second cases respectively. * current ( 149). 
 
Effect of 
 altering the 
 E.M.F. of 
 the Testing 
 Battery. 
 
 TESTING. 
 
 251. It may often occur that, although the angle of 
 deflection C in the first case with the resistance R in 
 circuit, may be a readable deflection, x, the unknown re- 
 sistance, may be so much greater than R that when x is in 
 circuit the needle may scarcely be deflected a single degree 
 on the scale, with the same battery. Hence it becomes 
 desirable to increase the electromotive force of the testing 
 battery, so that in the second case with the large resistance 
 x in circuit the strength of the current may be increased 
 sufficiently to produce a readable deflection, C', of the 
 needle for comparison with the deflection C, obtained in the 
 first case with the smaller electromotive force ' through 
 the smaller resistance R. 
 
 Calling the increased E.M.F., when x is in circuit, E f , 
 we have 
 
 X 
 
 R 
 
 ~c 
 
 E' C 
 EC' 
 
 Battery 252. It will be observed that the only resistances taken 
 
 Resistance j nto accoun t f n the above circuit are those external to the 
 
 when taken 
 
 into account, testing battery. 
 
 It must be remembered, however, that the batteries 
 themselves contain resistance, and under certain circum- 
 stances the battery resistance may form an important part 
 of that of the whole circuit. 
 
 For example, in the case when a single cell whose re- 
 sistance is 20 ohms * is joined in circuit with a very low 
 resistance, say 2 ohms, it is obvious that if another cell be 
 added to the first (in series), although the E.M.F. will be 
 doubled, the strength of the current as indicated by the 
 deflection of the galvanometer will not also be doubled. 
 
 This is because the resistance introduced by the second 
 cell (20 ohms) is so great in comparison with the external 
 resistance (2 ohms) that the increased E.M.F. does not 
 
 4 6. 
 
MANUAL OF TELEGRAPHY. 
 
 increase the strength of the current accordingly, on account 
 of the resistance introduced along with it, which acts on the 
 strength of current in the inverse or opposite way to E.M.F. 
 that is to say, by adding E.M.F., the strength of the 
 current C is increased ; but, by adding resistance, C is de- 
 creased ; thus it may happen that adding cells to a battery 
 may have no effect whatever upon the deflections of a 
 galvanometer in circuit, the strength or quantity of the cur- 
 rent (not its E.M.F. simply) being the property of the 
 current which does work, i.e. deflects needles, decomposes 
 water, heats wires, &C. 1 l Def. i. 
 
 This may be very simply explained by Ohm's law as 
 follows : 
 
 Now, in the first case described above, viz. when the 
 circuit is composed of one cell of 20 ohms, and an external 
 resistance of 2 ohms, 
 
 C= 1 = - 
 
 20 + 2 22 
 
 In the second case, when another cell is added, 
 
 , _ 2 2 __ 1 
 
 ~~^2~"21 ) 
 
 that is to say, the strength of the current (and consequently 
 the deflection) is almost precisely the same with two cells as 
 with one. 
 
 If, however, the external resistance be very large com- 
 pared with that of the battery for example, say 5000 ohms 
 then, with one cell, 
 
 20 + 5000 6020 
 and with two cells, 
 
 20+80+5000 50^0 2520 
 
 /. C' : C :: 5020 : 2520, 
 
 C 1 = 
 
 or =- = 2 nearly; 
 C 2520 
 
 that is to say, the current is twice as strong with two cells as 
 with one. 
 
Rule for 
 calculating 
 the Strength 
 of Current 
 in reference 
 to arrange- 
 ment of 
 battery 
 power. 
 
 Testing 
 Batteries 
 joined for 
 Tension or 
 Quantity. 
 
 Measure- 
 ment of Low 
 Resistances. 
 
 TESTING. 
 
 253. This enables us to make a rule which shall be a 
 guide in calculating the effect of the current in all cases of 
 testing by deflections, according to the number of cells used, 
 under various conditions of resistance. 
 
 This rule may be stated as follows : 
 
 If the resistance to be tested is so great that 
 the battery resistance is insignificant in com- 
 parison with it, then, by multiplying the E.M.F. 
 of the battery, the deflections will be multiplied 
 accordingly. 
 
 That is to say, if one cell causes the needle to deflect 
 10, two cells will produce a deflection of 20, and so on (in 
 instruments whose deflections are proportional to the strength 
 of the current). 
 
 254. The definition of E.M.F. 1 accounts for this fact, i Defs. 5 
 for, being that property of the current which overcomes re- and 6 - 
 sistance, it follows that when the external circuit is such that 
 
 the work of the testing current is that of overcoming high 
 resistance the battery should be given high E.M.F., which 
 is effected by joining its cells in series, or for ' tension ' as it 
 is sometimes called. 
 
 . It is obvious, on the other hand, that if the resistance of 
 the circuit to be measured is so small that the current has 
 practically nothing to overcome, then the property of high 
 E.M.F. is not required, and no increase of deflection would 
 result from joining the cells in series. 
 
 In this case, doubling the quantity of the current would 
 double the deflections. 
 
 Now as the quantity of the current depends upon the 
 quantity of material consumed, it is evident that by doubling 
 the surface of the plates of a cell or, which is the same 
 thing, joining two cells parallel to one another 2 the quan- 2 19. 
 tity of the current (and consequently the deflections) will be 
 doubled in those cases where the external resistance is insig- 
 nificant compared with that of the battery. Hence joining 
 cells parallel is called ''joining for quantity' 
 
 255. The fact of doubling the surface, by joining the 
 plates of the cells parallel, reduces their combined resistance 
 to one half that of a single cell, according to the law of de- 
 rived circuits, their joint resistance being the product of the 
 resistance of each cell divided by their sum. 3 For example, * Law 8, 
 the parallel resistance of two cells, each measuring 20 units, is App - A - 
 
 '85 
 
i86 
 
 MANUAL OF TELEGRAPHY. 
 
 Adjustment 
 of Battery 
 Power 
 according 
 to work to 
 be done. 
 
 Deflections 
 controlled by 
 Adjustment 
 ofE.M.F. 
 and by 
 Galvano- 
 meter 
 Shunts. 
 
 20X20 
 
 20 + 20 40 
 or half the resistance of one cell. 
 
 Thus, in testing low resistances, when it is desired to 
 reduce the battery resistance to less than that of one cell, 
 it can be effected by joining cells parallel. 
 
 256. It will be observed that the effects of joining 
 batteries for tension or for quantity respectively are in 
 agreement with the law for obtaining the maximum effects 
 from electro-magnets, viz. that the greatest magnetic force 
 is obtained when the internal resistance of the battery is 
 
 equal to the resistance of the coils of the electro-magnet. 1 1 Law 15, 
 
 These rules, regulating the adjustment of battery power, App * A> 
 are useful, not only in the case of testing currents, but in the 
 adaptation of working batteries to suit the circuits in which 
 they are placed. 
 
 257. From the above remarks 2 it will be understood 3 251 et seq. 
 how the addition of E.M.F. in the testing battery increases 
 
 the range of measurements which may be tested by deflec- 
 tions, in those cases where the unknown resistance x (para. 
 251) is so much greater than the known resistance ^?, as to 
 render the deflection of the galvanometer, when x is in 
 circuit, too small for comparison with its deflection when 
 was in circuit, the E.M.F. in both cases being the same. 
 
 On the other hand, it may occur that x may be so much 
 smaller than R that the deflection with x in circuit may be 
 so much greater than with R in circuit (the E.M.F. being 
 the same), that the needle may deflect to its full extent on 
 the scale, and readings be thus again incomparable. 
 
 Here it would be necessary to reduce E.M.F., so as to 
 reduce the deflections in a corresponding ratio until brought 
 within readable limits. 
 
 Under certain circumstances, however, the E.M.F. of 
 the battery may be already so small as not to admit of 
 further reduction, as, for example, in the case when one cell 
 only is used. 
 
 Other means must therefore be employed for reducing 
 the deflections within readable bounds, this being most con- 
 veniently done by shunting the galvanometer, so that a 
 known fraction of the current only shall pass through the 
 coils to deflect the needle ; these deflections being multiplied 
 again by the coefficient known as ' the multiplying power of 
 
R esistances 
 measured 
 with the 
 Thomson 
 Galvano- 
 meter. 
 
 TESTING 
 
 the shunt ' l in calculating the effective strength of the cur- 
 rent. 
 
 258. The Thomson galvanometer, 2 which, by its 
 extreme sensitiveness, indicates the weakest currents passing 
 
 For descrip- 
 
 through its coils, is used in accordance with the above prin- 
 
 ciples for the measurement of very high resistances. 
 
 Fig. 64 represents the instrument joined in circuit 
 with the testing battery E, and an unknown resistance x. 
 s represents the shunt. 3 
 
 instrument, 
 see 156. 
 
 157- 
 
 FIG. 64. MEASUREMENTS WITH REFLECTING GALVANOMETER. 
 
 Now, considering the deflections of the needle as directly 
 proportional to the strength of the current flowing through 
 the coils, 4 it follows from Ohm's simple law that 4 149. 
 
 E representing the number of cells employed, C the deflec- 
 tion produced thereby through the unknown resistance x. 
 
 Now supposing E and x in the above circuit to be re- 
 placed by a different electromotive force e t and a known 
 resistance ^?, and calling the deflection in this case C' 
 
 Then, by the same law, 
 
 or R = 4.. 
 
 And comparing the two circuits, we have 
 
i88 
 
 MANUAL OF TELEGRAPHY. 
 
 J& 
 
 2. _ ; 
 
 C' 
 E 
 
 EC' T 
 
 e C 
 
 Adjustment 
 of Circuit. 
 
 the above being a general formula which is applicable to any 
 E. M. forces and resistances which may be in circuit. 
 
 259. Now the deflection C' produced by the known 
 E.M. force e, through the known resistance 7?, may be ad- 
 justed to any value most convenient consistent with readable 
 deflections. 
 
 For example, supposing C' to be produced by the cur- 
 rent of one cell through a resistance of 10,000 ohms, the 
 galvanometer being shunted with the -^-g- shunt i.e. the 
 shunt which allows y^^ part of the current to go through 
 the galvanometer or, in other words, has the effect of 
 multiplying the value of the resistance in circuit by 1,000 ; 
 
 then 
 
 C' = 
 
 10000x1000 1 million 
 
 Now, supposing the unknown resistance x to be a very 
 large one, and, in order to produce a readable deflection C 
 when x is in circuit, we increase the number of cells to 100, 
 and take off the shunt so as to give the galvanometer its full 
 deflecting power ; then, in this case, 
 
 and, comparing this with the preceding equation, 
 
 / . 100 
 1 million ' x 
 
 -t- 
 
 c 
 
 / million 
 
 x 
 x 
 100 millions (or megohms), 
 
 and x = 100 (megohms}. 
 
To measure 
 Resistances 
 with the 
 Tangent 
 Galvano- 
 meter. 
 
 the~ tangent 
 
 TESTING. 
 
 260. To measure resistances with 
 galvanometer. 
 
 Having selected a suitable testing battery of known re- 
 sistance, / join up between its poles the resistance to be 
 measured, x, and the galvanometer, whose resistance may 
 be called G. 
 
 Then observe the deflection indicated by the galvano- 
 meter, which we may call a. 
 
 Now replace the unknown resistance oc by a known re- 
 sistance IV, the rest of the circuit remaining the same, and 
 observe the deflection in the second case, which we may call 
 a 10 . Then 
 
 189 
 
 SECTION 
 F. 
 
 x = 
 
 tan a 
 tan # c 
 
 * For proof 
 of this 
 formula, see 
 Solution X. 
 
 261. In measuring low resistances with strong currents, App- B 
 it is desirable to use a galvanometer of low resistance, be- 
 cause, in this case, the sensitiveness gained by increasing 
 
 the number of convolutions in the coil would be counter- 
 acted by the resistance they introduce into the circuit. 1 i 54. 
 
 The case is different, however, in measuring high resist- 
 ances when the resistance of the galvanometer bears but a 
 small proportion to the external resistance to be measured. 
 In this case a high resistance galvanometer should be used, 
 not that its resistance is of any advantage, but because it 
 implies a large number of convolutions in the coils, the 
 whole of the multiplying power of which is brought to bear 
 (as their resistance can be neglected) to render the needle 
 sensitive to the currents, enfeebled as they may be, in con- 
 sequence of the high external resistances they have to 
 traverse. 
 
 Thus, in the departmental tangent galvanometer, 2 which 2 152. 
 has two separate coils one of high, and the other of low 
 resistance it is advisable to use the thick coil for measuring 
 low resistances (i.e. up to 350 units), and for higher resist- 
 ances the thin. 
 
 262. As a rule, the various observations made in the 
 same test should be taken through the same coil of the 
 instrument ; it is possible, however, that the battery and 
 known resistance making up one of the circuits may allow 
 of a very readable deflection through one coil, when the de- 
 flection obtained through the same coil with the unknown 
 
MANUAL OF TELEGRAPHY. 
 
 resistance in circuit may be either too small or too large for 
 comparison ; hence it becomes necessary to take the read- 
 ing, in the case of the second circuit, through the other coil. 
 
 Now this shows that there is a certain property in dif- 
 ferent coils, depending on their respective resistance, form, 
 or construction, whereby the deflections of each differ, 
 although actuated by the same current through the same 
 external resistance. This property, peculiar to each coil, 
 which may be represented by the letter K, is called the 
 coefficient of the coil ; and as the value of the coefficient 
 K varies for each, so, in the same proportion, will the 
 deflections of various coils (or in the case of tangent galva- 
 nometers, the tangents of their deflections) differ. 
 
 For example, supposing it to be possible to find two 
 exactly similar coils, whose coefficients may be called K 
 and K', respectively, and that two equal currents C, C' are 
 sent through these coils, then in this particular case, as the 
 value of the coefficients K, K 1 is the same for both coils, 
 their deflections a and a' through the thick and thin coils, 
 respectively, will be equal. 
 
 Supposing .AT and K' , however, to be different; and, for 
 example, that C represent the current in the thick coil, and 
 C 1 that in the thin, produced by the same battery, through 
 no external resistance in either case ; 
 
 Then (by the principle of the tangent galvanometer) 
 
 C=Ktan a 
 and 
 
 C' == K' tan a 10 
 
 C K tan a 
 
 ' C' K' tana! 
 whence C K' tan a 
 
 C' K tan a! 
 
 . K' __ C 1 tan a 
 
 ' ' ~K ~ C tan a 10 
 
 = n. 
 
 This ratio (represented above by the letter n) may be 
 found once for all for any pair of coils ; and to reduce their 
 deflections to the same terms, the readings through the thin 
 coil must be multiplied by n, or those through the thick 
 coil divided by n, as is obvious from the above equation ; 
 hence n is called the reduction coefficient of the coils. 
 
TESTING. 
 
 To measure 263. In those cases in which the readings of the galva- 
 SngbotTcoiis nometer cannot be controlled by the alteration of the E.M.F. 
 of the Tangent of the testing battery l such, for example, as the measure- 
 Galvanometer. ment of earths of very low res i s tance, when increased E.M.F. 
 
 would give rise to polarisation, 2 and to variations in the 
 resistance in circuit the use of two coils becomes necessary. 
 
 In this case the natural current between the earths is 
 used as the testing battery, and the resistance (x) to be 
 measured is first placed in the circuit of the thick coil of 
 the galvanometer of resistance g whose coefficient is K, 
 
 Calling the deflection produced in the above circuit #, 
 
 191 
 
 SECTION 
 
 ._ F " 
 251. 
 
 222 - 
 
 Rules for 
 Testing with 
 the Tangent 
 Galvanometer. 
 
 Efficiency. 
 
 Accuracy of 
 Observations. 
 
 To control 
 Deflections. 
 
 Instrument 
 placed out of 
 the field of 
 
 then 
 
 Ktan a? = 
 
 g+ 
 
 Now, replacing the thick coil g in the above circuit by 
 the thin coil g', whose coefficient is K ', and noting the 
 deflection a', 
 then, in the second case, 
 
 K' tan a' = 
 
 from which, 
 
 
 tan a 
 tan a" 
 
 tan 
 
 tan a' 
 
 n 
 
 * For proof, see 
 Solution XT. 
 App. B. 
 
 264. The following will be found useful rules to be 
 remembered when testing with the tangent galvanometer. 
 
 (1) The tester should satisfy himself as to the perfect 
 efficiency of the instrument electrically, magnetically, 
 and mechanically. 3 
 
 (2) Errors of observation cannot be totally avoided, 
 but to reduce them to a minimum the deflections should 
 be controlled so as not to exceed 45, and to be as near 
 that angle as possible. 4 
 
 (3) The position of the needle on the scale is 
 controlled by the use of suitable resistances and electro- 
 motive forces in the circuit ; 5 and it should be remembered 
 that the most accurate results are obtained when the known 
 resistance inserted ( W) is equal to (x), the resistance to be 
 measured. 
 
 (4) The deflections of the needle being influenced 
 by neighbouring magnets or magnetic bodies, great 
 
 3 For tests of 
 efficiency, see 
 153- 
 
 4 The reasons 
 for this are 
 explained in 
 151. See 
 fig- 35- 
 
 5 251 and 
 257- 
 
SECTION 
 F. 
 
 neighbouring 
 Magnets. 
 
 Compensation 
 for Errors of 
 Zero Point. 
 
 Adjustment of 
 E.M.F. and 
 Resistance of 
 Testing 
 Battery. 
 
 To reduce the 
 Resistance of 
 the Testing 
 Battery. 
 
 MANUAL OF TELEGRAPHY. 
 
 care is necessary that such be removed to a distance (found 
 by experiment), where their movement to and fro does not 
 affect the galvanometer needle. 
 
 (5) Errors of zero point, observed by the needle 
 not returning to zero, in the course of testing, must be 
 overcome by using reversed currents for each observation, 
 and taking the mean of + and readings. 
 
 Earth currents may sometimes cause this, when the 
 earth forms part of the circuit. 
 
 (6) On the principle that the maximum effect is 
 obtained when the internal resistance of the bat- 
 tery is equal to the resistance external to it, 1 a J Law 15, 
 battery of high resistance, i.e. containing a number of cells App> A * 
 in series, should be used for measuring high resistances ; 
 
 but for the measurement of low resistances, the battery 
 resistance should be as low as possible, so as to reduce 
 errors in its computation to a minimum, but yet large 
 enough to maintain its constancy, for in batteries of low 
 resistance polarisation takes place, causing variation, both 
 in their resistance and electromotive force. 2 2 222. 
 
 (7) To reduce the resistance of a testing bat- 
 tery to less than the normal resistance of one cell, when 
 necessary, increase the surface by joining the number 
 
 of cells ''parallel] 3 and calculate their joint resistance 3 254. 
 
 ( j, 4 having carefully found the individual resistance 4 Law 8, 
 
 Testing Battery 
 the same 
 throughout 
 one test. 
 
 J 
 
 A pp. A. 
 
 \ sum 
 of each. 
 
 This calculated value will be more correct than the 
 measured resistance of a number of cells thus joined up ; 
 for the measurement of such a circuit, closed as it is through 
 a very small resistance, is liable to be rendered inaccurate 
 by virtue of polarisation, creating a false resistance. 
 
 In testing electromotive forces also, calculations are 
 likely to be more correct than measurements, the E.M.F. 
 of a Minotti cell being a constant quantity ; 5 and, further, 5 31. 
 the measurement of its E.M.F. is dependent on its re- 
 sistance, 6 and all measurements are liable to errors of 6 264 (2). 
 observation. 
 
 (8) For the various observations taken in the measure- 
 ment of one quantity, the same testing battery should 
 be used, unless absolutely impracticable, as in the case 
 when a very high resistance is to be measured. 7 ? 25I . 
 
Choice of 
 
 Galvanometer 
 
 Resistance. 
 
 Both Coils only 
 used when 
 absolutely 
 necessary. 
 
 Battery tested 
 before and after 
 Measurement^. 
 
 Measurements 
 made rapidly. 
 
 Resistances 
 measured with 
 the Sine Galva- 
 nometer. 
 
 Bridge used in 
 Measurement 
 of high Resist- 
 ance. 
 
 TESTING. 
 
 (9) In testing resistances up to 350 units the thick coil 
 is used, and above that the thin, for reasons explained in 
 para. 261. 
 
 (10) The same coil should be used for the various 
 observations in measuring the same quantity when practi- 
 cable, as explained in para. 262. 
 
 (n) When great accuracy is required, the resistance of 
 the testing battery should be measured both before and 
 after the tests. 
 
 (12) All measurements should be made as 
 quickly as possible, as the resistance of standard coils 
 may vary under the influence of continued currents ; 1 and 
 also in cases when the earth forms part of the circuit, in 2 3 6 - 
 order that the earth plates may not become polarised, and 
 thus render results inaccurate. 2 
 
 265. To measure a resistance with the sine 
 galvanometer, the same process is adopted as that de- 
 scribed in para. 260, for measurements with the tangent 
 galvanometer, the same formula holding true ; but with 
 this modification that the sines instead of the tangents of 
 the angles of deflection are used ; thus : 
 
 '93 
 
 _sin a' f 
 ~ sin a\ 
 
 It must be remembered in this case that the deflections 
 of the needle on the scale are not read off direct, but are 
 ascertained by turning the coil after the needle, so that the 
 zero point of the movable scale stands under the needle ; 
 in which case the number of degrees through which the coil 
 was turned, as indicated by the outer fixed scale, represents 
 the angle of deflection. 3 
 
 On account of this operation, it takes longer to execute 
 measurements with the sine, than with the tangent galvano- 
 meter. 
 
 266. The Wheatstone bridge may be used for 
 the measurement of a high resistance, in connection 
 with a sensitive galvanometer ; for, although when used as 
 a ' balance] it is only capable of measuring resistances up to 
 one million, 4 it can be made to form part of the circuit for 
 testing by deflections so that its branch resistances A and 
 B act as shunts to the galvanometer ; the adjustable resist- 
 ance w being used as described in para. 259. 
 
 o 
 
 161 and 
 
 240. 
 
 150. 
 
 245- 
 
194 
 
 MANUAL OF TELEGRAPHY. 
 
 Fig. 65 represents such an arrangement, in which the A 
 branch of the bridge is used as a shunt, whose resistance is 
 adjustable by 10, 100, or 1,000 units. B is plugged up so 
 that its resistance =o, and the terminals L and L' are 
 insulated. 
 
 To find the deflection with the known resistance w in 
 circuit, insert plugs in 1 and 6, the current reverser c being 
 plugged at 2 and 3 for a copper current, and at 4 and 5 for 
 a zinc current. 
 
 Observe the deflection a of the needle, with i cell 
 through the resistance w^ allowing for the multiplying power 
 of the shunt A. 1 
 
 According to the law of shunts 2 the resistance in circuit 
 would be represented by the expression 
 
 + \+G 
 
 J 159 
 
 2 Law 9, 
 App. A. 
 
 (G being the resistance of the galvanometer), and it is 
 easy to adjust the resistances of w and ^, so that the above 
 expression may be made equal to i million, 
 
 Then, 
 
 Join x, the resistance to be measured, between L and L' 
 
 making A = o 
 
 B = oo 
 W=. oo 
 
To measure 
 Resistances 
 containing 
 Electromo- 
 tive Force. 
 
 Measure- 
 ment of 
 Battery 
 Resistance. 
 
 TESTING. 
 
 Increase the electromotive force to n cells, and call the SECTION 
 deflection in this case b. F- 
 
 Then, if the instrument in use is a reflecting galvano- 
 meter it follows that 
 
 x = - n E*( millions j 
 or, if a sine galvanometer, 
 
 x = Sm a n E (millions}, 
 sin b Q \ ) 
 
 267. In treating of the measurement of resistances in 
 the foregoing paras. (244-266), the property of resistance 
 alone has been considered, with reference to the quantity x 
 to be measured. 
 
 It often occurs, however, that x may contain an electro- 
 motive force, as in the case of battery cells (the object of 
 which is to generate electromotive force), 1 or in the case of 
 lines in which natural currents may be flowing, 2 or between 
 earth plates, the result either of polarisation, 3 or of the 
 difference of potential of the plates ; 4 and it is obvious that 
 the effect of this electromotive force, when opposed to that 
 of the testing battery, is to create an apparent increase of 
 resistance in circuit ; or when acting in the same direction 
 as that of the testing battery, to indicate a corresponding 
 decrease of resistance. 
 
 For this reason it is necessary to take all measurements 
 of x both with + and currents, so that the apparent error 
 in one case is counteracted by the error in the opposite 
 direction, in the other case. 
 
 Further, it is sometimes found convenient to use the 
 electromotive force contained in x as its own testing battery. 
 
 268. Batteries are tested both for Resistance and Electro- 
 motive Force. 
 
 As the internal resistance of the battery often 
 forms an important part of the testing circuit in the measure- 
 ment of an electrical resistance or electromotive force ; 5 and 5 252. 
 further, as any alteration in the resistance of a working battery 
 is a sign either of exhaustion or of some specific fault ; 6 it is 6 27. 
 obviously important that this property of the battery should 
 be accurately ascertained. 
 
 The internal resistance of a battery may be measured by 
 
 1 2. 
 
 2 242. 
 
 3 240(4 
 
 4 242 (*). 
 
ig6 
 
 First 
 
 Method, with 
 the Tangent 
 Galvano- 
 meter. 
 
 Second 
 Method 
 (Poggen- 
 dorff's), with 
 the Wheat- 
 stone Bridge. 
 
 MANUAL OF TELEGRAPHY. 
 
 any of the following methods, which describe various pro- 
 cesses of measurement, with various instruments, one or 
 more of which will be found to apply to the case of any 
 telegraph office. 
 
 269. First Method. To measure the resistance of 
 a battery with the departmental tangent galvano- 
 meter. 
 
 As the battery contains an electromotive force (E) of its 
 own, it is used to supply its own testing current. 
 
 First, the battery and galvanometer are joined in circuit ; 
 and the deflection produced, which may be called a, is 
 noted. 1 
 
 Second, a known resistance w (adjusted so as to produce 
 a readable deflection a') is inserted in the above circuit. 
 Then, denoting by/ the battery resistance to be measured, 
 and by g the resistance of the galvanometer, 
 
 /= 
 
 tan a' 
 
 tana tan a' ( 
 
 w 
 
 270. Second Method. To measure the resistance of 
 a battery with the Wheatstone bridge (Poggendorff). 
 
 The standard cell is inserted between L and L' ; and 
 the branch B is made infinity. 
 
 The battery, whose resistance f is to be measured, is 
 joined up to the points m and #, as shown in the following 
 figure : 
 
 Standard cell 
 Battery to le teslcd (f) 
 
 1 For a single 
 cell the thin 
 coil may be 
 used, but for a 
 battery pro- 
 ducing too 
 large deflec- 
 tions through 
 the thin coil to 
 be read, the 
 thick coil is 
 used. 
 
 * For proof of 
 this formula, 
 see Solution 
 XII. App. B. 
 
 FIG. 66. 
 
 First, make .4 = 1110, and adjust w till balance is ob- 
 tained. 
 
 Second, make A = IO, and call this value A', and the 
 resistance unplugged in this case to obtain balance 
 
 then ~ 
 
 AW -AW. 
 J ~ W- W 
 
 ; f For proof of 
 this formula, 
 see Solution 
 XIII. App. B. 
 
TESTING. 
 
 Third 271. Third Method. To measure the resistance 
 
 wmiam (Si <> f the testing battery of the Wheatstone bridge. 
 
 Thomson's). (Sir William Thomson.) 
 
 The battery to be tested is first joined in circuit with the 
 galvanometer, g, and a resistance, w, its poles being shunted 
 by a resistance, A. w and A are adjusted so as to produce 
 a convenient deflection (a). 
 
 The shunt is then taken off, and the resistance of w in- 
 creased until the same deflection (a) is reproduced. 
 
 Calling the new resistance ^, and that of the battery to 
 be tested,/ 
 
 W'W 
 / = ^ 
 
 or, if the shunt A be made equal to w+g* 
 f=W- W. 
 
 The Wheatstone bridge itself supplies the necessary 
 apparatus. The positions of the battery and galvanometer 
 are interchanged ; 1 the A branch forms the shunt to the i Fig. 66, 
 battery, and its resistance is made =10, 100, or 1,000, so as 27 - 
 to produce a suitable deflection (a). 
 
 B = o. 
 
 W any suitable resistance. 
 
 X = 00. 
 
 After noting the deflection (0), the shunt is taken off by 
 
 making ^=00, which, of course, increases the deflection; * Forinthe 
 
 plugs are then withdrawn from w until the original deflection first circuit the 
 
 (a) is reproduced through the new resistance, w'. Stedby 8 m 
 
 Fourth 272. Fourth Method. To measure the resistance ^ ^ _ E 
 
 Method,with O f a battery with the sine galvanometer. 
 
 the bine Gal- - i i / i i i 11 i 
 
 vanometer. Join the poles of the battery to be tested to the terminals 
 
 of a sine galvanometer ; note the deflection (a) produced, 
 and find the sine of this angle. 
 
 Then insert resistance w until the deflection is reduced 
 to the angle whose sine is half that of the first deflection ; 
 then 
 
 Method, the 273. Fifth Method. To measure the resistance of 
 SfeTtant a battery with any tangent galvanometer. 
 
 Gaivano- The process is precisely similar to that prescribed in the 
 
 meter. 
 
198 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 F 
 
 preceding para, for measurements with the sine galvano- 
 meter, the only difference being that the tangents instead of 
 the sines of the angles of deflection are read ; and, by the 
 same reasoning, 
 
 Sixth 
 Method, 
 with the 
 Differential 
 Galvano- 
 meter. 
 
 274. Sixth Method. To measure the resistance of 
 a battery with a differential galvanometer. 1 
 
 First join the battery to be tested in circuit with one coil 
 only of the galvanometer, and note the deflection produced. 
 
 Then through both coils, in series, by which the deflec- 
 tion will be increased ; because, by doubling the convolu- 
 tions, sensitiveness is also doubled. 2 The resistance of the 
 added coil, however, has to be considered ; if the addition 
 of this in the second case exactly doubled the total resistance 
 in circuit, then the deflections in the first and second cases 
 would be the same. It will be found in practice, however, 
 that the deflection in the second case is greater than in the 
 first. This is due to the fact that / is not doubled at the 
 same time that the resistance of the galvanometer coils is. 
 If, then, in the third case resistance w be inserted, until the 
 first deflection is reproduced, it follows that w is equal tof, 
 the resistance of the battery ; thus, 
 
 165. 
 
 144- 
 
 Seventh 
 Method, 
 with a re- 
 flecting Gal- 
 vanometer. 
 
 275. Seventh Method. To measure the resistance 
 of a battery with Thomson's reflecting galvano- 
 meter, or any galvanometer whose deflections are propor- 
 tional to the current traversing the coils. 
 
 Join up the battery, /, and galvanometer, g, in circuit with 
 a resistance, w^ and note the deflection, a. Increase the 
 resistance to w', and note the corresponding deflection, b. 
 
 Then, from Ohm's law, and according to the principle of 
 the instrument, the current in the first case may be expressed 
 thus : ^ 
 
 and, similarly, in the second case : 
 
 W + g+f 
 
TESTING. 
 
 199 
 
 Eighth 
 
 Method, 
 
 Mance's 
 
 Bridge 
 
 Method. 
 
 ( W+g)=fb + b ( W+g) ; 
 -b)=b (W'+g)-a (W+g), 
 
 and/= 
 
 276. Eighth Method. Mance's bridge method of 
 testing the resistance of a battery. 
 
 Place the battery whose resistance f is to be measured 
 in the x branch of the bridge. 
 
 FIG. 67. 
 
 As the resistance to be measured possesses an electro- 
 motive force of its own, it is used to supply its own testing 
 current, and the usual testing battery is removed and re- 
 placed by a key in a circuit known as the F branch of the 
 bridge. 
 
 By depressing the key, the resistance of the F branch is 
 made nil, and, by opening it, infinity. 
 
 Adjust the resistance of the branches A, B, and w until 
 the deflection of the galvanometer is the same whether the 
 key be open or closed. 
 
 Now, as this can only occur when the condition of 
 balance is obtained, 1 i.e. when 
 
 it follows that 
 
 or, in this case, as the battery forms the x branch, 
 
 164. 
 
 Ninth 
 
 
 277. Ninth Method (Mance's). To measure the re- 
 by sistance / of a battery by opposing electromotive 
 force. 
 
200 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 F. 
 
 Join the poles of the battery to be tested to a galvano- 
 meter, and into this circuit introduce a smaller battery 
 (generally i cell) with its poles reversed so as to oppose the 
 battery under test. 
 
 FIG. 68. 
 
 Tenth 
 Method, 
 with a 
 Calibrated 
 Galvano- 
 scope or 
 Galvano- 
 meter. 
 
 Measure- 
 ment 
 
 of Electro- 
 motive 
 Force. 
 
 First 
 Method, 
 with the 
 Tangent 
 Galvano- 
 meter. 
 
 Shunt the poles of the battery whose resistance /is to be 
 measured through a variable resistance W, and adjust this 
 resistance until the galvanometer needle stands at zero. 
 Calling the number of cells in the battery under test . E 
 And that of the opposing battery e 
 
 Then 
 
 /= W-- W* 
 
 e 
 
 278. Tenth Method. To measure the resistance 
 of a battery with a calibrated galvanometer. 1 
 
 The battery to be tested is joined up in circuit with the 
 galvanometer, and the deflection rioted. 
 
 Look up the corresponding deflection on the card issued 
 with the instrument, and note the resistance marked opposite 
 to it. 
 
 This represents the average resistance per cell of the 
 battery under test, for reasons explained in para. 252, and 
 when multiplied by the number of cells in series represents 
 the resistance of the whole battery. 
 
 279. The unit of electromotive force is termed a 
 Volt, 2 and as the E.M.F. of a Minotti or any form of 
 DanielPs cell is almost identical with the volt, it is found 
 convenient to measure the electromotive force of working 
 batteries in terms of a standard Minotti cell. 3 
 
 280. First Method. To measure the electromotive 
 force, E, of a battery, with the tangent galvano- 
 meter. 4 
 
 Calling the E.M.F. of the battery under test E, and its 
 
 * For proof of 
 this formula, 
 see Solution 
 XIV. App. B. 
 
 1 For descrip- 
 tion of the 
 instrument and 
 its mode of 
 use, see 147. 
 
 2 Def. 7. 
 
 3 43- 
 
 4 For descrip- 
 tion of the 
 instrument and 
 its use, see 
 
TESTING. 
 
 2OI 
 
 Second 
 Method, with 
 the Wheat- 
 stone Bridge. 
 
 resistance/; and the corresponding properties of the standard 
 cell used for comparison E' and/' ; 
 
 Firs^ observe the deflection a', produced by the standard 
 cell through the galvanometer alone, the resistance of which 
 is g. If the departmental tangent galvanometer be the 
 instrument employed, use the thin coil. 
 
 Second. Observe the deflection a, produced by the 
 battery under test, through the same coil and a suitable 
 external resistance w. 1 
 
 Then 
 
 tana 1 
 
 __ 
 f' + g 
 
 281. Second Method. To measure the electro- 
 motive force, E, of a battery, with the Wheatstone 
 bridge (Poggendorff). 
 
 The circuit is arranged in precisely the same way as that 
 represented in fig. 66, para. 270. 
 
 When balance is obtained, no current flows through the 
 branches x and g of the bridge, but follows the circuit com- 
 posed of A, W) and / From the same observations which 
 are made to measure the resistance of the battery, its electro- 
 motive force is calculated by the following formula : 
 
 1 With the 
 Departmental 
 Tangent Gal- 
 vanometer, n r 
 may = 2,000 
 ohms. 
 
 * For proof of 
 this formula, 
 see Solution 
 XV. App. B. 
 
 Third Method, 
 with any Gal- 
 vanometer. 
 
 282. Third Method. To measure E, the electro- 
 motive force of a battery, in terms of that of the 
 standard cell, e, with any galvanometer. 
 
 First join the battery to be tested in circuit with the 
 galvanometer, g t and a certain resistance, w ; and note the 
 deflection produced. 
 
 Then replace the battery by the standard cell and adjust 
 the resistance until a value, iv', is found, when the same 
 deflection is obtained. 
 
 Then, by Ohm's law 
 
 E _g+f + W 
 e g+f + W 
 
 (f and / being the resistances of the battery and standard 
 cell respectively). 
 
 t For proof of 
 this formula, 
 see Solution 
 XVI. App. B. 
 
2O2 
 
 MANUAL OF TELEGRAPHY. 
 
 Fourth 
 Method, the 
 same with a 
 Shunt. 
 
 Fifth 
 Method 
 (Wheat- 
 stone's). 
 
 g+f + 
 
 Or, as e is the standard of unity 
 *_*+/ + 
 
 W 
 
 The above simple plan has the merit of being practi- 
 cable when only a galvanoscope is available. 
 
 283. Fourth Method. The same when a shunt is 
 necessary to bring the deflection produced by the stronger 
 battery within readable limits. 
 
 In this case, the expression g in the first circuit described 
 
 in the preceding para, becomes (s representing the 
 
 resistance of the shunt), according to the law of derived 
 circuits, 1 and the value of the deflection is multiplied by 
 
 , the multiplying power of the shunt 2 
 
 Thus : 
 
 +/+ W 
 
 e+s 
 
 +/'+ W s 
 
 284. Fifth Method. To measure the electromotive 
 force of a battery by Wheat'stone's system. 
 
 The battery, whose electromotive force E is to be 
 measured, is joined up with a galvanometer ; resistance 
 being inserted, if necessary, to produce a convenient deflec- 
 tion, say #, which is noted. 
 
 Resistance (iv) is then added until the deflection is 
 reduced to b, which reading is also noted. 
 
 Now replace E by e (the electromotive force of the 
 standard cell), and adjust the resistance in circuit so as to 
 reproduce the first deflection (a). 
 
 Then add resistance (w 1 ) until the second deflection 
 (b) is reproduced. 
 
 From Ohm's law it is obvious that 
 
 E 
 
 e ' 
 
 W_ 
 
 W 
 
 Or, as e is taken as unity, being the standard of com- 
 parison 
 
 * Where^-and 
 J/^are very 
 large compared 
 with/, the 
 latter may be 
 neglected. 
 
 1 Law 8, 
 App. A. 
 
 2 159- 
 
 . 
 
 w< 
 
Sixth 
 
 Method, by 
 opposing 
 equal Elec- 
 tromotive 
 Forces. 
 
 Seventh 
 Method 
 (Poggen- 
 dorff's). 
 
 TESTING. 
 
 285. Sixth Method. To measure the electro- 
 motive force of a battery by opposing to it a 
 battery of equal electromotive force. 
 
 Connect the poles of the battery to be tested with the 
 terminals of a galvanometer. 
 
 Then into this circuit insert cells of known electro- 
 motive force, with their poles in the opposite direction 
 to those of the battery under comparison, adjusting their 
 number until the deflection of the galvanometer is reduced 
 to zero. 
 
 Then, obviously, the electromotive force of the battery 
 under test is equal to that of the cells inserted. 
 
 286. Seventh Method (Poggendorff's). To measure 
 the electromotive force E of a battery by opposing 
 e, that of a standard cell or cells. 
 
 Join the poles of the battery to be tested (E) to an 
 adjustable resistance w. 
 
 To the terminals of w connect also another circuit, 
 consisting of a galvanometer, G, and a battery of smaller 
 electromotive force e (generally i cell), with its poles reversed 
 to those of E, as shown in the following figure : 
 
 203 
 
 FIG. 69. 
 
 Now, the direction of the currents of both batteries is 
 the same through w t but opposite through G. w forms a 
 shunt, by adjusting which the amount of current from , 
 flowing through c, can be regulated. By increasing w, 
 therefore, the deflecting current of E can be reduced to that 
 of e t when the needle will stand at zero ; in which case 
 
 (g+f) 
 
 (the strength of current from E, whose resistance is F) is 
 equal to 
 
204 
 
 SECTION 
 F. 
 
 
 
 Eighth 
 Method, 
 with a Re- 
 flecting Gal- 
 vanometer. 
 
 Regular 
 Tests of 
 Batteries. 
 
 MANUAL OF TELEGRAPHY. 
 
 e 
 
 (the current strength of e, whose resistance is / the resist- 
 ance of the galvanometer being g). 
 Whence 
 
 FW 
 
 gF + fF + gW 
 
 F + W 
 
 g+f 
 
 F + W 
 
 W+g+f 
 
 287. Eighth Method. To measure the electro- 
 motive force, E, of a battery by means of a re- 
 flecting galvanometer. 
 
 First join the standard cell e in circuit with the galvano- 
 meter and a certain resistance w> adjusted so as to cause 
 a readable deflection, a. 
 
 Then replace the starfdard cell by the battery E, and 
 observe the deflection produced, b. 
 
 Now, from the principle of the reflecting galvanometer, 1 
 the deflections a and b are proportional to the strength of 
 current in the first and second circuits respectively, i.e. 
 
 g 
 
 = a & 
 
 g+ 
 
 /. E - + 
 e F 4- 
 
 W+ F 
 
 g+ 
 
 Or if, as is probable, it be necessary to shunt the galvano- 
 meter in the second case (with the larger battery E\ to keep 
 the deflections within readable limits, 
 
 The expression g in the numerator becomes -, 
 
 g+ s 
 the value of the deflection b is multiplied by the coefficient 
 
 of the shunt g+ s * 
 s 
 
 288. Batteries are tested daily. When first set up, 
 each cell is tested in the battery room, both for resistance 
 and electromotive force, by one or other of the methods 
 
 156- 
 
 159- 
 
TESTING. 
 
 described in the preceding paragraphs, 
 apparatus available. 
 
 As the electromotive force of the Minotti reaches its 
 full extent immediately chemical action takes place in the 
 cell and remains constant, it becomes necessary to measure 
 the resistance only in taking the regular daily tests of 
 working batteries. 
 
 When the cells have been short-circuited l a sufficient 
 time to have reduced their resistance to the standard re- 
 quired for the purpose to which they are to be applied, they 
 are joined up together and the whole battery is tested in the 
 battery room. 
 
 The operation is then repeated in the instrument room, 
 and if the additional resistance of the leading wires makes 
 no appreciable difference (as it should not, these wires being 
 always as short as possible), further tests are continued at 
 the commutator. 
 
 In order that they may be made as expeditiously as pos- 
 sible, and that the whole of the office connections may be 
 included in the circuit, it is generally arranged as follows : 
 
 205 
 
 Line 
 
 FIG. 70. 
 
 The galvanometer G is joined up between the testing bar 
 of the commutator and the lower plate of the lightning dis- 
 charger, by which it is connected with the earth. 
 
 B and A' represent any line battery and signalling key 
 joined up in the ordinary circuit for s working. 2 
 
 In order to close the circuit of the line battery , it is 
 only necessary to throw off the line by removing the com- 
 mutator screw in x and replacing it in -/. 
 
 Then insert a plug in 2. 
 
 On depressing the key A~ the battery current traverses 
 the leading wires, key, commutator, galvanometer, and 
 
 180. 
 
20 5 MANUAL OF TELEGRAPHY. 
 
 lightning discharger earth, thence returning through the 
 battery earth to the zinc pole of the battery B. 
 
 As the natural tendency of working batteries is to fall in 
 resistance according to the consumptipn which goes on, any 
 sudden rise in resistance should be at once checked by a 
 test in the battery room, with a view to discovering whether 
 the increased resistance is due to an imperfect contact, or 
 to a defect in one of the leading wires. 
 
 If to neither, it will probably be traceable to one of the 
 causes mentioned in paras. 221, 222 (sec. E). 
 
 Strength of 289. The design of a line battery being the production 
 
 ri^^at^fe- of a current wnicn sha11 work a receiving instrument at the 
 
 tant ^Station, distant end of a telegraph line, it is of obvious importance 
 
 that the strength of the arriving current should be known, for 
 
 as the insulation of no line can be absolutely perfect, the 
 
 loss of current along it, due to leakage, must always render 
 
 the received current less than that which leaves the battery 
 
 at the transmitting end. 
 
 The unit current, that is, the standard by which cur- 
 rents are measured, has been already described as the Am- 
 pere or Oerstedt? such being the current produced by i i Def. 2. 
 volt (the unit of E.M.F.) through i ohm (the unit of resist- 
 ance). 
 
 As signalling currents, however, never exceed more than 
 a few thousandths of an oerstedt in strength, it is more con- 
 venient to express them in thousandths or millioerstedts 
 (moe). 2 
 
 It is found in practice that Siemens' relays work best 
 with a current of from 3 to 5 moe. 
 
 The minimum arriving current is thus fixed at 3 
 moe ; and when found to be less than that strength the 
 sending station is instructed to increase battery power, or if 
 this fails to bring the current up to 3 moe, an intermediate 
 station is called in to translate or repeat. 
 
 4 moe is considered the average, and 5 moe the 
 maximum arriving current. 
 
 If a current is received stronger than the maximum, the 
 sending battery is reduced until the current arriving falls to 
 about 4 moe, the average. 
 
 Measure- 2QO. The process by which these values of the strength 
 
 received * rece ^ ve ^ currents is determined may be described as 
 Currents. follows : 
 
TESTING. 2O7 
 
 Remembering that the current produced by i volt through SECTION 
 i ohm is called an oerstedt ; that is F * . 
 
 - 
 ohm 
 
 and that the E.M.F. of a Minotti cell is so nearly equal 
 
 to i volt that it is always taken as such ; * it follows that i 43. 
 
 iy___ = strength of current in oerstedts ; 
 
 ___ 
 
 Res. in circuit 
 Or 
 
 No. of cells x 1QOO= fa {n mi u ioer5 tcdts. 
 Res. in circuit 
 
 For example, the current from i cell through 300 ohms, 
 
 _^voU__ . 0033 oerste dts 
 300 ohms 
 
 or 3 '3 millioerstedts. 
 
 The current c arriving through any line is measured as 
 follows, by means of the departmental tangent galvano- 
 meter : 
 
 First, to form a standard of comparison, the current d 
 of the standard cell is measured through the amount of 
 resistance, w, necessary to produce, through the galvano- 
 meter, g (thin coil), a deflection of 45, whose tangent is 
 
 Supposing W=300 
 g = 100 
 /= 20 (res. of cell) 
 
 Then C' = 0'0023 oerstedts 
 ^20 
 
 = 2'3 moe, 
 
 which may be called the constant of the galvanometer ; and 
 this fixed value of d representing the strength of the con- 
 stant current, in moe, has only to be multiplied by the tan- 
 gent of the deflection (a), produced (in the same instrument, 
 of course) by any arriving current, to determine c, the 
 strength of that current in moe. 
 
 For, on the principle of the tangent galvanometer 
 
 C_ _ tan a _ tan a 
 C tan 45 T~ 
 
 :. C = C tan a. 
 
20 8 MANUAL OF TELEGRAPHY. 
 
 SECTION Now suppose, for example, the deflection a in the above 
 
 . F " __ . case to be 50. 
 
 Then c (the strength of the arriving current producing 
 this deflection), 
 
 = C tan 50 
 
 X 1 m 1()2 moe 
 = -^ moe. 
 
 Thus, the value of any arriving current is at once calcu- 
 lated from its deflection on the tangent galvanometer. 
 
 Again : 
 
 The constant value of c' for any galvanometer being 
 determined, it is easy to calculate the deflections which will 
 correspond to any given strength of arriving current in moe. 
 
 For as 
 
 C tan a 
 
 it follows that in the case of a current of $ moe, 
 
 _L = tan a 
 2-38 
 
 .'. tan a = 1-26, 
 
 which will be found from a table of tangents to represent an 
 angle of 52 (nearly). 
 
 Similarly for a current of ^ moe, 
 
 = tan 
 
 2-38 
 
 .'. tan a = 1'6$, 
 
 which is the tangent of 59 (about). 
 And for a current of '5 moe, 
 
 -5 = tan a 
 
 .*. tan a = 2'1, 
 
 which is the tangent of 65 (nearly). 
 
 Thus the galvanometer whose constant is 2*38 moe, as 
 described above, would indicate an arriving current of 
 
 3 moe by a deflection of 52 
 l> moe ., 59 
 3 moe 65 
 
Line testing 
 by Strength 
 of received 
 Currents. 
 
 TESTING. 
 
 In taking these measurements, the galvanometer is in- 
 serted between the receiving instrument and the earth. 
 
 2pl. The system of measuring received currents is ex- 
 tensively adopted in England, where it is used as a means 
 of estimating the insulation of the various lines. 
 
 The resistance of each line, and the E.M.F. of the bat- 
 tery used to work through it, being known, the maximum 
 current which can possibly be received is at once ascertained 
 by dividing the latter by the former : e.g. suppose a battery 
 of 10 cells to be used to work a circuit of 1,000 ohms ; 
 here 
 
 C = 
 
 10 
 
 1,000 
 
 = 0-01 oerstedts 
 
 Or 10 millioerstedts 
 
 209 
 
 Regular 
 Testing of 
 Lines. 
 
 Bridge used 
 for Testing. 
 
 which may be called the standard current of that circuit, i.e. 
 the current arriving when the insulation of the line is abso- 
 lutely perfect ; and any falling off from the standard current 
 (provided the battery is in order) must be due to leakage 
 along the line the greater the leakage, the greater being 
 the falling off of the strength of the arriving current from 
 the standard value ; thus the ratio between the arriving cur- 
 rent and the standard forms a comparative measure of the 
 state of insulation of the line. 
 
 292. In India, however, where a more perfect know- 
 ledge of the electrical state of the lines is required, to afford 
 data whereby faults occurring in long sections may be 
 localised electrically, regular tests of the actual conduc- 
 tion and insulation resistance of the lines are periodically 
 made and recorded ; whereby also the normal condition of 
 the line is constantly under observation, and a means is 
 thus afforded of often detecting irregularities before they 
 develop themselves into faults which would interrupt com- 
 munication. 
 
 293. The instrument universally used for regular test- 
 ing is the Wheatstone's bridge, described in para. 164. 
 
 The following figure represents the apparatus with the 
 connections and switches used for line testing. 
 
 -/ is a key by means of which the testing current can be 
 made to enter the bridge at m. 
 
 u is a battery reverser, by plugging 2 and 3 of which, 
 
 p 
 
210 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION the copper pole of the testing battery E is connected to m 
 Fj of the bridge. 
 
 A zinc current is applied by changing the plugs to 4 
 and 3. 
 
 8 
 
 Dislcml 
 Station 
 
 Adjustment 
 of Testing 
 Battery. 
 
 FIG. 71. 
 
 v is a switch by means of which the point n of the bridge 
 is connected either to the earth (plug in 7), or to the battery 
 (plug in 6), or to both battery and earth (plugs in 7 and $). 
 
 294. Another switch s (shown separately in fig. 72) is 
 often inserted between the block (z) of the battery reverser 
 17, and various zinc poles of the testing battery (e.g. the ist, 
 
 FIG. 72. 
 
 loth, and 3oth), so that by inserting a plug in 3 the E.M.F. 
 of the whole battery of 30 cells is applied, as required for 
 measurement of lines and other high resistances. 
 
 By changing the plug to e 1 the testing battery is re- 
 
TESTING. 
 
 211 
 
 Mode of 
 connecting 
 Lines to be 
 tested with 
 the Bridge. 
 
 The Conduc- 
 tion Test. 
 
 duced to 20 cells, for the measurement of lower resist- 
 ances. 
 
 When the plug is changed to e', then only one cell is in 
 circuit, forming a suitable battery for measuring very low 
 resistances. 
 
 295. It has been explained in para. 245 how any ordi- 
 nary metallic resistance x, joined up between the terminals 
 L and iJ of the bridge, can be measured. 
 
 The resistance of a telegraph line is measured in pre- 
 cisely the same way. 
 
 The end of the line at the testing station is joined to L 
 of the bridge : the farther end, although it cannot be brought 
 back direct to the L' terminal, is practically joined thereto 
 by using the earth as a return wire. 
 
 This is effected by connecting the distant end to the 
 earth, as shown in fig. 71, the return circuit to L' being 
 completed through the earth at the testing station joined to 
 the switch F, thence through the plug 7 to n and L' of the 
 bridge ; a plug being inserted in either # or 6 to connect 
 the testing battery with the earth. 
 
 296. Thus in the above circuit the resistance between 
 the points L and L' (to be measured) is that of the line 
 alone, the earth resistance being so small in comparison 
 that its value may be neglected. 1 
 
 The above measurement constitutes what is called the 
 Conduction Test, denoted by the letter ft). 
 Thus when balance is obtained 
 
 vfl 
 
 177 and 
 
 The Circuit 
 Test. 
 
 The Insula- 
 tion Test. 
 
 297. If the distant station inserts a relay between the 
 line and the earth at the point R (fig. 71), the resist- 
 ance under measurement between the points L and L' 
 comprises that of the line and relay, the measurement of 
 which constitutes the circuit test, denoted by the 
 letter 5181. 
 
 208. If, however, the distant station throws off earth by 
 insulating the end of the line, then the return circuit to L' 
 is altered, being no longer completed simply through the 
 resistance of the line wire and the insignificant resistance 
 between the earth-plates, but through each point of leakage 
 of the line to the testing office earth. 
 
 p 2 
 
2I2 MANUAL OF TELEGRAPHY. 
 
 SECTION The measurement in this case is that of the resistance of 
 
 . F ~ ^ the insulators, that of the wire being comparatively insignifi- 
 cant in the case of a short well-insulated line. 
 
 This measurement, which is called the insulation 
 test, is denoted by the letter g. 
 Objects of 299. Thus: 
 
 the Circuit -^ shows the measured conductor resistance of the line. 
 
 the measured insulation. 
 
 SO represents the measured resistance of the line and 
 
 distant relay together ; and by the difference, SO Jt), 
 
 affords a measure of the resistance of the relay itself. 
 
 Measured 300. This, however, is not the only object of the circuit 
 
 Relay Re- test j ts mos t important use being that the measured value 
 
 sistance an . 
 
 indication of of the relay R, as shown by the difference between SO and 
 insiSon^of ^' ^ orms a valuable criterion as to whether the measured 
 the Line. values of fl) and are correct. 
 
 For if SO tt), as found by the difference between the 
 measured values of the circuit and conduction tests, be 
 exactly equal to the known resistance of the relay inserted at 
 the distant end, then it follows that tt) represents the cor- 
 rect conductor resistance of the line. 
 
 This, however, is only the case when the insulation of 
 the line is very high. When the reverse is the case, the 
 testing current is shunted by the derived circuits formed at 
 each point of leakage, so that only a portion of it arrives at 
 the distant end ; the greater this leakage, the more nearly 
 will the value of tt) correspond with that of SO ; in other 
 words, the smaller will be the measured resistance of the 
 relay. 
 
 Thus it is that the measured value of the relay affords 
 an indication as to whether the measured resistance jt) re- 
 presents the real conductor resistance of the wire, or that of a 
 derived circuit made up of the wire and the paths of leakage. 
 
 In the same way, the measured value of SO W is a 
 check on the correctness of >, the measured insulation of 
 the line ; for if the measured resistance of the relay is much 
 less than its actual known resistance, it shows that the dif- 
 ference is due to escape of the current along the line, and 
 this may be so great that may not be a measure of the 
 insulation merely, as it should be, but that the actual re- 
 sistance of the wire may be so considerable in comparison 
 with the insulation that it cannot be neglected. 
 
Correction of 
 
 the measured 
 
 Values 
 
 rendered 
 
 necessary by 
 
 defective 
 
 Insulation. 
 
 Order of 
 Observations 
 made in 
 regular 
 Testing. 
 
 Adjustment 
 of Branch 
 Resistances 
 of Bridge. 
 
 Routine of 
 Testing. 
 
 Relay 
 Resistance. 
 
 TESTING. 
 
 301. In the above cases the values of f and tt> both 
 require correction : and the measured relay 212E tt) not 
 only points out when this is necessary, as explained above, 
 but, by the proportion it bears to the actual known value of 
 the relay, forms the basis on which the formulae for corrected 
 values are calculated, as described in paras. 311-315., 
 
 302. The regular tests of lines embrace the three 
 measurements described above, which are made in the fol- 
 lowing order : 
 
 I. Circuit. . . . 8181 
 II. Conduction . . . tD 
 III. Insulation . . 
 
 303. In taking the measurements of the circuit and con- 
 duction tests, equal branches are used in the bridge, A and B 
 being usually made 1,000 each, as the resistance of tele- 
 graph lines under test is generally between 1,000 and 10,000 
 units. 
 
 For the insulation test, however, where the absolute 
 insulation resistance of the line is often greater than 10,000 
 (the maximum resistance of the comparison coil w\ it is 
 necessary to adjust the branches A and B^ so as to gain a 
 multiplying power ; B being made 1,000 and A 100, when 
 the absolute insulation resistance of the line is between 
 10,000 and 100,000 units, in which case the resistance un- 
 plugged in wis multiplied by 10 ; or, when the resistance 
 
 to be measured is greater than 100,000, - is made ^^ -, in 
 
 A 10 
 
 which case the resistance unplugged in w is multiplied by 
 100. 
 
 304. The testing officer, before taking the circuit test, 
 informs the distant station by the signal * circuit] which is 
 replied to by the resistance marked on the relay in circuit. 
 
 Before the conduction test is taken, the signal ' Con- 
 duction minutes' is sent, which is acknowledged by the 
 readings of the wet and dry bulb thermometer. 
 
 The signal ' Insulation minutes ' is then given, and ac- 
 knowledged by the state of the weather at the distant station. 
 
 305. The importance of knowing the actual resistance 
 of the relay included in the circuit test, for comparison with 
 the measured value 812H tt>, has already been explained in 
 para. 300. 
 
 2I 3 
 
214 
 
 Reduction of 
 Resistances 
 to corre- 
 sponding 
 Units. 
 
 Reduction of 
 Resistances 
 to corre- 
 sponding 
 Tempera- 
 tures. 
 
 Positive and 
 Negative 
 Readings 
 differ owing 
 to Natural 
 Currents. 
 
 Calculation 
 of measured 
 Resistances 
 allowing for 
 the Effect of 
 Natural 
 Currents. 
 
 MANUAL OF TELEGRAPHY, 
 
 306. If the resistance given by the distant station be 
 expressed in B.A. units, and the comparison coil used for 
 measurement be composed of resistances in Siemens units 
 (as is the case with most, if not all, of the bridges used in 
 the department), it is necessary to reduce the B.A.U. to S.U. 
 by multiplying the former by i'o456, as i ohm or B.A. 
 unit = i -045 6 S.U. 1 
 
 307. The resistances marked on telegraph instruments 
 represent their value at 80 Fahrenheit, and as copper wire 
 increases in resistance about *2i per cent, with each degree 
 of temperature (Fahrenheit), it is necessary when accuracy 
 is required to reduce the marked resistances to their corre- 
 sponding value at the actual time of test. 
 
 This is done by the following formula : 
 
 ?/ _ rj+y-^),. 
 
 ' | v + (/ _ ^) a / *' 
 
 ^ 
 
 In the above 
 Rt = Resistance at temperature f '. 
 
 7~) __ j. 
 
 \. f .. . 
 
 1 Appendix 
 VIII. Vol. I. 
 Testing In- 
 structions, con- 
 tains a useful 
 table in which 
 the operation 
 is already per- 
 formed for 
 corresponding 
 values between 
 i and 100 units. 
 
 * For proof of 
 this formula, 
 see Solution 
 XXXVII. 
 App. B. 
 
 a = 'oo2i, the coefficient of increase of resistance of 
 copper for each degree (F.) of temperature. 
 
 308. The three measurements, viz. SJ, ft), and g, 
 from which the electrical condition of the line is known, are 
 all taken with positive and negative currents, the difference 
 between the + and readings representing a measure of 
 
 the strength of the natural currents existing in the line. 2 267 and 
 
 309. Expressing by w' the resistance unplugged in the Def - 29- 
 comparison coil w to obtain balance when testing with a 
 positive current, and w" the resistance unplugged when a 
 negative current is used, if the difference between w 1 -and 
 
 w" is small their mean is taken, i.e. : 
 
 B 
 
 and , the electromotive force of the natural current, is con- 
 sidered nil. 
 
 Rule. The difference between w' and w" is considered 
 small when less than 20% for resistances under 1,000 
 units, 
 
 or less than 15% for resistances under 5,000 units 
 , 10% over 5,000 
 
TESTING. 
 
 215 
 
 When the difference is greater than stated above, and e is SECTION 
 sufficiently large to be a multiple (N) of , the electromotive _ ; _ , 
 force of the testing battery e = NE, and the following for- 
 mula is used for the calculation of x, the resistance to be 
 measured : 
 
 w 
 
 + w' w" -w' * Int , his 
 
 , u 
 
 X = - - N formula the 
 
 2 2 branches A and 
 
 the value of N being as under : % e s ^PP ose( 
 
 _ __ ^L^' _ f t /being the 
 
 W" + W> + 2(A + *ff 
 
 For proof of 
 
 Electro- 310. When the strength of the natural current is con- these formulae, 
 
 ^e Force stable, as in the last case, its electromotive force e is cal- xvn^Ap a 
 Natural culated in terms of the testing battery E, and expressed by 
 the following formula : 
 
 _ W" W' _ E + t For proof of 
 
 w " 4. w > J_ 2(A {- 2f) ' this formula, 
 
 see Solution 
 
 The above calculations are only made in the case of the App. B. 
 circuit and conduction tests. 
 
 The simple mean of positive and negative readings is 
 always taken in the case of the insulation measurements, 
 for, as the distant end of the line is disconnected from the 
 earth, the difference between w' and w" is likely to be 
 the result more of electrolytic action l than of natural cur- l 242 (d). 
 rents. 
 
 Correction of 311. It has been explained, in para. 300, that in the case 
 measured o f p er f ec t insulation, $211 tt> (the difference between the 
 measured circuit and conduction resistances) is equal to x, 
 the actual resistance of the relay in circuit, in which case 
 the measured values may be accepted as correct, but in pro- 2 By an error 
 portion to the leakage of the line, so does TO - w become g^Kj^ 1 
 
 less than A'. 2 be greater than 
 
 To discover, therefore, whether corrections are necessary, ^Jg ajew^ 
 
 the first thing to do is to compare 212H tt) with /?, the case of high 
 
 actual resistance of the distant relay. 3 tfSSSi?* 
 
 If 512H tt> = ^> or very nearly, and at the same time circumstances, 
 
 the insulation is so great that the conduction resistance can be J^y^t? by 
 
 3 , the test should 
 
 neglected against it, that is, in practice, say when -g- > 36, be rejected. 
 
 the measured values suffice, and no corrections are neces- temperature of 
 sary. test : see 37- 
 
216 
 
 MANUAL OF TELEGRAPHY. 
 
 Corrected 
 Relay 
 Resistance 
 a Test of 
 Uniformity 
 of Line. 
 
 Correction 
 of measured 
 Values of 
 ' Conduc- 
 tion ' and 
 ' Insulation ' 
 in the case of 
 a uniform 
 Line. 
 
 312. When the above, however, is not the case, g, the 
 corrected value of the measured relay resistance, is calcu- 
 lated by the following formula : 
 
 - 
 
 As the above formula is based on the supposition that 
 the electrical condition of the line is uniform throughout its 
 length, it becomes, conversely, by its approximation to R 
 (the actual value of the relay), a criterion of the uniformity 
 of the line. 
 
 Thus, if the value found for^ agree within 15% of R (at 
 temperature of test), the line may be considered uniform, and 
 the resultant fault, consequently, at the centre of conduction, 
 i.e. / = I' (I and /' representing the conduction resistance 
 from testing station to resultant fault, and from resultant 
 fault to distant station respectively), as explained in Solution 
 XIX. App. B. 
 
 313. In this case, the actual corrected insulation 
 resistance of the line (/) is found by the following formula : 
 
 and 
 
 * For proof of 
 this formula, 
 see Solution 
 XIX. App. B. 
 
 ) f t For proof of 
 
 this formula, 
 
 iM=j (the corrected insulation per mile) see Solution 
 the corrected conductor resistance (L) of the 
 
 and 
 
 same line, by the following : 
 
 and T r L 
 
 Resistance 3 J 4' Or, expressing the same in terms of reduced 
 
 per Mile length, the convenience of which will be understood from 
 
 expressed in , , , . , r ,, 
 
 reduced tne remarks which follow : 
 Length. 
 
 Whence 
 
 m 
 
 m representing the reduced length in miles 
 per mile 
 
 K average conduction resistance per mile 
 
 M actual length in miles ; 
 
TESTING. 
 
 M 
 and, denoting by q the ratio , i.e. the average gauge 
 
 m 
 
 of the wire of which the line is composed, 
 
 217 
 
 The plan of expressing the average conduction resistance 
 per mile of the whole line in reduced length i.e. in terms of 
 No. i wire (Indian Iron Wire Gauge) is most convenient for 
 comparing the results of regular tests when a knowledge of 
 the average only is required, but is not a sufficient record 
 from which to calculate the distance of faults, unless it so 
 happen (which is rarely, if ever, the case) that the whole 
 line is composed of wire of the same gauge. 
 
 For the localisation of faults it is necessary to know the 
 reduced length of each gauge, which is found by dividing 
 its length by its I.W. gauge. 
 
 Thus the reduced length of a section containing two 
 
 miles of No. i wire I.W.G. would be - = 2 ; and to find M, 
 
 the reduced length, of a line containing various gauges, a', 
 a", a'", &c. (expressed by their numbers according to the 
 I.W.G.), of lengths /', I", I 1 ", c. respectively, 
 
 + &c. 
 
 It is most important that the various lengths and gauges of 
 wire in every line that is tested should be accurately known ; 
 which with their corresponding reduced length should be 
 recorded 
 
 The following will be found a convenient form : 
 
 LINE No. 
 
 FROM 
 
 TO 
 
 
 
 
 
 sections and 
 gauge of wire 
 (I.W.G.) 
 
 Actual length of 
 section 
 
 Actual distance 
 to end of section 
 
 Reduced 
 length of 
 section 
 
 Reduced dis- 
 tance to end 
 of section 
 
 () 
 
 co 
 
 (A) 
 
 (m= -a) 
 
 00 
 
 50 
 
 177-666 
 
 177-666 
 
 3 '5533 
 
 3-5533 
 
 24 
 
 9-250 
 
 186-916 
 
 0-3854 
 
 3*9387 
 
 6 
 
 6-048 192-964 
 
 1-0080 
 
 4-9467 
 
 This table shows at once in case of a fault (the conduction 
 resistance to which (L) has been found) in what section the 
 fault lies, for 
 
218 
 
 Measured 
 Values, how 
 corrected in 
 the case of 
 a line not 
 uniform in 
 Insulation 
 and Con- 
 duction. 
 
 MANUAL OF TELEGRAPHY. 
 
 f reduced distance ") _ L 
 \ to the fault J ~~ M 
 
 from which n (the actual distance) is calculated, by the 
 formula explained in para. 323 on ' Fault Testing.' l 
 
 315. If the value found for g (para. 312) is not within 
 15 per cent, of 21, it proves that the line is not uniform in 
 
 1 To find r, the actual resistance per mile, of any particular section of 
 wire, of length /, and diameter d, in a line made up of various lengths, /, /', 
 and diameters d, d r , from x, the measured absolute conduction resistance 
 of the whole line, 
 
 For, asTthe resistance of any wire is inversely proportional to the square of 
 its diameter [App. A, Law n (ii)], the resistance of any section 
 
 / /' 
 
 ;***#! 
 
 whence, the per-mile resistance of the same section 
 
 i r 
 T* + d^ 
 
 Similarly, for any other length /', of diameter d' 
 
 x 
 
 I I 1 
 
 or, in a line composed of any number of sections of wire, of diameters 
 d, d', d" ...... &c. 
 
 /, /', /" . . &c. 
 
 of lengths 
 
 to find the per-mile resistance of any particular gauge, it is only necessary 
 to divide x by the product of d' 2 (the square of the diameter of that parti- 
 cular gauge), and a constant c (c being the sum of all the lengths divided 
 by the squares of their respective diameters), i.e. 
 
 &c. 
 
Displace- 
 ment of 
 resultant 
 Fault. 
 
 Localisation 
 of resultant 
 Fault. 
 
 Section 
 Tests. 
 
 TESTING. 
 
 insulation and conduction, the measured values for which 
 are therefore corrected by the following general formula, in 
 which want of uniformity is taken into account : 
 
 = A/ 
 
 (g - 
 
 and j (the insulation per mile) is found by multiplying / (the 
 absolute insulation) by M (the actual length of the line), 
 
 And . (3 
 
 K being found by dividing L (the absolute conduction re- 
 sistance) by ;// (the reduced length of the line). * 
 
 316. If the value of the corrected relay g (para. 312) be 
 greater than R, the resultant fault is beyond the centre of 
 conduction ; if g be less than /?, the resultant fault is nearer 
 (towards the testing station) than the centre of conduction. 
 
 317. The real conduction resistance (/) of the line, from 
 the testing station to the resultant fault, is found by the 
 following formula : 
 
 -v>. representing its distance in miles. 
 K 
 
 Or, calculated from the measured values, 
 
 219 
 
 * For proof of 
 this formula, 
 see Solution 
 XX. App. B. 
 
 f For proof of 
 this formula, 
 see Solution 
 XXL App. B. 
 
 1 Def. 38. 
 
 =V ( 
 
 Further, the position / and the resistance z of the resul- 
 tant fault can be found by the following formulae : 
 
 Z = 
 
 G) 
 
 1 
 
 G) 
 
 y representing the resistance of any fault ; 
 
 r being the conduction resistance from the testing station 
 to the fault. 
 
 318. The conduction and insulation of separate sections 
 of the same line may be calculated from measurements 
 
 I For proof of 
 this formula, 
 see Solution 
 XIX. App. B. 
 
 See 
 
 Equation (3), 
 Solution XIX. ; 
 
 || For proof 
 of these 
 formulae, see 
 Solution XX II 
 App. B. 
 
22O 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION made at one end of the whole line by the following 
 formulae : 
 
 In the above figure A c represents a line consisting of 
 two sections A B and B c. 
 
 Let gj and |t)j represent the measured insulation and 
 conduction respectively of the section 
 A B ; 
 
 and 2 an d tt>2 tne same for the whole line, joined 
 direct at B ; 
 
 and and #) the required corresponding values for 
 the distant section B c, 
 
 Then, in the case of an imperfectly insulated line, the 
 measured values of which require correction, 
 
 tt> = 
 
 1 (n>a 
 
 -n> 2 
 
 * For proof of 
 these formulae, 
 see Solution 
 XXIV. 
 App. B. 
 
 Or when the measured values suffice, 
 
 Fault 
 Testing. 
 
 To discover 
 whether 
 Fault is in 
 Office. 
 
 = tt> 2 - n>i t 
 
 And 
 
 319. The faults to which the line circuit is liable l 
 viz. ' disconnections/ ' earths,' and contacts, total or partial, 
 are also common to the circuit of the office, including bat- 
 teries, instruments, earths, or connecting wires ; \hzfirst step 
 to be taken, therefore, on the existence of a fault becoming 
 known, is to ascertain whether it is in any part of the office 
 circuit, i.e. in the batteries, instruments, connecting wires, or 
 earth plates, or whether it is on the line, i.e. that part of the 
 circuit exterior to the terminal post of the office. 
 
 320. To ascertain this : First. Close the front contacts of 
 the signalling key by means of a lead pencil or the blade of 
 a penknife, without depressing the handle ; so that contacts 
 
 f For proof of 
 these formulae, 
 see Solution 
 XXIII. 
 App. B. 
 
 1 241. 
 
Nature of 
 Line Faults 
 and their dis- 
 tinguishing 
 Features. 
 
 TESTING. 
 
 1 and 2 of the key are both closed together. 1 If the sounder 
 works, it shows that there is no fault in the back contact of 
 the key, nor in the relay coils, nor in the circuit in which 
 they are joined, and that the relay contacts and every part 
 of the local circuit are in order. 2 
 
 Second. Take a regular test of the line battery at the 
 commutator, as explained in para. 288. If the deflec- 
 tion agrees with the regular readings recorded, it proves 
 that there is no fault in the battery, earth-plates, key, switch, 
 or contacts 1 and 2 of the commutator, nor in any of the 
 wires connecting them. 3 
 
 Third. Replace the commutator screw as usual, for 
 working ; disconnect the wire between the testing bar of the 
 commutator and the galvanometer, and join that terminal 
 of the galvanometer by a direct wire to the line at the ter- 
 minal post 4 
 
 Then depress the key ; if the deflection now observed 
 agrees with the former reading, it proves that the continuity 
 of the working bars of the commutator and that of the 
 upper plate of the lightning discharger, and of the leading 
 wire therefrom to the terminal post, is perfect, and that there 
 is no fault in any part of the office circuit between the 
 earth-plate and the terminal post. 5 
 
 321. The line must therefore be tested for earth? contact? 
 or discontinuity? as the case may be. 
 
 Any or all of these classes of faults may occur together ; 
 for example, a line may be broken, its ends to earth, and 
 the wire in contact with a working line. It must, therefore, 
 first be decided which class of fault to test for. 
 
 The case of a contact, when the signals of one line are 
 shown upon another, is sufficiently clear to distinguish it 
 from earth or disconnection. 
 
 The tester should, however, be sure that the duplicate 
 signals are due to contact and not to induction, which can 
 be ascertained at once by depressing the key on one line ; 
 if the signal produced on the instrument of the other line 
 continues so long as the key is depressed, there is contact ; 
 but if the signal is only a transient one, observed at the 
 instant of depressing and releasing the key, then induction 
 is the cause. 7 
 
 A discontinuity, however, in the case when both ends of 
 the broken wire rest on the ground, is liable to be mistaken 
 
 221 
 
 1 Fig. 45, 
 180. 
 
 2 Fig. 46, 
 181. 
 
 3 Fig. 70, 
 288. 
 
 4 The other 
 terminal of the 
 galvanometer 
 remains joined 
 to earth 
 through the 
 lower plate of 
 the lightning 
 discharger, as 
 before. 
 
 5 Where a good 
 line enters the 
 office, as well 
 as the faulty 
 line, the first 
 step taken may 
 be simply that 
 of looping the 
 two wires at 
 the terminal 
 post. If perfect 
 signals can be 
 exchanged 
 between the 
 instruments of 
 these lines, the 
 complete office 
 circuit must be 
 in order, and 
 the fault is on 
 the line. If the 
 contrary be the 
 case, the fault 
 inmost probably 
 in the office, 
 and should be 
 traced by the 
 processes de- 
 scribed above. 
 If not actually 
 in the office, it 
 will be found 
 close to it. 
 
 6 See 241, 
 Section E. 
 
 7 n8. 
 
222 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 F. 
 
 General 
 principle on 
 which Faults 
 are localised: 
 (i) In a Line 
 of uniform 
 Gauge. 
 
 (2) In a Line 
 composed 
 of Wires of 
 various 
 Gauges. 
 
 for an earth fault, for the result may be that the measured 
 conduction resistance to the fault is even less than the 
 normal value of the whole line, instead of being greater, as 
 would be the natural result of a more or less insulated 
 break. 
 
 It then becomes necessary to loop the faulty wire with a 
 good line at the distant station, and to measure the resist- 
 ance of the loop without any earth on the bridge. 
 
 If the resistance of the loop be higher than the normal 
 conduction resistance of the wires, then the fault must be 
 discontinuity with earth ; but if it agree with the normal 
 resistance, the fault is probably earth alone. 
 
 The latter result, however, might occur in the case of a 
 broken wire, the ends of which made perfect, i.e. dead earth 
 at the fault, which in this case would itself offer no resist- 
 ance. 
 
 It would, however, give rise to a strong natural current 
 of polarisation, which would distinguish it from earth on an 
 unbroken wire. 1 
 
 322. To find the distance, , of a fault from the 
 testing station, it is necessary first to measure the absolute 
 conduction resistance, x, of the line up to the fault ; and in 
 the case of a line of uniform gauge simply to divide this by 
 A-, the average resistance per mile ; 
 
 Thus: 
 
 n = miles. 
 K. 
 
 323. In the case of a line composed of various 
 
 gauges, however, the reduced length to the fault must be 
 found by dividing x by r, the reduced conduction resistance 
 per mile. 2 
 
 This reduced length is then compared with the last 
 column of the table (explained in para. 314), prepared 
 for the particular line under test, from which it is seen up 
 to what section the reduced length, thus found, extends. 
 
 then 
 
 Calling \ the actual 1 length up to the farther 
 and j. the reduced J end of the section 
 
 n S\ + 
 
 (a representing the number [I.W.G.] of the wire in which 
 the fault occurs). 
 
 1 In testing- 
 faults in which 
 natural cur- 
 rents exist, the 
 testing currents 
 should be made 
 as small as 
 possible, and 
 the readings 
 should be taken 
 as quickly as 
 possible, so as 
 not to polarise 
 the fault more 
 than can be 
 avoided. 
 
 2 314- 3i5- 
 
 * For proof of 
 this formula, 
 see Solution 
 
TESTING. 223 
 
 Dead Earth 324. To localise a dead earth it is sufficient to take 
 a si m pl e conduction test, x (mean of + and readings), 
 and if the line be of one gauge, 
 
 Or if composed of various gauges, 
 
 * For proof of 
 this formula, 
 
 as explained in para. 323. xxv luti n 
 
 Partial Earth 325. When the fault offers resistance, however, in which App. B. 
 wire Smle case it: is called partial earth, the above value for x would 
 obviously be too high. 
 
 In such a case, the resistance of the fault itself is an 
 important and often a varying element. 
 
 Further, it is subject to variation owing to polarisation, 
 caused by the testing currents. 
 
 To reduce the effects of this to a minimum, a testing 
 battery as small as possible is used, and + and readings 
 are taken, as explained in paras. 267 and 337. 
 
 Further, the reverse currents should be applied as fol- 
 lows : 
 
 First, a +, or copper current, should be kept on so long 
 as the resistance of the circuit is observed to increase ; } l Def. 26 and 
 when approximately constant, note the + readings, and I95- 
 apply the or zinc current, which will dissolve the copper 
 salt formed at the surface of the fault by electrolytic action, 
 and will gradually reduce its resistance until the metallic 2 There will be 
 surface becomes bare, when the resistance will of course be no difficulty in 
 at its minimum, and the reading should be noted at this JjJSkelhe 
 
 moment. 2 reading, as 
 
 As a partial earth admits of communication with the minimum is 
 distant station, the insulation i of the line is first measured, reached the re- 
 
 , sistance of the 
 
 and then the conduction resistance w. fault begins to 
 
 In this case, the resistance to the fault, rise a s ain ' . 
 
 owing to pola- 
 risation by the 
 
 \T \ j. 
 -W) (I- W) f 
 
 hydrogen being 
 
 L being the normal conduction resistance of the line, formed at the 
 
 taken from regular tests under corresponding conditions of Suit! 06 
 
 climate, &c. t For proof of 
 
 x may be still more accurately determined by taking a ^^[JJ,^' 
 
 circuit test w in addition to the above, xxvi. App. B. 
 
224 
 
 MANUAL OF TELEGRAPHY. 
 
 SECTION 
 F. 
 
 Earth on a 
 multiple line. 
 The Loop 
 Test. 
 
 When 
 
 -'-V 
 
 (I-W) (I-w 
 W-w 
 
 * For proof of 
 this formula, 
 see Solution 
 
 R denoting the resistance of the relay inserted, reduced XXVL A PP- R 
 to the temperature at which the test was taken. 1 1 3?- 
 
 n (the distance of the fault in miles) is found as ex- 
 plained in para. 322, if the line be of uniform gauge ; or, 
 as in para. 323, in the case of different gauges. 
 
 326. An earth dead or partial should, when a second 
 good line is available, always be tested by the loop 
 method ; for, as the following figure explains, the testing 
 current enters the fault at z, by both branches of the bridge, 
 in opposite directions ; its resistance, therefore, or any 
 natural current in it, can exercise no influence on the 
 test. 
 
 FIG. 73. 
 
 In the above figure, x represents the conduction resist- 
 ance to the fault through the faulty wire ; and y that of the 
 whole of the good line and the section of the faulty line 
 beyond the fault. 
 
 The two lines are looped at the distant end, care being 
 taken that the loop is perfectly insulated from the ground, 
 and the two near ends are brought to the bridge ; the 
 faulty wire to b and the good wire to L, so that x (which 
 must always be less than y, in the case of similar lines) 
 may be in the same branch as the comparison coil 
 w. The branches of the bridge must be always made 
 equal. The resistance of the comparison coil, w, is now 
 
TESTING. 
 
 225 
 
 adjusted till balance is obtained, with the testing battery 
 to earth : plugs in 
 
 f 1, 2, 3, for + current 1 
 U, 4, 5,$ - J 
 
 The resistance of the looped lines, L, is next measured, 
 with no earth on the testing battery : plugs in 
 
 f 1, 2, 3, 6 for + current "I 
 U, 4, 5, - / ' 
 
 The resistance up to the fault is at once found by the 
 
 following formula : 
 
 T TXT 
 
 X = Y * * For proof 
 
 2 of this formula, 
 
 see Solution 
 
 And #, the distance in miles, is calculated by the for- 
 mula either in para. 322 or 323, according as the line is 
 made up of wire of one or more gauges. 
 
 Contact on a 327. To localise a contact between two wires (when 
 
 First* G inC a third wi re is n t available for a loop\ it is necessary to 
 
 Method. connect one of the two wires in contact to L of the bridge, 
 
 the other to L', and to take two measurements of their 
 
 resistance, the first (^) when their distant ends are insulated, 
 
 and apart ; 
 
 The second (w) when their distant ends are insulated, 
 but looped together. 
 
 Let L ' and L " represent the real conduction resistance 
 of these lines, respectively, to the dis- 
 tant station, 
 
 and x ' and x " the conduction resistance, to the fault, 
 of the corresponding wires, 
 
 z the resistance of the contact itself. 
 Then if z = o (i.e. perfect contact), 
 
 Or expressing (x ' + x ") as x, which thus represents the 
 conduction resistance of the two wires between the testing 
 station and the contact, 
 
22 6 MANUAL OF TELEGRAPHY. 
 
 and if the lines are of uniform gauge, of resistances A* and 
 K ' per mile respectively, 
 
 n (the distance of the fault) =- > - 7/ (miles). 
 
 Or, if K ' and K " are equal, 
 
 W 
 
 In the above, the lines in contact have been considered 
 as being of equal length ; but if, in consequence of a diver- 
 sion, one be longer than the other ; for example, suppose 
 the wire whose resistance to the fault is expressed by x " to 
 be longer than x ' by, say, d miles, then 
 
 X= x'+ x"= nK + (n + d) K" 
 X-dK" 
 
 And when the lines are composed of various gauges, 
 
 X-8r" . 
 
 8 being the reduced length of the diversion, the other 
 letters of the formula corresponding with those explained 
 in para. 323. 
 
 We have now to consider the case in which the contact 
 itself offers resistance, as indicated by the measurement w 
 being greater than w. 
 
 In this case, 
 
 X W */(R w} ( W w}* * For proof 
 
 of this formula, 
 
 /? being the sum of the resistances L' + L". 
 
 n (the distance to the fault) is found precisely as in the A PP- B - 
 
 preceding cases explained in this paragraph. 
 
 Correction 328. When the resistance of the contact (z) is not zero, 
 
 for Leakage. an( j ^Q insulation of the lines in contact is imperfect, a 
 
 second fault (/) viz. earth due to leakage is introduced, 
 
 the magnitude of which depends upon the ratio Z -. 
 
 When this is considerable, the value obtained for x, by 
 the formula in the preceding paragraph, requires correction 
 as follows : 
 
TESTING. 
 
 227 
 
 Expressing by e the ratio - 
 
 and by 
 
 5) 
 
 Then 
 
 i' the corrected absolute insulation of one 
 
 line, 1 
 
 /" that of the other, 1 
 /' the resistance of the first line up to the 
 
 resultant fault due to leakage, 1 
 I" the same for the second line, 1 
 /the sum of /' + /". 
 
 1 Known from 
 record of pre- 
 vious tests. 
 
 and the combined resistance (z') of the two faults / and z 
 becomes 
 
 whence 
 
 _-_ 
 
 +* 
 
 
 * Law 8, 
 App. A. 
 
 and the corrected value for x becomes 
 
 X = {w- 
 
 Second 
 Method : 
 Contact on 
 a two-wire 
 line localised 
 by the Loop 
 Method. 
 
 -w) (W-w}}(i + e)cl. 
 
 329. A contact may be localised on the principle 
 of the loop test as follows : 
 
 
 M^ 
 
 
 
 \/ 
 
 j 
 
 2*^~ 
 S ^ Jt 
 
 
 
 /\ 
 
 II 
 
 "Xaw 
 
 yN-ii" 
 
 E 
 P 
 
 
 7 
 
 
 W 
 
 FIG. 74. 
 
 The end of one of the wires in contact is brought to L 
 of the bridge. 
 
 Q 2 
 
228 
 
 MANUAL OF TELEGRAPHY. 
 
 Third 
 Method : 
 Contact 
 localised by 
 the Loop 
 Method 
 when a third 
 good wire is 
 available. 
 
 The distant end of the same wire is joined to earth. 
 
 The distant end of the other wire is insulated, the home 
 end being connected to the testing battery (through the 
 battery reverser), as shown by the dotted line. 
 
 L 1 of the bridge is joined to earth through plug 7. 
 Then, when balance is obtained (branches A and B being 
 equal, as they are always made in the loop test), the testing 
 current enters the contact from the A branch through w 
 and y, and from the B branch through #, so that x = w +y, 
 
 Then disconnecting the second wire from the battery (both 
 ends insulated), and taking an ordinary conduction test of 
 the wire connected to L, and calling the resistance un- 
 plugged to obtain balance in this case w 1 
 
 x+y = W. . . . (2) 
 But, from equation (i), x y = W. 
 Whence, by adding (i) and (2), 
 
 x W + W = resistance to fault, 
 
 and 
 
 x 
 
 n = -- miles, f 
 J\. 
 
 Or, in the case of a line of different gauges, 
 
 330. When a third good wire is available the 
 
 following connections are made : 
 
 The good wire and one of the faulty wires are joined to 
 L and L' of the bridge respectively, their other ends being 
 looped at the distant station. 
 
 The second faulty wire is joined to the testing battery, 
 its distant end being insulated. Then, at balance (A and B 
 being equal), x + w'= y ; 
 
 * If balance is 
 not obtainable, 
 with line to L, 
 and earth to L', 
 reverse the con- 
 nections. But 
 if balance is 
 obtainable in 
 both cases, the 
 best value of x 
 is obtained by 
 taking the 
 measurement 
 first with line to 
 /- and earth to 
 L', and then 
 vice versd, w 
 being taken as 
 the mean of the 
 two measure- 
 ments. 
 
 t 322- 
 
 t 323- 
 
 y - X = 
 
 (I) 
 
 Then, disconnecting the second faulty wire from the battery 
 
TESTING. 
 
 )oth 
 circuit of the loop test, and 
 
 Subtracting (i) from (2) 
 
 229 
 
 reverser and leaving both ends insulated, we have the SECTION 
 
 (2) 
 
 * 326. 
 
 Whence , the distance in miles, is found as described in 
 the preceding case (para. 329). 
 
 Discon- 
 nection. 
 Localisation 
 by Insula- 
 tion Test. 
 
 Localisation 
 by Capacity 
 Test. 
 
 FIG. 75. 
 
 331. A perfectly insulated break can be localised 
 by an ordinary insulation test, on the principle that the 
 length of line to the fault (n) is to the length of whole line 
 (jv) as the absolute insulation of the whole line (/) l is to 
 the absolute insulation of the section up to the fault, i.e. 
 
 n 
 
 'N 
 
 n = , 
 i 
 
 It is assumed in the above case that the insulation of the 
 line is uniform throughout its length. 
 
 332. In the case of a break in a single wire the method 
 of localisation by the insulation test is open to the objection 
 that, owing to the great changes which the normal insulation 
 of lines undergoes, it is very difficult to decide what value of 
 
 1 Known from 
 regular tests 
 taken under 
 similar condi- 
 tion of weather 
 and time of 
 day ; or if there 
 be a second 
 good wire by 
 comparison 
 with tests of 
 that, at the 
 time of fault 
 test. 
 
230 
 
 SECTION 
 F. 
 
 MANUAL OF TELEGRAPHY. 
 
 i to select from the regular tests as most likely to correspond 
 with the state of insulation of the line when the fault test is 
 taken. 
 
 In such a case the capacity test is of great advantage 
 as a check on the results obtained by the insulation tests. 
 
 It is based upon the principle that the length of line 
 up to the fault (n) is to N the length of the whole line as 
 the inductive capacity of the line up to the fault (s) is to 
 that of the whole line (s), 1 i.e. 
 
 334- 
 
 Imperfectly 
 
 Insulated 
 
 Break. 
 
 Measure- 
 ment of 
 Capacities. 
 
 s being known by regular capacity tests recorded in terms 
 of a standard condenser. 
 
 333. In the case of a break, when the end of the wire is 
 not perfectly insulated from the ground, correct localisation 
 from the testing station is impossible, as there is no means 
 of ascertaining what proportion of the measured insulation 
 is due to the fault itself and what to the line ; thus, a 
 distant fault offering considerable resistance may be mis- 
 taken for a near fault offering infinite resistance (i.e. an 
 insulated break). 
 
 Here, again, the capacity test forms a useful criterion, 
 for a near disconnection is generally characterised by high 
 insulation and low capacity, and a distant disconnection by 
 low insulation and high capacity. 
 
 334. The capacity of a line is proportional to the 
 quantity of current (Q) with which it can be charged by a 
 given electromotive force. 
 
 The amount of this current is proportional to the transient 
 deflection or throw of the needle of a galvanometer placed 
 in its circuit. This first swing is, of course, its maximum 
 deflection, which we may call a. 
 
 Again, the force which produces this deflection (i.e. the 
 current of charge or discharge) is proportional to the sine of 
 half the angle of deflection. 2 
 
 Whence, 
 
 Q = sin \ a. 
 
 Thus the capacity s, of a line or condenser, can be measured 
 
 2 This follows 
 the law relating 
 to a force 
 actuating a 
 pendulum, to 
 which the 
 momentary 
 current of 
 charge or dis- 
 charge deflect- 
 ing the needle 
 to its full ex- 
 tent may be 
 compared. 
 It is assumed 
 that the motion 
 of the needle is 
 not sensibly 
 impeded by 
 friction, or by 
 the resistance 
 of the air. 
 
TESTING. 
 
 either by the current of charge or discharge. The following 
 figures will sufficiently explain the circuit in either case : 
 
 FIG. 76. CHARGE. 
 
 Make momentary contact (i) and 
 observe the throw of the needle 
 
 FIG. 77. DISCHARGE. 
 
 With key at rest, the current is charging 
 the line (through 2). Allowing time 
 for the line to be fully charged, make 
 momentary contact (i) and observe a. 
 
 Then, in either case, as explained above, 
 Q = sin \ a. 
 
 Suppose the above to represent the charge of a con- 
 denser of capacity c, with which it is desired to compare the 
 unknown capacity, s, of a line, and supposing a' to repre- 
 sent the throw of the needle in the case of the line (the dis- 
 tant end of which is of course insulated), the current being 
 denoted by Q'. 
 
 S _ Q' _ sin i- a' 
 
 C ~ Q ~~ sin \ a 
 
 Then 
 
 and 
 
 c 
 
 o 
 
 sin 
 
 or, with unequal electromotive forces, E and E', in the first 
 and second cases respectively, 
 
 231 
 
 SECTION 
 F. 
 
 Q 
 
 sn a 
 
 C E'Q E' sin a 
 
 stn a 
 
 I T 
 
 ' sin a 
 
 C 
 
 It is presumed that the above readings, a and 0', are 
 taken at the same time and with the same galvanometer, so 
 that the constant of the instrument is the same in both cases. 
 
232 
 
 SECTION 
 F. 
 
 Earth 
 Testing. 
 
 MANUAL OF TELEGRAPHY. 
 
 The state of insulation of the line under test has an im- 
 portant influence on the results of capacity tests, for leakage 
 obviously tends to increase the deflection when the current 
 of charge is applied, and to decrease it in the case of the 
 discharge current j hence, to reduce this source of error to 
 a minimum, it is advisable to take both charge and discharge 
 tests and to take the mean of a series of successive deflec- 
 tions of both, alternately. 
 
 335. The importance of a good earth has been already 
 remarked upon (para. 178). 
 
 The maximum resistance a telegraph earth should offer 
 is TO units. 
 
 As every office is provided with two earths, 1 it is easy to 
 test whether they fulfil the above condition by measuring 
 their total resistance by any of the simple methods described 
 in the preceding paragraphs, 2 their two leading wires forming 
 the terminals of the circuit (x + y) to be measured. Call 
 this w' . 
 
 . Then, if w' be less than 10 units, of course the resistance 
 of either of the two earths x or y alone must be within the 
 required limit. 
 
 This result does not, however, show how much of the 
 resistance measured is due to one earth and how much to 
 the other. 
 
 In order to ascertain this, it is necessary to use a third 
 earth (z) for comparison, so that by measuring each pair of 
 earths together we obtain the following equations : 
 
 x + y = w' 
 x + z = w" 
 
 y + Z = W'". 
 
 Care must be taken that there is no metallic connection 
 between one earth and another, for the object of the test, 
 which is to measure the resistance offered by the surface 
 of the soil to the diffusion of the current from each plate, 
 would then of course be defeated. 
 
 The absence of natural currents between any two earth 
 plates at once indicates that the plates are in metallic con- 
 tact, unless the measurement proves the resistance between 
 them to be so great as to account for the natural current not 
 being observed. 
 
 182. 
 
 243-266. 
 
First 
 Method : 
 with Tangent 
 Galvano- 
 meter and 
 no Battery. 
 
 TESTING. 
 
 The resistance of the leading wires is, of course, always 
 as small as possible. 
 
 336. When the E.M.F. of the natural current between 
 each pair of earth plates is sufficiently strong, it is used as 
 its own testing battery, whereby the polarisation likely to 
 result from the use of a galvanic cell is avoided, together 
 with the inaccuracy attendant thereon. 1 
 
 To measure earth resistance when the natural 
 current between each pair of earths is sufficiently 
 strong to produce readable deflections (say not less 
 than 10) with the tangent galvanometer. 
 
 Note the mean of + and readings, obtained from 
 each pair of earths, both through the thick and thin coils of 
 the galvanometer. 
 
 a representing deflections through the thick coil 
 a' thin 
 
 in the case of each couple. Then, as explained in para. 
 263, the resistance of each pair of earths 
 
 tan a 
 ~ tan a' ' 
 
 w' or w" or w'" = 
 
 tan a 
 ion a r ~ 
 
 From which the separate values of x, y, and z are found, as 
 follows : 
 
 / _j_ /' 
 
 t 
 
 w' + w'" - w" 
 
 
 w 
 
 > _ w < 
 
 325- 
 
 * For proof of 
 this formula, 
 see Solution 
 XL App. B. 
 
 f For proof of 
 these equa- 
 tions, see Solu- 
 tion XXIX. 
 App. B. 
 
 Second 
 Method : 
 with the Tan- 
 gent Galva- 
 nometer and 
 a Testing 
 Battery. 
 
 337. In the case when the natural current be- 
 tween each pair of earth plates is not sufficiently 
 strong to produce readable deflections through the 
 thin coil, it becomes necessary to use a testing battery. 
 
 This should be as small as possible, in order that polari- 
 sation of the plates may be avoided, 2 and, with the same 
 object, the tests should be executed as rapidly as possible. 
 
 The standard cell, of resistance /, will, as a rule, suffice, 
 and the readings should be taken through the thick coil 
 (resistance g), with no external resistance. 
 
 240. 
 
234 
 
 Third 
 Method : 
 with the 
 Calibrated 
 Galvano- 
 scope. 
 
 Fourth 
 Method : 
 with the 
 Wheatstone's 
 Bridge and 
 no Testing 
 Battery. 
 
 MANUAL OF TELEGRAPHY. 
 
 Call the deflection resulting from the above circuit, 
 c, which should be measured before and after the earth 
 circuits. 
 
 Then insert each pair of earths (w\, and note the mean 
 of the deflections in each case, with opposite currents 
 
 * 
 
 Then 
 
 w' or w" or w"' = 
 
 tan C \ 
 
 tan a + tan b " (/+ g).' 
 
 From which the separate values of x, y, and z are found, as 
 explained in para. 336. 
 
 338. To measure the resistance of earths with 
 the calibrated gal vanoscope, first join up the instrument 
 in circuit with the standard cell and the two leading wires 
 to be used, and by comparing the deflections (mean of + 
 and readings) with the calibration table, 1 note the re- 
 sistance (r) due to the battery, leading wires, and galvano- 
 meter, which make up this circuit, and deduct it from the 
 subsequent value obtained in measuring each pair of earth 
 plates. 
 
 Then separate the two ends of the leading wires, which 
 are joined to each other, connecting the free end of one to 
 one earth plate (x, say) and the other to y. 
 
 Then observe the decreased deflection (mean of + and 
 readings), and, comparing it again with the table, note 
 the increased resistance due to the addition of the pair of 
 earths (w') t remembering to exclude the value noted for r. 
 
 Repeat the same operation for each pair, viz. w" and 
 w'", and from these measurements find the value of x, y, 
 and z, as explained in para. 336. 
 
 339. When a Wheatstone's bridge is available, and the 
 natural current between each pair of earths is sufficiently 
 strong to render a testing battery unnecessary, Mance's 
 method, described in para. 276, is the simplest and most 
 accurate. 
 
 Each pair of earths is successively joined up between the 
 terminals L and L' of the bridge ; and 
 
 * Each pair of 
 earths should 
 be inserted, 
 first with one 
 to the + and 
 the other to the 
 pole of the 
 testing cell, 
 and then 
 vice versa. 
 
 t For proof 
 of this formula, 
 see Solution 
 XXX. 
 App. B. 
 
 147- 
 
 w' or w" or w'" = W 
 
Fifth Method : 
 with the Diffe- 
 rential Galva- 
 nometer and 
 no Testing 
 Battery. 
 
 Sixth Method : 
 with the 
 Wheatstone's 
 Bridge and 
 a Testing 
 Battery. 
 
 Seventh 
 Method : with 
 the Differential 
 Galvanometer 
 and a Testing 
 Battery. 
 
 To Test 
 
 Lightning 
 
 Conductors. 
 
 TESTING. 
 
 w being the mean of + and readings ; and the separate 
 values for x, y, and z are found as explained in para. 336. 
 
 340. The differential galvanometer may be used 
 for the above test, in which case 
 
 w 1 or w" or w'" = W* 
 
 341. In measuring the resistance of earths with 
 a Wheatstone's bridge in the case when a testing 
 battery is necessary, it is reduced to one cell, 1 and each 
 pair of earths is successively joined up between the terminals 
 L and L! of the bridge, and measured with + and cur- 
 rents like any ordinary resistance ; and 
 
 w' or w" or w 1 " = ? A W*t 
 A 
 
 x,y, and z being found as explained in para. 336. 
 
 342. The same measurements may be made with the 
 differential galvanometer, in which case 
 
 w 1 or w" or w'" = W,% 
 
 x, y, and z being found as before. 
 
 343. The principal object of a lightning conductor being 
 to afford a path of least resistance to the discharge (either 
 disruptive or silent) of the electricity of the clouds to the 
 earth, it is obvious that the conductor should be continu- 
 ous, and that it should make as perfect a connection with 
 the earth as possible. 
 
 The foregoing rules for earth testing apply, therefore, 
 to the measurement of the resistance of lightning 
 conductors. 
 
 Looking upon the conducting rod and its earth (x) as 
 one, a leading wire should be first connected with the point, 
 so as to include the whole of the rod in the circuit, and 
 another leading wire to each of two separate earths, y and z, 
 successively. 
 
 If the resistance be found unduly high, then disconnect 
 the leading wire from the upper extremity, and join it to the 
 rod just above the level of the ground, and repeat the test ; 
 if the resistance in circuit be now reduced, there is a fault 
 in the rod ; if, however, it be found the same as before, the 
 rod is perfect, and the fault is underground, being probably 
 due to imperfect earth. 
 
 235 
 
 SECTION 
 F. 
 
 * 248. 
 1 If this is not 
 sufficient to 
 overcome the 
 natural current 
 opposing either 
 measurement 
 of the battery 
 current, two or 
 more cells may 
 be joined 
 parallel, so as 
 to keep the 
 resistance of 
 the battery 
 down. 
 
 t 
 
 J 248. 
 
236 
 
 SECTION 
 F. 
 
 Tests of 
 Instruments 
 and Con- 
 nections. 
 
 MANUAL OF TELEGRAPHY. 
 
 Faults in the rod are likely to be due to bad joints ; 
 hence the necessity for carefully testing them when the rod 
 is first constructed, and if possible before erection, when it 
 should be ascertained that every joint is soldered. 
 
 The points attached to the upper part of the rod, the 
 object of which is to facilitate the combination of the oppo- 
 site potentials of the clouds and the ground by silent dis- 
 charge, should also be in perfect contact with the rod, 
 and should be kept free from corrosion. 
 
 In testing the efficacy of lightning conductors it should 
 be ascertained that they are perfectly connected with 
 all masses of metal about the building, such as iron 
 pipes, gutters, and other lightning rods, if more than one is 
 used. 
 
 In the case of the earth connection being found defec- 
 tive, the precautions recommended with regard to a tele- 
 graph earth may be observed with advantage. 1 
 
 344. The electrical resistance of instruments 
 and connections can be measured, according to the 
 apparatus available, by either of the methods described in 
 paras. 245, 248, 250, 258, 260, 265, by which also the 
 efficiency of the insulation of those parts which are pur- 
 posely kept free of contact with one another may be 
 tested. 
 
 The range of instruments is ascertained by the pro- 
 cesses described in para. 71. 
 
 The resistance (w) to be added in order to give any 
 range (n) may be calculated by the following formula : 
 
 278. 
 
 * For proof 
 of this formula, 
 see Solution 
 
 71- 
 
 p representing the number of cells used (without external App> B . 
 resistance) to produce the strong current c ; 2 
 
 q the number of cells used (with external resistance w) 
 to produce the weak current c ; 2 
 
 R the resistance of the instrument ; 
 
 /the average resistance per cell (which may be neglected 
 when R is large). 
 
 In the case when only one fixed resistance w is avail- 
 
 able, it becomes necessary to adjust the ratio ^, i.e. the 
 
TESTING. 237 
 
 relative battery power used in the two circuits, to find #, 
 the required range. 
 
 Here p- H( 1 R __ * For proof of 
 
 R + W ( n 1)qf this formula, 
 
 see Solution 
 XXXII 
 
 Usually q = 1 for instruments whose resistance is under App. B.' 
 1,000 units, and 2 for greater resistances. 
 
 Capacity is ascertained by the mode explained in 
 para. 334, and the prolonging effects of extra currents 
 as described in para. 124. 
 
APPENDIX A. 
 
 LAWS AND PRINCIPLES 
 
 I- 7. LAWS OF CURRENTS 
 
 8-12. LAWS OF CIRCUITS 
 13-19. LAWS OF ELECTRO-MAGNETISM 
 20-22. LAWS OF INDUCTION 
 23-26. LAWS OF MAGNETISM 
 
LAWS AND PRINCIPLES. 
 
 LAWS OF CURRENTS. 
 
 1. Ohm's 
 Law. 
 
 (i.) THE strength or intensity of a current c is equal to 
 the electromotive force E divided by the total resistance in 
 circuit R : 
 
 (ii.) Or, letting ?- represent the external resistance and/ 
 the internal resistance of each battery cell joined in series, 
 and expressing by n the number of cells (of resistance/), 
 
 C = 
 
 2. Kirch- 
 hoff' s Laws. 
 
 3. Bosscha's 
 Laws, or 
 Corollaries 
 to Kirch- 
 hoff ' s Laws. 
 
 (iii.) If the n cells are joined parallel, 
 
 nE> 
 
 ^. __ 
 nr+f 
 
 (i.) The sum of the current strengths in all those wires 
 which meet in a point is equal to nothing. In other words, 
 the sum of all the currents approaching the point is equal 
 to the sum of those going away from it. 
 
 (ii.) In any inclosed figure the sum of all the products 
 of the currents into the resistances of the wires conveying 
 them is equal to the sum of all the electromotive forces in 
 the circuit. 
 
 (i.) If in any system of circuits containing any electro- 
 motive forces there is a conductor in which the current = o t 
 the currents in the remaining circuits are not altered if the 
 circuit of the conductor in question is taken away, together 
 with the electromotive force contained in it. 
 
 (ii.) If the conductor in question contains no electro- 
 motive force it may be removed, and the points between 
 which it previously existed may be connected directly with 
 
 R 
 
 * Because in 
 this case 
 
 E 
 
 nE 
 nr+f 
 
242 
 
 4. Laws of 
 Mutual 
 Action of 
 Currents. 
 
 MANUAL OF TELEGRAPHY. 
 
 each other without affecting the currents in any part of the 
 circuit. If, on the other hand, it contain an electromotive 
 force, an equivalent electromotive force must be inserted 
 between the points before they can be joined again. 
 
 (iii.) In a system of linear conductors containing electro- 
 motive force, the current set up in any conductor a by an 
 electromotive force contained in any other conductor b will 
 be identically the same as that which would be set up in b 
 by an equal electromotive force in a. 
 
 (iv.) If in a system of linear conductors there are two 
 of them, a and I, in which an electromotive force in a 
 produces no current in b, then a may be divided or re- 
 moved without altering the current in b, and likewise b 
 may be divided or removed without altering the current 
 in a. 
 
 (i.) Two parallel wires in which currents flow in the 
 same direction attract one another ; or, more simply, 
 * Parallel currents in the same direction attract one another? 
 
 (ii.) ''Parallel currents in opposite directions repel one 
 another? 
 
 (iii.) When the wires are straight, but not parallel, they 
 (i.e. the currents in them) attract one another, if both 
 currents flow towards or away from the point of con- 
 vergence ; 
 
 (iv.) But repel one another if one current flows towards 
 and the other from the point of convergence. 
 
 Thus, it is obvious that the tendency of angular currents 
 is to become straight, so that, in the following figure, the 
 points a and c would tend to coincide ; likewise b and d. 
 
 FIG. 78. 
 
 [If two wires conveying currents be coiled, the above 
 effects are multiplied in proportion to the number of con- 
 volutions ; and if one coil be large enough to embrace the 
 other, the larger will, if the currents are flowing in the same 
 direction in both, draw the latter into it ; but if the currents 
 are flowing in contrary directions through the coils, repulsion 
 will take place between them. 
 
 A magnet presented to the interior of the larger coil will 
 behave similarly to the smaller coil, bearing out the resem- 
 blance between magnets and solenoids.] 1 
 
 Def. 25. 
 
5. Maximum 
 Current of a 
 Battery. 
 
 6. Laws of 
 Current 
 Strength 
 
 in Derived 
 Circuits. 
 
 7. Laws of 
 Electrolytic 
 Decomposi- 
 tion. 
 
 LAWS AND PRINCIPLES. 
 
 In a telegraphic circuit the best effect is obtained from 
 the battery when the internal resistance of the battery 
 approximates most nearly that of the circuit exterior to it. 
 
 (i.) The sum of the currents in the derived parts of a 
 circuit is equal to the strength of the main current. 
 
 (ii.) The strength of current in each branch of a derived 
 circuit is inversely as the resistance of the branch. 
 
 (i.) The quantity of any particular electrolyte decom- 
 posed in a given time is proportional to the strength of the 
 electrolysing current. l 
 
 (ii.) With the same current the quantities decomposed 
 of various electrolytes are proportional to their chemical 
 equivalents. 
 
 243 
 
 1 Def. 26. 
 
 LAWS OF CIRCUITS. 
 
 8. Combined (i.) The joint resistance R of any two branches of a 
 Resistance of derived circuit 2 of resistances x and y respectively is equal 2 Defs. 21, 22. 
 CircuTor to tne product of the two resistances divided by their sum, 
 Multiple i.e. 
 
 Arc. R = _^__.* 
 
 X +/ 
 
 (ii.) Denoting by R the joint resistance of any three 
 circuits, x, y, and 0, 
 
 * For proof, see 
 Solution I. 
 App. B. 
 
 xy + yz + #s 
 
 i.e. their product divided by the sum of the products of 
 each pair. 
 
 (iii.) The joint resistance of any number of resistances 
 in derived circuit is found by dividing unity by the sum of 
 their reciprocals : 
 
 p- 1 
 
 * 
 
 t For proof, see 
 Solution II. 
 App. B. 
 
 For proof, see 
 Solution III. 
 App. B. 
 
 [The reciprocal of any number is 1 divided by that 
 number.] 
 
 [Conductivity is the reciprocal or converse of resistance, 
 and vice versa thus, if x = the resistance of a circuit, 
 
 - = its conductivity or conduction.] 
 
 oc 
 
 9. Multiply- The multiplying power of any shunt of resistance s on 
 ing Power of a galvanometer of resistance G is represented by the follow- 
 ing expression : 
 
 + , 
 
 ~5~' $ 
 
 R 2 
 
 For proof of 
 this formula, see 
 Solution IV. 
 App. B. 
 
244 
 
 10. Resist- 
 ance of 
 Shunts. 
 
 11. Resist- 
 ance of 
 Telegraph 
 Wire. 
 
 12. Laws 
 which govern 
 Speed of 
 Signalling. 
 
 MANUAL OF TELEGRAPHY. 
 
 To prepare a galvanometer shunt having a multiplying 
 power n, the resistance of the shunt must be 
 
 n 1 
 
 (i.) The resistance of any wire of one quality is directly 
 proportional to its length. 
 
 (ii.) It is inversely proportional to its weight per mile, 
 and also inversely proportional to the square of its diameter. 
 
 (i.) In cables (or well-insulated land lines) of similar 
 make, the speed of signalling in each, s and s f , is inversely 
 proportional to the squares of their respective lengths ; thus 
 
 * For proof, see 
 Solution V. 
 App. B. 
 
 S' ~ ' I*' 
 
 (ii.) If the lengths are the same, the speed is proportional 
 to the expression 
 
 X lo. 
 
 Where d represents the diameter of the conductor, and D 
 that of the insulator or dielectric. Thus, for two cables of 
 equal lengths but of different diameters, 
 
 S 
 
 S' 
 
 ** 
 
 (iii.) If their lengths differ as well as their diameters, 
 
 s_ _ l '* d * l s- d 
 
 CV 
 
 / 2 d 1 * tL. 
 
 LAWS OF ELECTRO-MAGNETISM. 
 
 13. Action of 
 Currents on 
 
 (Oefstedt's 
 discovery). 
 
 Ampere's Rule. 
 
 A suspended magnet brought within the influence of a 
 current will take up a position across it. 
 
 Ampere's rule for the direction of the magnet in such a 
 case is as follows : 
 
 ' Imagine an observer swimming with the current, with 
 his face always turned towards the needle, the north pole is 
 always deflected to his left? 
 
14. Polarity 
 of Electro- 
 Magnets. 
 
 15. Best Re- 
 sistance for 
 Electro- Mag- 
 netic Coils. 
 
 16. Laws of the 
 Magnetic In- 
 tensity of Elec- 
 tro-Magnets. 
 
 17. Principle 
 of Tangent 
 Galvanometer. 
 
 18. Deflection 
 of Galvano- 
 meter Needle. 
 
 19. Principle 
 of Sine Galva- 
 nometer. 
 
 LAWS AND PRINCIPLES. 
 
 In electro-magnets the south pole is always found at 
 that end where the positive current enters a right-handed 
 helix. 1 
 
 The maximum force is obtained from an electro-magnet, 
 when the resistance of the coils is equal to the resistance of 
 the battery. 
 
 The magnetic force developed in the soft iron core of 
 an electro-magnet is proportional to the strength of the 
 battery current and to the number of turns of the wire. 2 
 
 A circular current flowing in the plane of the magnetic 
 meridian deflects a needle, which is infinitely short in com- 
 parison with the radius of the current, so that the tangent of 
 the angle of deflection is proportional to the strength of the 
 current? 
 
 The angle of deflection is not determined in any way by 
 the strength of the magnetism in a single needle. 
 
 If the circular conductor or coil be turned after the 
 deflected needle until the latter lies again parallel with the 
 coil, the current strength is proportional to the sine of the 
 angle through which the conductor is turned.^ 
 
 LAWS OF INDUCTION. 
 
 245 
 
 '50. 
 
 2 It must be re- 
 membered that 
 this law treats 
 only of the force 
 of magnetic at- 
 traction in an 
 electro-mag- 
 net, irrespec- 
 tive of its 
 efficiency for 
 use as a signal- 
 ling instrument 
 in the matter of 
 speed, where 
 other condi- 
 tions, such as 
 extra currents, 
 magnetic 
 inertia, resi- 
 dual magnet- 
 ism, &c., come 
 into effect and 
 must be con- 
 sidered. 
 
 3 iSi. 
 * 150. 
 
 20. Laws and 
 Principles of 
 Induced Cur- 
 rents. 
 
 21. Laws of 
 Extra Currents. 
 
 22. Laws of 
 Extra Currents 
 in Coils of Elec- 
 tro-Magnets. 
 
 (i.) A current at the moment of the circuit being closed 
 produces in a neighbouring conductor an inverse induced 
 current. 5 
 
 (ii.) A current at the moment it ceases produces a direct 
 induced current. 
 
 (iii.) A current which is removed, or which diminishes in 
 strength, gives rise to a direct induced current. 
 
 (iv.) A current which is approached, or which increases in 
 strength, gives rise to an inverse induced current. 
 
 (v.) A continuous and constant current does not induce 
 any current. 
 
 (vi.) The strength of induced currents is proportional to 
 that of the inducing currents. 
 
 (i.) With the same strength of primary currents, the 
 extra currents obtained on opening and closing the circuit 
 have the same electromotive force. 
 
 (ii.) The E.M.F. of the extra current is proportional to 
 the strength of the primary current. 
 
 The strength of the extra currents induced in the coils 
 of electro-magnets, by the working currents which pass 
 through them, is proportional to the number of convolutions 
 in the electro-magnet and to the mass of iron in the core. 
 
 5 In the case 
 of an electro- 
 magnet, the 
 inverse current 
 exists during 
 the time the 
 magnetism is 
 increasing ; the 
 direct, while it 
 is decreasing. 
 
246 
 
 MANUAL OF TELEGRAPHY. 
 
 23. Mutual 
 Action of 
 Magnets. 
 
 24. Law of 
 Magnetic At- 
 tractions and 
 Repulsions. 
 
 25. Law of the 
 Direction of 
 the Amperian 
 Currents in 
 Magnets. 
 
 26. Law of 
 Portative Force 
 of a Magnet. 
 
 LAWS OF MAGNETISM. 
 
 Like poles repel and unlike poles attract one another. 
 
 Magnetic attractions and repulsions are inversely as the 
 square of the distances between the bodies exerting their 
 influence on one another. 
 
 At the north pole of a magnet the direction of the 
 Amperian currents 1 is opposite to that of the movement of l 118 (last 
 the hands of a watch ; but at the south pole the direction is clause), 
 the same as that of the movement of the hands of a watch. 
 
 The portative force p 2 of a saturated horse- shoe magnet 3 2 Def. 45. 
 is proportional to its own weight / and to a coefficient #, 5 Def. 47. 
 depending upon the coercive force of the steel. 4 * Def. 43. 
 
 This is expressed mathematically by Hacker's formula 
 
APPENDIX B. 
 
 FORMULAE 
 
 AND THEIR SOLUTIONS 
 
PORMUL^E. 
 
 To those who do not possess the knowledge of mathematics 
 necessary for the comprehension of algebraic formulae, the 
 study of electrical science is likely to be rendered difficult 
 by reason of the unintelligible symbols which are often 
 introduced into the thread of explanations such as may 
 relate to the working of instruments, the action of currents, 
 the testing of batteries, lines, or earths ; in fact, the most 
 ordinary electrical matters. Moreover, the fact of not 
 understanding these formulae renders such explanations un- 
 attractive, and too often induces the reader at the sight of 
 them to give up all attempt to follow the reasoning any 
 further. 
 
 This is a mistake which is usually the result of a wrong 
 impression as to what formulae really are ; and it is the 
 object of the following remarks and proofs to show that, by 
 the exercise of a little intelligence and perseverance, the 
 reader who will take the pains to master even the very 
 rudiments of arithmetic and algebra may follow and under- 
 stand many of the electrical truths and principles algebrai- 
 cally expressed which were at first quite incomprehensible 
 to him. 
 
 He will then discover that the real object of mathe- 
 matical formulae is not intricacy, but simplicity and brevity r , 
 and that each letter represents some distinctive electrical 
 feature, such as a resistance, electromotive force, deflection, 
 &c., which, when combined with arithmetical or algebraical 
 signs, represents some electrical law in a much more concise 
 and intelligible form than could be expressed in a great 
 number of words. 
 
 The sight of letters of the Greek alphabet, which are 
 frequently met with in formulae, need create no alarm, for 
 they are employed in precisely the same manner as English 
 letters, i.e. simply as conventional signs representing certain 
 quantities, to avoid the necessity for writing these quantities 
 in full ; for example, the English letters L and / may have 
 
250 
 
 MANUAL OF TELEGRAPHY. 
 
 APPENDIX been used to denote the lengths of two conductors under 
 
 , B - certain conditions ; then, to express the lengths of the same 
 
 conductors under different conditions, it may be obviously 
 
 convenient to employ the corresponding Greek letters A 
 
 and X. 
 
 As all formulae are based upon laws and principles, it is 
 essential that the fundamental laws which govern the action 
 of electrical currents &c. should be thoroughly understood, 
 and with this object they are included in the preceding 
 appendix (A). 
 
 It will be found that the laws of currents and circuits 
 (Nos. i to 12, App. A) comprise the principles on which 
 testing formulae generally are based, two of which, in par- 
 ticular, are of the greatest use and importance, viz. 
 
 No. i, Ohm's Law, and No. 8, On the joint 
 resistance of derived circuits. 
 
 The first is expressed by the simple formula 
 
 the truth of which is confirmed by experiment, as manifested 
 by the deflections of a galvanometer needle under various 
 conditions of electromotive force and resistance. (For 
 examples see paras. 250 and 290, sec. F.) l 
 
 The laws for the joint resistance of derived circuits 
 admit of such simple mathematical proof as to be readily 
 understood by the beginner, and their development will 
 form a suitable introduction to the solution of electrical 
 formulae generally. 
 
 * Law i, 
 App. A. 
 
 1 See 
 
 Appendix II. 
 Testing In- 
 structions. 
 Ohm's Law 
 mathemati- 
 cally proved. 
 
FORMULA AND THEIR SOLUTIONS. 
 
 251 
 
 APPENDIX 
 B. 
 
 SOLUTION I. 
 
 joint Resist- The joint resistance R of two conductors x 
 D n eriv?d and ft forming a derived circuit, is equal to the 
 circuits. product of the resistances of each conductor di- 
 vided by their sum. 
 
 R - *y 
 
 x +y 
 
 Let x and y represent the resistances of the two branches 
 of the derived circuit between the points a and b. 1 * Defs. 21, 22. 
 
 Then 
 
 Let x be the sum of the two resistances x and j, i.e. 
 X=x+y. 
 
 _ i _ 
 
 X x y xy 
 
 i.e. ? - = the joint conductivity of the derived circuit 
 
 (conductivity being the reciprocal of resistance). 2 2 Law 8 > 
 
 And, for the same reason, the joint resistance R will be App ' A ' 
 the reciprocal of this, or 
 
 , 
 
 x + y 
 i.e. the product of x and y divided by their sum. 
 
APPENDIX 
 J3. 
 
 MANUAL OF TELEGRAPHY. 
 
 SOLUTION II. 
 
 The joint resistance # of any number of con- 
 ductors in derived circuit is equal to the product 
 of their resistances divided by the sum of each 
 resistance multiplied into each of the other re- 
 sistances. 
 
 Thus, in the circuit represented below, 
 
 z>_ xy* 
 
 FIG. 80. 
 
 It will be observed that the above figure represents the 
 circuit described in the preceding proof, with the addition 
 of a third wire of resistance z. 
 
 It was shown by Solution I. that the expression for the 
 joint resistance of x and_y is 
 
 . xy 
 
 Its joint resistance, therefore, with that of z is found, as 
 explained in the preceding case, by dividing their product 
 by their sum, i.e. 
 
 xy 
 
 X Z 
 
 x 
 whence 
 
 xyz 
 
 xy + xz 
 
 x + y 
 
 xy +yz + xz 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 SOLUTION III. 
 
 The joint resistance R of any number of resist- 
 ances in derived circuit is found by dividing unity 
 by the sum of their reciprocals. 1 i Law 8, 
 
 Thus, in the preceding case, A PP- A - 
 
 x y z 
 For the sum of the above resistances would be 
 
 x + y z. 
 Therefore the sum of their conductivities would be 
 
 x y z 
 
 (resistance and conductivity being reciprocals of one 
 another). 
 
 For the same reason, 
 
 x y 
 
254 
 
 APPENDIX 
 B. 
 
 MANUAL OF TELEGRAPHY 
 
 SOLUTION IV. 
 
 To find n the multiplying power of a shunt of 
 resistance s on a galvanometer of resistance G. 
 
 * Law 9, 
 App. A, and 
 158, 159- 
 
 A galvanometer and its shunt form a derived circuit 
 containing two branches of resistances G and s respectively. 
 A current c of E.M.F. E flowing through this circuit will, 
 therefore, divide between these branches into two currents, 
 CD ^2) the strength of which may, in accordance with Ohm's 
 Law, 1 be expressed as follows : l Law i, 
 
 App. A. 
 
 Ci = . = current in G branch ; 
 G 
 
 c - E - S 
 
 ^~'S ' " " " 
 
 But C = 
 
 /. (substituting the above value for <r 2 ) 
 
 That is, the value of the whole current c is found by multi- 
 plying the current in the galvanometer (as shown by the 
 deflection) by 
 
 G + S 
 
 (~* -L 9* 
 Hence, is called the multiplying power of the shunt. 
 
FORMULA AND THEIR SOLUTIONS. 
 
 SOLUTION V. 
 
 To find s the required resistance for a shunt in 
 order to produce a given multiplying power n on a 
 galvanometer of known resistance G. 
 
 G * 
 
 1' 
 
 The above formula follows from the general rule that 
 
 S = 
 
 n 1' 
 
 as proved by the preceding solution. Whence 
 
 Sn = G + S 
 Sn-S = G 
 
 and S = -. 
 
 n 1 
 
 255 
 
 APPENDIX 
 B. 
 
256 
 
 MANUAL OF TELEGRAPHY. 
 
 SOLUTION VI. 
 Proof of the theory of the Wheatstone bridge. 
 
 245. 
 
 PROOF. 
 
 The principle of the bridge is based upon the equalisa- 
 tion of the potentials at the points/ and q by adjusting the 
 relative resistances of the branches A, s, w, and x. 
 
 FIG. 8 1. 
 
 Now, in the derived circuit between the points m and n t 
 made up of the two branches (A + w) and (B + x) the 
 potential of the current at any point in either circuit falls in 
 direct proportion to the resistance up to that point from m. 1 
 
 First, let us consider the upper branch (A -t- w\ which 
 may be represented by the straight line mpn, as under ; v^ 
 u, and v 1 denoting the potential at the points m, /, and n 
 (the line v u v 1 thus showing the fall of the potential from 
 m to n) : 
 
 V 
 
 163. 
 
 FIG. 82. 
 Then, in the similar triangles vm v 1 and up y t ', 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 257 
 
 Vm \ mV \\ Up \ pV APPENDIX 
 
 . Vm Up 
 
 mV " pV 
 
 Now, the vertical lines in the above figure represent 
 potentials, and the horizontal ones resistances. 
 
 Thus vm represents the difference between the potential 
 at the points m and , i.e. 
 
 Vm = V - V, 
 and, similarly, Up = U- V . 
 
 mv' represents the resistance (A + w 7 ) 
 and/F 7 represents the resistance w. 
 
 Thus, substituting these values in the above equation, 
 
 V V U-V 
 A'+W = W 
 
 TT V ' 
 
 V-V = U . ./ . A + W. 
 
 W 
 
 Now, considering the lower branch of the bridge made 
 up of the resistances B and x t it may be proved in precisely 
 the same way that 
 
 V-V _ U-V 
 B +x x 
 
 whence 
 
 TT V 
 
 V-V = . + x. 
 
 x 
 
 But, in the first case, 
 
 x W 
 
 Ax + Wx = J3W+ Wx 
 
 Ax = BW 
 
 and x - W* * Formula I. 
 
 A Testing In- 
 
 structions. 
 
MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 B. 
 
 SOLUTION VII. 
 
 Proof of the theory of the differential galvano- 
 meter. 
 
 x = W* * 248. 
 
 PROOF. 
 
 The principle of the differential galvanometer is also 
 based upon the equalisation of potentials by the equalisation 
 of the external resistances x and w. 
 
 For, it follows from Ohm's Law and the construction of 
 the instrument 1 that, in the case of c 1 , the portion of current l 165. 
 passing through one coil r l and the resistance x external to it, 
 
 And in the case of the current c 2 , through the other coil 
 and external resistance w t that 
 
 cr= 
 ~ 
 
 But, as c 1 and c 2 are equal when balance is obtained, i.e. 
 when the needle stands at zero, and E is the same in both 
 cases, 
 
 r l +x = r 2 + W 
 
 x = 
 
 But as r 1 = r l 
 
 x = W. 
 
FORMULA AND THEIR SOLUTIONS. 
 
 259 
 
 SOLUTION VIII. 
 
 Proof of the theory of the tangent galvano- 
 meter. 
 
 FIG. 83. 
 
 Let AB represent the plane of the coil in the magnetic 
 meridian, 1 and NS the needle (suspended at c) which is 
 deflected by a current in the coil, so as to form the angle a 
 with the magnetic meridian. 
 
 In this position the needle is held in equilibrium by two 
 forces : 2 one, the horizontal component of the earth's mag- 
 netism, represented by the line NE parallel to the magnetic 
 meridian, the other the resultant of the forces exercised 
 on the needle by the current in the circular coil, which, 
 owing to the breadth of the coil and the shortness of the 
 needle, acts practically at right angles to the magnetic 
 meridian, and is thus represented by the line ND. 
 
 Now, denoting by t the directive force of the earth's 
 magnetism on the pole jv, as represented by the line NE> 
 and by i the force of the current as represented by JVD, and 
 resolving / into the two forces bE (parallel to the plane of 
 the deflected needle) and Nb (at right angles to it), and 
 similarly the force / into the components an and aw, then, 
 as the components an and bE act parallel to the needle, 
 they exercise no influence on its deflection; the only rer 
 maining forces therefore acting on the pole N are the two 
 opposite forces NO, tending to increase the angle a, and 
 Nb, tending to diminish it. 
 
 2 Note. Atten- 
 tion is here 
 confined to the 
 action of the 
 forces on one 
 pole, N, of the 
 needle ; the 
 same reasoning 
 of course 
 applies to the 
 action of the 
 pole 6 1 . 
 
 S 2 
 
2 6 MANUAL OF TELEGRAPHY. 
 
 APPENDIX But, when the needle is in equilibrium, 
 
 Na = M. 
 
 Now Na = i . COS a* * The angles 
 
 NCB, Nab, 
 
 and Nb = t . sin a* . aND, are by 
 
 construction 
 
 /. / COS a = t . Sin a equal, and 
 
 thus-o . 
 
 z" a , o 
 
 i t . tan a. 
 
 That is, the tangent of the angle of deflection is 
 proportional to the strength of the current in the 
 coil. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 26l 
 
 SOLUTION IX. 
 
 Proof of the theory of the sine galvanometer. 
 
 In the preceding proof it has been shown that 
 
 and that 
 
 Nb = t sin a 
 Nb = Na. 
 
 Each of the forces therefore acting on the deflected needle 
 and holding it in equilibrium may be denoted by the 
 expression 
 
 t sin a. 
 
 Now suppose the coils of the sine galvanometer to be turned 
 after the needle. l 
 
 The current in the coils will cause the needle to be 
 deflected still farther from the magnetic meridian until a 
 point is reached where the needle lies parallel to the coils, 
 in which position they are shown in the following figure, a 
 representing the angle through which the needle and coils 
 have been turned out of the magnetic meridian. 
 
 150. 
 
 The deflecting force /, represented by the line jra, thus 
 acts at right angles to the needle as well as to its own plane, 
 viz. that of the coils. 
 
 Na being counteracted by the equal and opposite force / 
 represented by the line Nb, tending to draw the needle back 
 to the magnetic meridian AB. 
 
2 6 2 MANUAL OF TELEGRAPHY. 
 
 But it has been shown above that 
 Nb t sin a 
 .'. Na = / sin a 
 i.e. i = t sin a. 
 
 And it may be similarly shown that, for any other strength 
 of current /', requiring the coils to be turned after the 
 needle through an angle # /0 , that 
 
 /' = / sin a'* 
 
 That is, the currents are proportional to the sines 
 of the angles through which the coils are turned 
 after the needle. 
 
FORMULA AND THEIR SOLUTIONS. 263 
 
 SOLUTION X. 
 
 To measure a resistance x by the deflections of 
 a tangent galvanometer. 
 
 PROOF. 
 
 In the first circuit, described in para. 260, it follows from 
 Ohm's Law that 
 
 tan a = 
 
 - , 
 + G+f 
 
 and in the second circuit that 
 
 " 
 
 /O 
 
 tan a x +G+f 
 ThuS = " 
 
 W + G+f 
 
 tana 
 
 " tana r x+G+f 
 Whence 
 
 x (tan a) + (G +/) tan a = ( W + +/) tan a' 
 
 .-. x (tan a) = (W + G +/) /a a'-(G +/) to 
 r +/ )^^o_ ^^ 
 
 tan a ' tan a 
 
 APPENDIX 
 B. 
 
 '260. 
 
 X = taH a ' (W+G+f}-(G +/).t t Formula 
 
 tan a ^ i.xix. 
 
 Testing In- 
 structions. 
 
264 MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 P. 
 
 SOLUTION XL 
 
 To measure a resistance x using both coils of 
 the tangent galvanometer. 
 
 tana* 
 * tan a' 6 * * 263. 
 
 tan a _ 
 tatTa 70 
 
 PROOF. 
 
 It has been shown in the case of the first circuit described 
 in para. 263 that 
 
 Ktana* = _^ 
 g+x 
 
 and in the second circuit that 
 K' tan a' = 
 
 g 1 + x 
 
 K tan a _ g' + x 
 K' tan a' "" ~g+x 
 
 tana _ g' + x K' 
 
 tan a' " Y+^ ' ^ 
 and as 
 
 K 1 
 
 -= =n ..... (see para. 262) 
 
 tan a _ 
 
 ~ 
 
 tan 
 
 whence 
 
 X = i tan a ' .f t Formula 
 
 tan a ^ XXIV v 
 
 Testing In- 
 
 tan a n structions. 
 
FORMULAE AND THEIR SOLUTIONS. 2 6$ 
 
 SOLUTION XII. 
 
 To measure / the internal resistance of a 
 battery with the departmental tangent galvano- 
 meter. 
 
 f _^ tan a 10 _ * '269. 
 
 J ~ tan a - tan a 10 ' 
 
 PROOF. 
 
 In the case of the first circuit described in para. 269, it 
 follows from Ohm's Law that 
 
 c - E 
 
 ~ 
 
 and, in the second circuit, 
 E 
 
 C' = 
 
 f+g+ 
 E 
 
 f+g 
 
 f+g 
 
 But as, according to the principle of the tangent galvano- 
 meter, 1 i 151. 
 
 C_ __ tan a 
 
 C 7 == tan a' 
 
 then by substituting the latter expression for the former 
 
 tan a _ f + g 4- w 
 tan a' ~ f+g 
 
 (tan a) f + (tan a) g = (tan a') (f+g+w) 
 .'. f(tan a - tan a') = w (tan a') - g(tan a - tan a') 
 
 f ( tan a ') w & ( tan a ~ tan a ') 
 
 " (tan a tan a 10 ) " (tan a tan a') 
 
 f __ tan a ' . W F.t t Formula 
 
 J tan a - tan a' LXV. Testing 
 
 Instructions. 
 
266 
 
 MANUAL OF TELEGRAPHY. 
 
 SOLUTION XIII. 
 
 To measure / the resistance of a battery with 
 the Wheatstone's bridge (Poggendorff). 
 
 f AW -A'W* 
 /= W-W ' 
 
 * 270. 
 
 PROOF. 
 
 By reference to fig. 66, para. 270, it will be observed 
 that when balance is obtained no current flows through the 
 x and g branches of the bridge, but is closed through the 
 circuit composed of A, w, andf. 
 
 Now, by KirchhorFs second law, l which states that the * Law 2 (ii), 
 total electromotive force in any circuit is equal to the sum App. A. 
 of all the resistances multiplied by the sum of all the cur- 
 rent strengths (c) in the same circuit, it follows that, in the 
 first circuit described in para. 270, 
 
 E = C(A 
 
 rr),T f and.fi' 
 
 represent the 
 and for the same reason, in the closed circuit made up of electromotive 
 
 w> x, and P-, as no current flows in x and r, forces in the 
 
 first and second 
 
 ., CW observations 
 
 respectively of 
 
 whence E _ A +/+ W t 
 
 7 = W 
 
 and similarly in the second case described in para. 270, 
 E' _ A 1 + f+W 
 
 W 
 
 and as 
 
 E 
 
 e 
 
 E^_ 
 
 ~7 
 
 A' +/+ W 
 
 W 
 
 A+f+W 
 W 
 
 A'W + fW+WW = AW +/W + WW 
 
 fW-fW = AW -A'W 
 f(W-W'} = AW -A'W 
 
FORMULA AND THEIR SOLUTIONS. 
 
 SOLUTION XIV. 
 
 To measure / the resistance of a battery by 
 Mance's method of opposing E.M.F. 
 
 f=W~-W* 
 e 
 
 PROOF. 
 
 Referring to fig. 68 (para. 277), it is obvious that when 
 the galvanometer needle stands at zero, c (the current 
 strength of the battery E on the needle) must be equal to c 
 (the effective strength of the battery e). 
 
 Now c = E x W 
 
 J * 7^ I rr? 
 
 'he multiplying power of the shunt w on 
 
 W+G 
 the galvanometer G\ 
 and 
 
 
 
 whence 
 
 * 
 
 r> _L fW 
 
 e 
 
 + 'f+w 
 
 f GW 
 
 G'+W 
 
 EW 
 
 W + G c 
 
 7 fw 
 
 + f+w 
 
 e 
 
 / (G + W)+GW +-ftr G(f+W)+fW 
 
 EW 
 
 fG+fW + GW" Gf+GW+fW 
 
 APPENDIX 
 B. 
 
 and 
 
2 68 MANUAL OF TELEGRAPHY. 
 
 SOLUTION XV. 
 
 To measure E the electromotive force of a 
 battery in terms of the standard cell with the 
 tangent galvanometer. 
 
 tan a 
 
 tana' ' f+g 
 
 PROOF. 
 
 Calling d the current strength of the standard cell in 
 the first circuit described in para. 280, and c the current 
 strength of the battery under test in the second circuit, it 
 follows from Ohm's Law l that i Law i, 
 
 App. A. 
 
 c= 
 
 and that r , _ 
 
 ~ 
 
 And, from the principle of the tangent galvanometer, 2 that 2 151. 
 C tan a 
 
 C 7 ~~ tan a 
 
 10 
 
 tana f+g+W 
 'ta^Ta 7 * = E' 
 
 f+g 
 
 Hence <^ == f . _Z*L 
 
 tan a' E 1 
 
 ^L tan a f+g+ w 
 
 " "E'~ tana!* f'+g 
 
 and E = . ..E'.t t Formula 
 
 tan a' f ' + g LXVII. 
 
 Testing In- 
 structions. 
 
FORMULAE AND THEIR SOLUTIONS 269 
 
 SOLUTION XVI. 
 
 To measure E the electromotive force of a 
 battery with the Wheatstone bridge (Poggendorff). 
 
 ^=4=4^ + '-* * 281 - 
 
 PROOF. 
 
 The circuit is described in fig. 66, para. 27O. 1 l See also 
 
 In the first case described in para. 270 it follows from 28x - 
 
 Kirchhoffs second law 2 that \ L *w 2 (ii), 
 
 App. A. 
 
 E= C(A+f+W) 
 and e=CW 
 
 (E and e representing the electromotive forces of the battery 
 under test and of the standard cell respectively) 
 
 E _A+f+W 
 7 = ~W~ 
 
 and 
 
 Now, by the formula in para. 270, proved by Solution 
 XIII. 
 
 f _ AW'-A'W 
 
 s = w-w ' 
 
 /. Substituting the above value of/, 
 
 F ^[A + W A W'-A' W 
 = l W ' W(W-W 
 
 Hence (e, the standard, being taken as unity) 
 
 F - A + W 4- 
 
 ' 
 
 W(W-W) 
 
 _ (A + W)(W-W) + A W'-A' W 
 W(W- W 1 } 
 
 AW+ WWA W- WW +A W'-A' W 
 W(W-W>} 
 
MANUAL OF TELEGRAPHY. 
 
 AW-A'W + WW- WW 
 W(W - W) 
 
 W(A-A')+W(W-W) 
 W(W-W') 
 
 W(A-A') W(W-W') 
 
 W(W-W] " W(W-W') 
 
 A A' A * * Formula 
 
 7/r7 JM + 1-* VIII. Testing 
 
 W W Instructions. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 271 
 
 APPENDIX 
 B. 
 
 SOLUTION XVII. 
 
 To find x the corrected resistance of a con- 
 ductor influenced by natural currents. 
 
 _ 
 
 w" + w' 
 
 *39- 
 
 Denoting by the letters A, , w, G, x, and F the currents 
 which flow through the corresponding resistances #, t>, w, l g, 
 x, and / respectively in the figure represented above, we 
 have, when balance is established (i.e. G = o), the following 
 equations of current strength according to KirchhorT's first 
 law. 2 
 Considering the 
 
 A-W=.o 
 
 point/ 
 
 A=W (a) 
 
 B=X ....() /. X-B =o 
 
 . (t) /. F-A-B=o 
 
 1 w' expressing 
 the value of w 
 when measured 
 with a + cur- 
 rent, and w" 
 its value with 
 
 a current. 
 
 2 Law 2 (i), 
 App. A. 
 
 m .... * 
 
 Suppose the natural current (e) to be in the same direc- 
 tion as the testing current (E), as shown by the arrows, then 
 by KirchhorT's second law 3 we have the following three s Law 2 (ii), 
 equations : A PP- A - 
 
 In the closed figure mpqm . . . . aA b = o . . (i) 
 No E.M.F. in a or b. a A = bB. 
 
272 MANUAL OF TELEGRAPHY. 
 
 APPENDIX In the closed figure npqn .... xX w' W= e . (2) 
 . _ B ; . (e) the E.M.F. in x being the only E.M.F. in the circuit. 
 
 f mpqn and ] 
 In the closed figure^ testing \/F+aA+xX=E + e (3) 
 
 [ battery J 
 E in /and e in x being the total E.M.F. in the circuit. 
 
 Substituting in (3) the values F=-A+B 
 
 wdX=*& 
 and in (2) the values W=A 
 
 and^=J? 
 
 equations (3) and (2) may be written 
 
 . . . (3) 
 and ^^ w'A e ..... (2) 
 Dividing (3) by (2) 
 
 E+e == , 
 
 "7~ xB-w'A 
 
 e xB w'A 
 
 . E+e __ B(x+f) + A(f+a) 
 e xBw'A 
 
 Now, by equation (i), a A = bB> 
 
 .: A = b B. 
 
 a 
 Substituting this value of A in equation (4), 
 
 *B-B 
 a 
 
 Dividing numerator and denominator by B> 
 
 
 ax-bw' 
 
FORMULA AND THEIR SOLUTIONS. 273 
 
 7? APPENDIX 
 
 Let = y B. 
 
 then _ a(x+f)+b(f+a) 
 
 # * - bw' + y (ax- bw') = a (x +f) + 
 
 j (ax.bw 1 ) = ax + af + bf + ab ax+bw' 
 
 y (ax-bw') = a(b+f) + b (f+w 1 ). . . (6) 
 
 Now, suppose the testing battery to be reversed 
 
 then, by a similar process, 
 
 1 - E = 1 - y = 
 
 ax bw" 
 
 . . (8) 
 ,x 
 
 Now, from equations (6) and (8), 
 
 y (ax - bw') = a (b + f) + b 
 and y (bw" -ax) = a (b + /) -I- b (f + w") 
 .'. by addition 
 
 jy (fatf 1 bw') = ^^ + 2af + ^w' + bw" 
 
 Again, equations (6) and (8) may be written 
 axy bw'y= ab + of }- bf + bw' 
 
 and fluey + bw"y = al> + a/ + l>f + bw" 
 
 .'. by subtraction, 
 
 2axy bw'y bw"y = bw' bw" 
 2axy = by (w' + w") + b(w' w") 
 
 _b(w' + w") b(w'-w") 
 ~~ -~ -- 
 
274 MANUAL OF TELEGRAPHY. 
 
 APPENDIX Substituting the value ofy from equation (i 
 
 b (w'-w") 
 
 x ^ ' + 
 
 2a b (w 1 + w") + zf(a + b) 2ab 
 
 b(w"-w') 
 
 x = t>(' + "} , b* (w' -w") (w" -w') 
 
 %a ' za{b(w' + w") + 2f(a + b) + 2ab] 
 
 _b w' + w" b(w' \ 
 
 a' 2 2[b (w 1 + w") + 2{ab+f(a + b}}} 
 
 Now (w' w") (w" w'} = 
 
 b (w"-w') (w"-w') 
 
 ' 
 
 Now the following proof (Solution XVIII.) goes on to 
 show that the ratio 
 
 e_ _ N= b(w"-w') _ 
 
 E b w ' + 
 
 and, substituting ^for this expression in equation (n), 
 
 b fw' + w" A7 W"-W'\* 
 X = - J -r-r-5 -- ^V -- J- * 
 a \ 2 2 J 
 
 or, with equal branches a and , 
 
 * Formula III. 
 
 Testing 
 
 Instructions. 
 
 * T W" W' . t Formula V. 
 
 X = - - - N. .t Testing 
 
 r * Instructions. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 275 
 
 APPENDIX 
 B. 
 
 SOLUTION XVIII. 
 
 To find e the electromotive force of a natural 
 current in the line in terms of E the E.M.F. of the 
 
 testing battery. 
 
 w"-w' 
 
 3 J o. 
 
 PROOF. 
 
 Referring to the preceding proof (Solution XVII. ), it has 
 been shown by equation (10) that 
 
 _ E_ _ b (w 1 + w") 
 =Z 7 ' 
 
 b) + 2ab 
 
 b(w"-w') 
 b(w"-w') 
 
 E b (w' + w') + t>{ab + f(a + b)} 
 and calling the ratio -^, = N, 
 
 N = 
 Or, with equal branches a and b^ 
 
 <,!," _ . 
 
 N = 
 Or, 
 
 b ( w "-w') 
 
 e = 
 
 E. 
 
 .t 
 
 t Formula IV. 
 
 Testing 
 
 Instructions. 
 
 t Formula VI. 
 
 Testing 
 
 Instructions. 
 
 T 2 
 
276 
 
 MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 B 
 
 SOLUTION XIX. 
 
 To find g the corrected relay resistance from 
 the measured values , 5123, and n>. 
 
 *r=^ . ,' w -* '3". 
 
 PROOF. 
 
 FIG. 86. 
 
 Let AB represent a telegraph line, of which L = the 
 corrected conduction resistance ; 
 
 Let / = the corrected conduction resistance from A to 
 the resultant fault (at c) ; 
 
 And /' = the corrected conduction resistance from c to B. 
 
 (Thus L = l + F) 
 
 and let i = the corrected insulation resistance of the whole 
 line. 
 
 Then, in the case of the circuit test, in which a relay R 
 is included, it follows, from the law of derived circuits, 1 that * Law 8, 
 
 App. A. 
 
 i+l'+R 
 
 In the conduction test 
 
 > = ' +7 f ? , .... w 
 
 and in the insulation test 
 
 =/+*. - (3) 
 
 And suppose the insulation of the line to be uniform, i.e. 
 /=:/', 
 
!SSE LlB^s 
 
 Of THE r 
 
 UNIVERSITY 
 
 FORMULAE AND 
 Subtracting (i) from (3) 
 
 277 
 
 APPENDIX 
 B. 
 
 - a = 
 
 
 (4) 
 
 And subtracting (2) from (i) 
 
 am m - + 
 
 . ... _ _ 
 
 Dividing (4) by (5) 
 
 Now by (3) = / + / 
 
 /. /=-/. 
 And substituting this value of / in equation (6) 
 
 R 
 
 And expressing by ^ this corrected value of /?, 
 
 = giae^-jm) * 
 
 * Formula 
 S g 
 
 Instructions. 
 
2 7 8 
 
 MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 B. 
 
 SOLUTION XX. 
 
 To find / the corrected absolute insulation of 
 any line from the measured values g, SO, and w> 
 
 'V 
 
 * 3x5. 
 
 PROOF. 
 
 Subtracting equation (i) from (3) in the preceding proof 
 (Solution XIX.) 
 
 And subtracting (2) from (3) (Solution XIX.) 
 
 .-. g/'+g/'-ito -/'> = / . . (8) 
 
 Thus, from equations (7) and (8) 
 
 Now, it has been shown above, that subtracting (2) from 
 (3) (Solution XIX.) 
 
FORMULA AND THEIR SOLUTIONS. 
 
 279 
 
 APPENDIX 
 R 
 
 And substituting in this equation the value of I' found by 
 equation (9) 
 
 iR (8 - fflgi) - i ' (ggg ~ tt) 
 
 . _ m , 
 
 . J? (g - gigE) - * (fflO - tt>) 
 
 ae(g- 
 
 (g - 
 
 = V ( 
 
 XXIV. (a) 
 
 Testing 
 
 Instructions. 
 
2 g MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 B. 
 
 SOLUTION XXL 
 
 To find L the corrected absolute conduction 
 resistance of any line from the corrected insulation. 
 
 
 PROOF. 
 
 It has been already shown by fig. 86, Solution XIX., 
 that 
 
 L = / + /', 
 
 and by equation (3), Solution XIX., that 
 
 / = -/, 
 
 and by equation (9), Solution XX., that 
 
 je(g 
 
 Whence 
 
 
 - 8 
 
 xxv. (</) 
 
 Testing 
 
 . The value of z is found by the formula in the preceding 
 solution (XX.) 
 
 When g is within 15 per cent, of R, and consequently 
 
 1 = *'* * 312. 
 
 the following simpler values for i and L are true : 
 
FORMULA AND THEIR SOLUTIONS. 
 
 For, when / = /', 
 
 by equation (2), Solution XIX. 
 
 and by equation (3), Solution XIX. 
 
 / = g - 
 and substituting this value of / in (a) 
 
 il 
 
 2 8i 
 
 APPENDIX 
 
 a 
 
 Now, as 
 from ^ 
 
 -1) 
 
 * See figure 86, 
 Solution XIX. 
 
2 3 2 MANUAL OF TELEGRAPHY. 
 
 SOLUTION XXII. 
 
 To find / the conduction resistance of a line 
 from the testing station to the resultant fault, and 
 z the resistance of the fault. 
 
 / _. V V J * * 317. 
 
 <;-) 
 
 21 - 
 
 z = 
 
 PROOF. 
 
 Let AB represent the circuit of the conduction test of a 
 uniformly insulated line. 
 
 First, supposing the line to contain only two faults, y, /', 
 all other points of the line between A and B being insulated 
 from the earth : 
 
 FIG. 87. 
 
 Expressing by small letters the resistances of the five 
 branches a, y ', 8, y, l>, and by corresponding capitals the 
 currents which flow through them respectively : 
 
 A being thus the current starting from A, flowing through 
 #, and arriving at y', and B the amount of current arriving 
 at B through b, we have, by the laws of current strength in 
 derived circuits, 1 the following expression for the amount of A LaAV A 9 ' 
 current flowing through 8 to the second fault y" : Solution IV. 
 
 App. B. 
 
FORMULAE AND THEIR SOLUTIONS. 283 
 
 D . A y* +yy 
 
 -*-^ ^^ -* 1 / ; (\\ 7 
 
 Similarly, the current flowing through b and reaching B is as 
 follows : 
 
 B-D-1L 
 
 y + b 
 
 B - A 
 
 ' 
 
 y+W 
 
 ^ = A . 
 
 y'y" b+ y'y"* 
 
 y'y" 
 
 . B= A 
 
 ' 
 
 Now suppose that in the line AB instead of the two faults 
 y, y" there be only one fault, of resistance z (I being the 
 resistance of the line from A to z\ and /' the resistance 
 beyond, 
 
 I I' 
 
 FIG. 88. 
 
 Then the current flowing through /' and arriving at B is as 
 follows : 
 
 L 1 = L^, 
 
 But L< = B, 
 
 and L = A. 
 
 y'y" 
 
 Next, suppose the earth to be removed from #, i.e. the 
 line A B to be insulated at its 'distant end, then by the same 
 reasoning, in the case of two faults y' and y", 
 
 A _ Constant 
 
 A 
 
 n 4- -^ 
 
 V+y'+s 
 
284 MANUAL OF TELEGRAPHY. 
 
 ne fault z, 
 Constant 
 
 APPENDIX Or, in the case of only one fault z, 
 B. 
 
 / + * ' 
 
 But = A, 
 
 '-^m 
 
 Further / + /' = a + 3 + b . . . . (3) 
 
 And, from equations (i), (2), (3), 
 
 Now suppose y and y ' to be invariably so close together 
 (as they are in an ordinary line containing an innumerable 
 number of faults, each of great resistance) that 8 can be 
 neglected against b, 
 
 Then the right-hand factor in equation (4) approximates 
 to unity very rapidly, and we have 
 
 and, substituting this approximate value of /in equation (5), 
 
 ylyll 
 
 Now, in a working line, as y' andy must be so large, as 
 compared with b, that b can be neglected, much more can B 
 (a small portion of b) be neglected against the sum of them. 
 Thus, 
 
 <> 
 
 Now, calling r 7 and r' the resistances from A toy and 
 y" respectively, 
 
 Then, from equation (8), 
 
 / = - .... (io) 
 
 /+/' 
 
 i.e. for any faulty whose distance from A (in resistance) =?- 
 
j) 
 
 2= 
 
 ~ 
 
 FORMULA AND THEIR SOLUTIONS. 
 
 , the sum of(ry) S (ry) APPENDIX 
 
 ~ the sum of (y) == 
 
 whence, dividing numerator and denominator by j 2 , 
 
 z(r , 
 
 * Formula 
 XXIV. (c) 
 Testing 
 Instructions, 
 From equation (9) App. XI. (a). 
 
 And dividing again by jy 2 , 
 
 -/ 
 
 Z = - -T t Formula 
 
 App. XI. (a) 
 
 Testing 
 
 Instructions. 
 
2 86 MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 B. 
 
 SOLUTION XXIII. 
 
 To find n> and g the absolute conduction and 
 insulation resistance of the distant section of a 
 highly insulated line, where measured values do 
 not require correction. 
 
 tt> = tt>2 tDl * * 318. 
 
 Referring to the figure in para. 318, it is obvious in the 
 case when the insulation of the line ABC is so good that 
 measured values are correct, that 
 
 n> = tt>2 Wi- 
 lt will be observed here that tt> occupies the same position 
 with regard to the line A B that x, the measured relay, does 
 in the case of a circuit test of the line A B. 
 Thus, the above equation, 
 
 tt) = tt>2 tt>l> 
 
 is identical with 
 
 R $2i - ttJ-t t 3". 
 
 And as 8123 corresponds with JD 2 in figure (para. 318), and 
 tt)i corresponds with jt> in the same figure, 
 
 tt> (the resistance of J3C) . 
 
 Now let g, g ly 2 represent the absolute insulation 
 resistance of the sections whose absolute conduction resist- 
 ance is expressed by JD, tt>i> tt>2 respectively. 
 
 Then, by the law of derived circuits, T ' Law 8, 
 
 App. A. 
 
 - 33. 
 
 33^332+313; 
 
FORMULA AND THEIR SOLUTIONS. 287 
 
 SOLUTION XXIV. 
 
 To find n> and > the absolute conduction and 
 insulation resistance of the distant section of an 
 imperfectly insulated line. 
 
 w = gi(tt>2-tPi) * * 3I8< 
 
 3l-tt>2 
 
 and Si (fj->i)* 
 
 ^iP^ir 
 
 PROOF. 
 
 Again referring to the figure in para. 318, it has been 
 shown in the preceding proof (Solution XXIII.) how (in 
 the case of measured values) tt> 5 the resistance of the section 
 /? c, corresponds with R the measured relay resistance in the 
 circuit test. 
 
 Similarly, in the case where corrections become neces- 
 sary on account of defective insulation, n> would correspond 
 with g the corrected relay resistance. 1 * 312. 
 
 Now it has been proved (Solution XIX.) that 
 
 3- 
 
 And substituting for g, f, SO, and tt>, the values as repre- 
 sented in the figure (para. 318), 
 
 
 Similarly, for the corrected insulation of the section B c, 
 Putting ^ = 
 
288 
 
 MANUAL OF TELEGRAPHY. 
 
 SOLUTION XXV. 
 
 To calculate n the distance in miles of a fault in 
 a line composed of wires of different gauges from 
 x the measured conduction resistance. 
 
 * 323. 
 
 PROOF. 
 
 . Let AB represent a telegraph line, between any two points 
 c and D of which a fault exists. 
 
 The references above the line relate to actual length 
 (in miles), and those below the line to. reduced length (in 
 miles). 
 
 Thus n = actual distance from A to fault. 
 A. = actual distance from A to end of section CD in 
 
 which fault lies. 
 
 /u = reduced length from A to end of section CD in 
 which fault lies. 
 
 Let x the absolute conduction resistance up to the fault, 
 
 and r = the reduced conduction resistance per mile up to 
 the fault. 
 
 Then, obviously, 
 
 * 
 
 the reduced length in miles to the fault. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 Hence /x - = the reduced length from the fault to D. 
 
 And as / - 
 
 w =-> 315- 
 
 i.e. the reduced length of 1 _ 
 any gauge of wire j " 
 
 .'. Actual length from fault to D 
 
 Hence, from the figure, 
 
 = X - a i. - - 
 
 Now, supposing the portion of the line A D to be made 
 up of several sections of different gauges, of which CD is one, 
 
 Then X = the sum of their lengths = 2A 
 
 And ya = the sum of their reduced lengths = 2/x 
 
 n = 2A -j- a ( - 2u J.f 
 
 \r ) 
 
 289 
 
 f Formula 
 
 xxxix. 
 
 Testing 
 Instructions. 
 
290 
 
 APPENDIX 
 B. 
 
 MANUAL OF TELEGRAPHY. 
 
 SOLUTION XXVI. 
 
 To find x the conduction resistance to fault in 
 the case of a partial earth on a single wire. 
 
 x = w N/ (Z w) (I w}* * 325- 
 
 or 
 
 W-w 
 
 PROOF. 
 c y 
 
 FIG. 90. 
 
 Let A B represent a telegraph line containing a fault at c. 
 As the fault is & partial earth it contains resistance in itself. 
 
 Let this resistance = z, 
 
 And the resistance of the line from A to c = x, 
 And the resistance of the line from c to B = y. 
 
 First. To find x from a conduction test only. 
 
 Let w = the conduction resistance of the faulty line mea- 
 sured from A (B to earth). 
 / = the resistance of the faulty line when insulated 
 
 at B. 
 Z = the known resistance of AB when the line is in 
 
 order. 
 
 Then L = x + y . . . . (i) 
 
 I=x + z . . . . (2) 
 
 W = X + ~y^ f . . . (3) f La\v 8, 
 
 y + z ' A P p. A. 
 
 Whence y = L x 
 
 Z = I - X. 
 
FORMULA AND THEIR SOLUTIONS. 
 And substituting these values in (3) 
 
 2QI 
 
 APPENDIX 
 R 
 
 L 
 
 w = x + 
 
 L + / 2x 
 
 'L~+ I 2x 
 
 LIx* 
 
 w = 
 
 L + I 2x 
 Lw + Iw 2xw = LI x 2 
 x 2 2xw = LI Lw Iw. 
 Adding + w 2 to both sides of the equation 
 
 x 2 Sxw + w 2 = LI Lw Iw + w 
 (x - wY = (L - w) (/ - w} 
 x w = + \/(L w) (I w) * 
 
 X = W \/ (L W) (I ?.'). f 
 
 Second. To find x when the circuit test is prac- 
 ticable. 
 
 In this case / = / + z $ 
 
 (*" being the corrected insulation of the line A B). 
 And further it has been proved that 
 
 = / 
 V 
 
 W-w 
 Now, comparing the above figure with that in Solution XIX. 
 
 /=* 
 
 and * = 2, 
 
 /. in this case 
 
 z 
 
 * The - sign 
 is always 
 taken, as the 
 + value would 
 obviously 
 make x too 
 large. 
 
 t Formula 
 XLIII. 
 Testing 
 Instructions. 
 
 J See fig. 86, 
 Solution XIX. 
 
 See Solution 
 
 x-x. 
 
 = / 
 
 = V" 
 
 W-w 
 And by equation (2), above, 
 
 * = /-* 
 
 (/- W}(I-w}R 
 W-w 
 
 II Formula 
 XLII. 
 Testing 
 Instructions. 
 
 U 2 
 
MANUAL OF TELEGRAPHY. 
 
 SOLUTION XXVII. 
 
 To find x the conduction resistance to fault in 
 the loop test. 
 
 2 
 
 PROOF. 
 Referring to fig. 73, para. 326, it is obvious that 
 
 (i) x 4- y = Z, the total resistance of the looped wires when 
 not connected anywhere with the earth. 
 
 And (ii) x + W=y when balance is obtained. 
 /. adding (i) and (ii), 
 
 2x + W + y = L + y 
 .-. ax = L - W 
 
 *: = ^ ~ f t Formula 
 
 2 XLVI. 
 
 Testing 
 Instructions. 
 
FORMULAE AND: THEIR SOLUTIONS 
 
 APPENDIX 
 B. 
 
 SOLUTION XXVIII. 
 
 To find x the conduction resistance between 
 testing station and fault of two wires in contact in 
 the case when the contact itself offers resistance. 
 
 X = w -^( - w)(W-w)* * 327 . 
 
 PROOF. 
 
 Referring to para. 327, in which the letters composing 
 the above formula are explained, it follows that when the 
 distant ends of the two lines in contact are insulated 
 
 lV=x'+x"+z . . . (a) 
 and when looped at the distant, end that 
 
 w =x ' + *" + 7^-^T^'' f ' <*> 1 ^V 
 
 * T * -r -L> X App. A. 
 
 (By definition, L' x 1 = resistance of one line beyond the 
 
 contact 
 
 L" x" resistance of the other line beyond 
 
 the contact) 
 
 And calling x' + x" == X 
 
 then from equation (a) 
 And from (b) 
 
 Now from (c) 
 
 z= W- X. 
 
 And substituting this value in (d} 
 
 W-X+ R-X 
 
294 
 
 MANUAL OF TELEGRAPHY. 
 
 APPENDIX WX 
 
 R .-. w 
 
 W-2X + R 
 w W 2wX + wR = X* + WR 
 X 2 - vwX = WR-wW - wR. 
 
 * The -sign 
 
 Adding + iv 1 to both sides of the equation must be taken - 
 
 as the + value 
 
 X*- wX + fi =WR-wW- wR + : b v tusly 
 
 (X- m ? = (W-U,)(R-U:) ,ar g k e. A ' t0 
 
 v _i_ /7-*?jr TTTi t Formula 
 
 X W >^/(W W)(R w)* XLVII. 
 
 Testing 
 
 X*stW-r*/\Ww)(R w).-f Instructions. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 295 
 
 SOLUTION XXIX. 
 
 To find the values of x t y y and z the several 
 resistances of three earths from their collective 
 resistances when measured in pairs. 
 
 Let x +y w' 
 
 ( * ) j These values are found by 
 
 . (2) I either of the methods de- 
 
 / 3 \ scribed in paras. 335-338 
 
 , _ 
 
 w < + w >/>_ w 
 
 z = 
 
 w" 
 
 To find x. 
 
 By adding (i) and (2) 
 
 From (3) j 
 
 Subtracting (3) from (4) 
 
 2x = w' + w" w 
 
 (4) 
 
 APPENDIX 
 B. 
 
 336. 
 
 2 
 
 Similarly, to find y. 
 By adding (i) and (3) 
 
 2y + x + z = w'+ w'" 
 From (2) x + z w". 
 
 Subtracting (2) from (5) 
 
 2y = w' + w'" w" 
 
 = 
 
 _/ ~ 
 
 (5) 
 
 2 
 
296 MANUAL OF TELEGRAPHY. 
 
 APPENDIX Similarly, to find z. 
 
 B. 
 
 By adding (2) and (3) 
 
 2z + x + y = w" + w" r (6) 
 
 From (i) x + y = w'. 
 
 Subtracting (i) from (6) 
 
 2z w" + w"' w 1 
 
FORMULA AND THEIR SOLUTIONS. 
 
 297 
 
 APPENDIX 
 
 a 
 
 SOLUTION XXX. 
 
 To measure the resistance of earths by means 
 of the tangent galvanometer and a testing battery. 
 
 / tan c \ 
 
 * 337- 
 
 tan 
 
 PROOF. 
 In the above formula 
 
 w represents the resistance of any pair of earths ; 
 
 c the mean of + and deflections from the testing battery 
 
 through thick coil of galvanometer ; 
 
 a "I the deflections through the pair of earths with reverse 
 b ] currents ; 
 
 /the resistance of the testing battery ; 
 fits E.M.F.; 
 
 e the electromotive force of the natural current ; 
 g the galvanometer resistance (thick coil). 
 
 First, considering the testing current and natural current 
 flowing in the same direction 
 
 E + e oc tan a (/+ g + w). 
 When opposite in direction 
 
 E e oc tan b (/+ g + w). 
 /. (by addition) 
 
 2E oc (tan a+ tan b) (/+ g + w) 
 
 Now suppose the deflection c to be produced by another 
 battery of electromotive force E' and resistance/ 7 through a 
 known external resistance w f . 
 
 Then by the former reasoning 
 
 ^'oc tan f (/'+ g + w'\ 
 
298 MANUAL OF TELEGRAPHY. 
 
 APPENDIX Whence tan a + tan 
 
 ,_ R ... E - 
 
 E ' tan c (j 
 
 . f + g + w E tan c 
 
 f'+g + W' " ~E' ' tana+ tanT 
 
 And if E' = E \ 
 
 tanc 
 
 $ 
 Further, when w' o (i.e. no external resistance) 
 
 * Formula 
 LXXII. 
 Testing 
 Instructions. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 299 
 
 SOLUTION XXXI. 
 
 To calculate the external resistance w to be 
 added in order to test any required range n. 
 
 ? + (n-l)qf* * 344- 
 
 PROOF. 
 
 Referring to para. 7 1 (sec. C) and para. 344 (sec. F) and 
 using the nomenclature employed therein, 
 
 The strong current c in the second observation may be 
 expressed : 
 
 and the weak current c in the first observation 
 
 c - 4 e 
 
 qf+R + w 
 
 But ~ represents the range of the instrument (n) 
 _ P 
 
 ~ 
 
 whence wp + pgf + pR _ n 
 
 pqf+qR 
 
 / 
 = n <^-R+nqf-qf 
 
 f Formula 
 XCII. 
 
 U> = (9* - l] R + (n - 1} qtf Teeing 
 
 \-P ' Instruct 
 
 Instructions. 
 
300 MANUAL OF TELEGRAPHY. 
 
 APPENDIX 
 B. 
 
 SOLUTION IXXXIL 
 
 To calculate the ratio ^, i.e. the relative bat- 
 
 tery power to be used, in order to find a given 
 range n with a fixed external resistance w. 
 
 p = _ * * 344 
 
 PROOF. 
 
 In the preceding proof (Solution XXXI.) it is shown 
 that 
 
 whence 
 
 n (P<lf + ?) = 
 
 p (nqf- qf) + nqR = p (R + w] 
 p (R + w) - (n - 1) gf= nqR 
 
 p -= - - r ,f t Formula 
 
 R + w (n 1}qf LXXXIX. 
 
 Testing 
 Instructions. 
 
FORMULAE AND THEIR SOLUTIONS. 
 
 3 OI 
 
 SOLUTION XXXIII. 
 
 To calculate R? the value of any resistance R at 
 temperature t' from R t the known resistance of R at 
 the temperature t 
 
 307- 
 
 PROOF. 
 
 32 Fahrenheit being taken as the standard of tempera- 
 ture, it follows that a resistance of one unit at that tempera- 
 ture becomes 7+ art at one degree above 32, and at any t The co- 
 given temperature / it becomes 1 + (t .32) a units. Simi- 
 larly, at any other temperature /' it becomes 
 
 1 + (/' 32) a units. 
 
 Expressing by R t the value of a certain resistance at tem- 
 perature /, and by R t > the value of the same resistance at 
 temperature /', it follows that . 
 
 being=-oo2i, 
 as explained 
 in | 307. 
 
 - 32 a 
 
 _ 
 
 Rt 
 
 whence 
 
 _ 32) a 
 _ 3g ) g 
 
302 
 
 MANUAL OF TELEGRAPHY. 
 
 TABLE OF 
 
 NATURAL SINES & NATURAL TANGENTS 
 
 Deg. 
 
 Sine 
 
 Deg. 
 
 Sine 
 
 Deg. 
 
 Tan Deg. 
 
 Tan 
 
 I 
 
 0175 
 
 46 
 
 7193 
 
 I 
 
 0175 46 
 
 1 '0355 
 
 2 
 
 0349 
 
 47 
 
 73H 
 
 2 
 
 0349 47 
 
 I -0724 
 
 3 
 
 0523 
 
 48 
 
 743* 
 
 3 
 
 0524 48 
 
 1106 
 
 4 
 
 0698 
 
 49 
 
 7547 
 
 4 
 
 0699 49 
 
 1504 
 
 5 
 
 0872 
 
 50 
 
 7660 
 
 5 
 
 0875 50 
 
 1918 
 
 6 
 
 1045 
 
 5i 
 
 7771 
 
 6 
 
 1051 
 
 51 
 
 2349 
 
 7 
 
 '1219 
 
 52 
 
 7880 
 
 7 
 
 1228 
 
 52 
 
 2799 
 
 8 
 
 1392 
 
 53 
 
 7986 
 
 8 
 
 '1405 
 
 53 
 
 3270 
 
 9 
 
 *5 6 4 
 
 54 
 
 8090 
 
 9 
 
 1584 
 
 54 
 
 3764 
 
 10 
 
 1736 
 
 55 
 
 8192 
 
 10 
 
 1763 
 
 55 
 
 4281 
 
 ii 
 
 1908 
 
 56 
 
 8290 
 
 ii 
 
 1944 
 
 56 
 
 4826 
 
 12 
 
 2079 
 
 57 
 
 8387 
 
 12 
 
 2126 
 
 57 
 
 '5399 
 
 13 
 
 2250 
 
 58 
 
 8480 
 
 13 
 
 2309 
 
 58 
 
 6003 
 
 14 
 
 2419 
 
 59 
 
 8572 
 
 14 
 
 2493 
 
 59 
 
 6643 
 
 15 
 
 2588 
 
 60 
 
 8660 
 
 15 
 
 2679 
 
 60 
 
 7321 
 
 16 
 
 2756 
 
 61 
 
 8746 
 
 16 
 
 2867 
 
 61 
 
 8040 
 
 I? 
 
 2924 
 
 ' 62 
 
 8829 
 
 17 
 
 3057 
 
 62 
 
 8807 
 
 18 
 
 3090 
 
 63 
 
 8910 
 
 18 
 
 3249 
 
 63 
 
 9626 
 
 19 
 
 3256 
 
 64 
 
 8988 
 
 *9 
 
 '3443 
 
 64 
 
 0503 
 
 20 
 
 3420 
 
 65 
 
 9063 
 
 20 
 
 3640 
 
 65 
 
 2-1445 
 
 21 
 
 3584 
 
 66 
 
 9i35 
 
 21 
 
 3839 
 
 66 
 
 2-2460 
 
 22 
 
 "374 6 
 
 67 
 
 9205 
 
 22 
 
 4040 67 
 
 2 '3559 
 
 2 3 
 
 3907 
 
 68 
 
 9272 
 
 2 3 
 
 4245 68 
 
 2-475I 
 
 24 
 
 4067 
 
 69 
 
 9336 
 
 24 
 
 4452 69 
 
 2-6051 
 
 25 
 
 4226 
 
 70 
 
 '9397 
 
 25 
 
 4663 70 
 
 2-7475 
 
 26 
 
 43 8 4 
 
 7 1 
 
 9455 
 
 26 
 
 4877 j 71 
 
 2-9042 
 
 27 
 
 4540 
 
 72 
 
 '95 IT 
 
 27 
 
 5095 72 
 
 3-0777 
 
 28 
 
 4695 
 
 73 1 -9563 
 
 28 
 
 5317 
 
 73 
 
 3-2709 
 
 29 
 
 4848 
 
 74 
 
 9613 
 
 2 9 
 
 '5543 
 
 74 
 
 S'4874 
 
 30 
 
 5000 
 
 75 
 
 9659 
 
 30 
 
 '5774 
 
 75 
 
 37321 
 
 31 
 
 5150 
 
 76 
 
 9703 
 
 31 
 
 '6009 
 
 76 
 
 4-0108 
 
 3 2 
 
 5299 
 
 77 
 
 "9744 
 
 32 
 
 6249 
 
 77 
 
 4-33I5 
 
 33 
 
 5446 
 
 78 
 
 9781 
 
 33 
 
 6494 
 
 78 
 
 4-7046 
 
 34 
 
 5592 
 
 79 
 
 9816 
 
 34 
 
 6745 
 
 79 
 
 5-1446 
 
 35 
 
 '573 6 
 
 80 
 
 9 8 4 8 
 
 35 
 
 7002 
 
 80 
 
 5 >6 7i3 
 
 36 
 
 5878 
 
 81 
 
 9877 
 
 36 
 
 7265 
 
 81 
 
 6-3138 
 
 37 
 
 6018 
 
 82 
 
 9903 
 
 37 
 
 7536 
 
 82 
 
 7'S4 
 
 38 
 
 6i57 
 
 83 
 
 9925 
 
 38 
 
 7813 
 
 83 
 
 8-1443 
 
 39 
 
 6293 
 
 84 
 
 '9945 
 
 39 
 
 8098 
 
 84 
 
 9'5i44 
 
 40 
 
 6428 
 
 85 
 
 9962 
 
 40 
 
 8391 
 
 85 
 
 1 1 "43 
 
 4i 
 
 6561 
 
 86 
 
 997 6 
 
 41 
 
 8693 
 
 86 
 
 14-30 
 
 42 
 
 6691 
 
 87 
 
 99 86 
 
 -42 
 
 9004 
 
 87 
 
 19-08 
 
 43 
 
 6820 
 
 88 
 
 '9994 
 
 43 
 
 9325 
 
 88 
 
 28-64 
 
 44 
 
 6947 
 
 89 
 
 99 9 8 
 
 44 
 
 9657 
 
 89 
 
 57-29 
 
 45 | 7071 
 
 90 
 
 I 'OOOO 
 
 45 
 
 I'OOOO 
 
 90 
 
 Infinite 
 
INDEX 
 
INDEX. 
 
 N.B. The numbers refer to the paragraphs in which the items occur, 
 not to the pages. 
 
 PARAGRAPH 
 
 * A,' working circuit . . . . . . .187 
 
 A. B.C. Instrument . . . . . . . 172, 197 
 
 faults in ........ 237 
 
 Absolute units . ..... (Def. 35, Sec. A.) 
 
 Accurate signalling, rules for . . . . .90 
 
 Action : 
 
 ( Chemical] of Bunsen's battery . . . . 12 
 
 Daniell's battery . . . . . .12 
 
 Grove's battery . . . . . 12 
 
 Leclanche's battery . . . . .12 
 
 Minotti battery . . . . 12 
 
 (Electric] of alarum or trembling bell . . . . 98 
 
 electro-magnetic shunt . . . . . 122 
 
 polarised relay . . . . . .60 
 
 sounders and non-polarised instruments generally . . 74 
 (Electrolytic}', cables, lines, and earth- wires (Def. 36, Sec. A), (App. A, 
 
 Law 7) .194, 195. 2 3 2 239 
 
 (Electro-magnetic] of a current on a magnet . . . 49-54 
 
 (Local] in battery cells . . . . . . 33 
 
 (Magnetic} of one magnet on another . . (App. A, Law 23) 
 
 (Magneto-electric} of a magnet on a current . . 118, 166, 167 
 
 Adjustment of battery power . . . 41,251,256,294 
 
 Adjustment of instruments : 
 
 Electric bells ....... 99, 102 
 
 Relays . . . . . . . . . 66 
 
 Sounders . . . . . . 76,81,83,86 
 
 Air, in batteries, effect of . . . . . 23 
 
 Alarum : 
 
 Adjustment of . . . . . . . 99 
 
 Principle of ....... 98-100 
 
 Alphabet (Morse) . . . . . . . 88, 89 
 
 X 
 
306 MANUAL OF TELEGRAPHY. 
 
 PARAGRAPH 
 
 Amalgamation of battery zincs .... (Def. 33, Sec. A) 
 
 Ampere unit of current .... (Defs. 2, 34, Sec. A) 
 
 Ampere's Law (App. A, Law 13). . > . . . -53 
 
 Amperian currents (Def. 49, Sec. A), (App. A, Law 25) . . . 118 
 
 Anode ... ... (Def. 26, Sec. A) 
 
 Arrangement : 
 
 ' Calling in ' . . . . . . . 187 
 
 For testing batteries from signal room ..... 288 
 
 Of batteries for local circuits ..... 42, 181 
 
 Arriving currents, to measure . . . . . 289-291 
 
 Astatic : 
 
 Galvanometer (Def. 32, Sec. A) . . . . . 154 
 
 Needle to remagnetise ....... 238 
 
 Atmospheric electricity, its effect on lines . . . . 242 
 
 Attraction, magnetic .... (App. A, Laws 23 and 24) 
 
 Balance : 
 
 Permanent ....... 201, 207, 210 
 
 Transient ...... 203, 207, 210 
 
 Wheatstone's (or bridge) 163, 164, 246, 247, 293-298, 303, 322-333, 339, 341 
 
 Batteries (see Section B) : 
 
 Adapted to circuit for which required . . . 41, "25 1-257, 294 
 
 Air or gas in, effects of, and remedy . . . 23, 27 
 
 Bichromate of potassium . . . . . .10 
 
 Constant (Def. 20, Sec. A) ... 15, 30, 31 
 
 Changes manifested by working . . . -27, 28, 223 
 
 Conditions to be fulfilled by . . . . 13 
 
 Conservancy of, rules for . . . . . -45 
 
 Copper deposit in, treatment of . . . 39, 40 
 
 Diaphragm, use of . . . . . 17? 18 
 
 Defects in, and their remedy . . . .23, 216-224 
 
 Dismantled . . . . . . . 39, 40 
 
 Efficiency of . . . . . ... 46 
 
 Electromotive force of (Def. 6, Sec. A) . .2, 14, 30, 31, 43, 251, 253 
 
 E.M.F. of, methods of testing: ..... 279-287 
 
 1. With tangent galvanometer ..... 280 
 
 2. With the bridge . . . . . . . 281 
 
 3. W T ith any galvanometer ..... 282 
 
 4. With any galvanometer and shunt . . . . . 283 
 
 5. Wheatstone's .... ... 284 
 
 6. By opposing equal E. M. F. . . . . 285 
 
 7. Poggendorff s method . . . . . . 286 
 
 8. With a reflecting galvanometer . . . 287 
 E.M.F. of Minotti, constant . . . . . 30, 31 
 Equating for P work . . 186 
 Exhausted . . 27, 28, 39, 40 
 Faults in . . - 216-224 
 
INDEX. 307 
 
 Batteries continued. PARAGRAPH 
 
 For ' line ' circuits ...... 26, 46 
 
 For ' local ' circuits ..... 26, 42, 46 
 
 For testing purposes . . . . . . .46 
 
 How adapted to circumstances . . . . 41, 42, 251, 256 
 
 Insulation of . . . . . . . .38 
 
 Joined for ' tension ' or ' quantity ' . . . . . 254 
 
 Leakage of, cause, effect, and remedy .... 37, 219, 220 
 
 Local action in ....... 33, 218 
 
 Management of, rules for . . . . -45 
 
 Mixing of liquids ...... 29,40,217 
 
 Polarisation of, by current (Def. 19, Sec. A) . 13, 15, 23, 222 
 
 Portable . . . . . . . . . 47 
 
 Preparation of (rules for) . . . . . -44 
 
 Reserve, for emergencies . . . . . 22 
 
 Resistance of . . . . . . 17, 22, 27, 28, 30 
 
 Resistance of: Methods of Testing ..... 267-278 
 
 1. With departmental tangent galvanometer . . . 269 
 
 2. Poggendorff s with the bridge . . . . 270 
 
 3. Sir W. Thomson's method . . . . .271 
 
 4. With sine galvanometer . . . 272 
 
 5. With tangent galvanometer , . . . . . . 273 
 
 6. With differential galvanometer . . . . 274 
 
 7. With reflecting galvanometer . . . . . 275 
 
 8. Mance's bridge method . . . . . 276 
 
 9. Mance's method of opposing E.M.F. . . . .277 
 10. With the calibrated galvanoscope . . . . 278 
 
 Salt, effect of adding . . . . . . .20 
 
 Shaking of cells, why to be avoided . . . 29, 40 
 
 Standard cell, use of . . . . . 43, 45, 46 
 
 Testing of, for E.M.F. ...... 279-287 
 
 ,, resistance ..... 267-278 
 
 To bring into working order . . . . 19, 30 
 
 ,, ,, rapidly . . . . .21 
 
 Zinc sulphate in, harm and remedy . . . 35 
 
 Battery : 
 
 Bunsen's, description and use ...... 9 
 
 Bichromate of potassium . . . . . 10 
 
 Cells, best deflection for . . . . .46 
 
 Best resistance for . . . . . 46 
 
 , , Current of, why unequal . . . . . 32 
 
 ,, < Short circuited ' . . . . . . . 19 
 
 ,, In series' . . 19, 254 
 
 ,, 'Parallel' . ... 19, 254 
 
 Circuit ........ I, 173 
 
 Commutator . . . . . . 140 
 
 Connections . . . . . . . 24, 25, 239 
 
 Daniels, description and use . . . . . . 5 
 
 Fuller's, ,, . . . . . II 
 
 X 2 
 
308 MANUAL OF TELEGRAPHY. 
 
 continued. PARAGRAPH 
 
 Grove's, description and use . . . . 8 
 
 Lee lane he* s, ,, ,, . . . . .7 
 
 MinottfS) description and advantages of . . . . 4, 6, 16 
 
 Plates, to clean . . . . . . 36 
 
 Polarisation of (Def. 19, Sec. A) ... 13, 15, 23, 222 
 
 'Poles' ........ I 
 
 ' Power,' adaptation of, to circuit . . . 41, 42, 251-256, 294 
 
 Resistance, when taken into account ... . . . 252 
 
 ,, To measure by various methods . . . 268-278 
 
 Reverser . . . ..... . 138, 208 
 
 Syringe, use of . . . . . . . 34 
 
 Testing . ... . . . 267-288 
 
 Zincs, composition for . . . . 35 
 
 ,, corrosion of ..... . . -35 
 
 Beats : 
 
 ' Contact ' and ' induction,' to distinguish between . . - . . 321 
 
 On instruments, ' sticking ' 69, 70, 226, 227, 242 
 
 Bells : 
 
 Electric. (See Alarum.) . . . . . 98-103 
 
 single stroke . ... . . . 101-103 
 
 faults in . . . . . . . 230 
 
 Best deflection for battery cells through tangent galvanometer . . 46 
 
 Best resistance for : 
 
 Battery cells, line . . . . . . . . 46 
 
 ,, local .. . . . . . .46 
 
 ,, testing . . . . . . . 46 
 
 ,, standard . . .. .. . . .46 
 
 Earth plates . . . 335 
 
 Electro-magnets ., . . . . (App. A, Law 15) 
 
 Best wire for : 
 
 Office connections . .... . . . 211 
 
 Resistance coils . . . . . . . .161 
 
 Bias to relay tongue ... . . . . 61 
 
 Bichromate of potassium battery . . . . .10 
 
 Bifilar, winding of coils . . . . . 162 
 
 Bosscha's corollaries ..... (App. A, Law 3) 
 
 Bridge, Duplex ., .. .. .. . . . 199-204 
 
 Bridge : 
 
 Wheatstone's . . 163, 164, 246, 247, 293-298, 303, 3 2 2-33.3> 339, 341 
 Battery testing with .... 270,271,276,281 
 
 Earth testing with ...... 339, 341 
 
 Line testing with, regular ...... 293-318 
 
 for faults . 319-333 
 
 Resistances measured by . . . . . . 245, 266 
 
 Bunsen's Battery .... -9 
 
 B.W.G. (Birmingham wire gauge) . . . . 41 
 
INDEX. 309 
 
 PARAGRAPH 
 
 Cable core, insulating materials of . . . . . .211 
 
 Cables and land lines, faults in . . . 241, 242, 321-333 
 
 Calibrated galvanoscope ..... 147, 278, 338 
 
 Calibration . . . . . . . . . 146 
 
 Calling-in arrangement . . . . . . .187 
 
 Capacity : 
 
 Electrical ...... (Def. 15, Sec. A) 
 
 Unit of. . . . ' . " . . (Def. 16, Sec. A) 
 
 Specific inductive . . . . . (Def. 18, Sec. A) 
 
 Test, for insulation of line ...... 332 
 
 To measure . . . . . . . 334 
 
 Capillary action in battery cells . . . . . 35> 37 
 
 Cells: 
 
 Arrangement of ..... 41, 42, 251-256, 294 
 
 Best resistance for . , . . .46 
 
 Bubbles of air or gas in, their effect and remedy . . . 23, 27 
 
 Capillary action in . . . 35, 37 
 
 Connecting wires of, to join . . . 24, 25, 239 
 
 Current of, why not uniform . . . . . 32 
 
 Difference between ' line ' and ' local ' . . . 26 
 
 Dismantled . . . . . . . 39, 40 
 
 Exhausted . . . . . . . 27, 28, 39, 40 
 
 Joined for quantity, in series, and on short circuit . . 19, 254 
 
 Leakage between, cause, effect, and remedy . . 37, 219, 220 
 
 Local action in . ...... 33,218 
 
 Minotti . . . . . . . . 4, 6, 16 
 
 Mixing of liquids in ...... 29,40,217 
 
 Newly prepared . . . . . . 19, 20, 21 
 
 Parallel . . . . . . .19, 254 
 
 Preparation of, rules for . . . . . . . 44 
 
 Reserve ........ 22 
 
 Resistance of . . . . .27, 28, 32, 46 
 
 ,, to adjust ..... 17,30,222,223 
 
 ,, to measure ...... 267-278 
 
 Shaking of, why avoided ..... 29, 40, 217 
 
 Standard cell, object of . . . . 43, 45, 46 
 
 ,, right resistance for . . . . .46 
 
 Sulphate of copper, quantity required for . . . 26 
 
 ,, ,, remaining in . . . -39 
 
 ,, zinc, harm and remedy . . . 35 
 
 To bring into working order . . . . . 19, 30 
 
 ,, ,, rapidly on emergencies . 21 
 
 To replenish ........ 34 
 
 Changes manifested by working batteries . . .27, 28, 223 
 
 Charge, static, of telegraph lines, &c. .... 109-121 
 
 Chemical action of various batteries . . . . 12 
 
 Circuit : 
 
 Electrical (Def. 13, Sec. A) . . . 173, 174, 177 
 
310 MANUAL OF TELEGRAPHY. 
 
 Circuit continued. PARAGRAPH 
 
 'A' working . . . . . . . 187 
 
 'ABC' working ....... 197 
 
 Battery . . . 173, 174 
 
 Calling-in arrangement . . . . . .187 
 
 Closed system of working . . . , . . 179, 189 
 
 Current strength at all points of . . . . .176 
 
 ' D ' working . . . . . . . 185 
 
 Derived or divided (Def. 21, 22, Sec. A) . . . . 163 
 
 Double current working . . . . . 65, 191-1 94 
 
 Duplex, bridge method ..... 199-204 
 
 ,, differential ...... 205-208 
 
 ,, split battery . . . . . . 209, 210 
 
 Earth . . . . . . . . . 177 
 
 'G' working ........ 185 
 
 Local, and arrangement of cells * . . . . . 42, 181 
 
 Microphone ........ 172 
 
 Office . . . . . . . . 182 
 
 Open system of working . . . . 179, 180, 182 
 
 ,, ,, looped . . 188, 190 
 
 'P' working . . . . . . . .186 
 
 Needle instrument . . . . . . . . 190 
 
 Reverse current, system of working .... 65, 191-194 
 
 ' S ' working ...... 179, 180, 182 
 
 Telephone ........ 1720 
 
 Test, of lines ....... 297, 299 
 
 ' T ' or translation working ..... 183, 184 
 
 Worked with positive signalling currents . . . . . . 194 
 
 ,, negative signalling currents . ' . . 195 
 
 Working, faults in, and their remedy (see Sec. E) . . . 214-242 
 
 Zinc sending . . . . . . . 196 
 
 Circuits : 
 
 (See Sec. D) . 173-213 
 
 Symbolically represented . . . . . . 175 
 
 Cleaning battery plates . . . . . 36 
 
 ' Closed circuit ' . . . . . . .179,189 
 
 Coefficient of coils ... . . 262 
 
 Coercive force . . . . . . (Def. 43, Sec. A) 
 
 Coils : 
 
 Bifilar winding of . . * . . . . . 162 
 
 Electro-magnetic . . . . . .54 
 
 Galvanometer, use of two ...... 261-263 
 
 Heated by current . . . . . . .161 
 
 Induction . . . . . . . ..118 
 
 Magneto-electric ...... 166, 167 
 
 Reduction, coefficient of .... . 262 
 
 'Resistance' . . . . . . . 160 
 
 ,, best wire for . . . . . . 161 
 
 Communication, natural causes, obstructing, and their remedy . . 242 
 
INDEX. 311 
 
 Commutator : PARAGRAPH 
 
 For batteries . . . . . . . . 141 
 
 For lines . . . ... . . . 140 
 
 For magneto -electric currents . . . . . . 169 
 
 Faults ..... ... 234 
 
 Composition for battery zincs . . . . . 35 
 
 Condensers : 
 
 Description and use of . . . . . . 116, 117 
 
 For duplex working . . . . . 202 
 
 Faults in ........ 231 
 
 Conduction, test, of lines . . 296,313,314 
 
 Conductivity, reciprocal, of resistance . . . (Def. 9, Sec. A) 
 
 Conductor . . . ... . . (Def. 8, Sec. A) 
 
 For lightning, to test ....... 343 
 
 Conjugate conductors ..... (Def. 37, Sec. A) 
 
 Connections : 
 
 Batteiy. . . . . . . . 24, 25, 239 
 
 Office . . . . . .211, 212, 213, 238, 239 
 
 Conservancy of batteries, rules for ..... 45 
 
 Constant : 
 
 Batteries (Def. 20, Sec. A) . . . . . 15, 30, 31 
 
 of galvanometer ..... (D'ef. 27, Sec. A) 
 
 Resistance key . . . . . . . . 107 
 
 Contacts : 
 
 Platinum ..... 79, 226, 232, 233, 238 
 
 Between lines . . . . . . . . 241 
 
 On a double line, to localise .... 327, 328, 329 
 
 On a line where a good wire is available, to localise . . . 330 
 
 Convolutions, effect of, in galvanometer . . . . .54, 144, 261 
 
 Copper : 
 
 Deposit in batteries, treatment of . . . . . 39, 40 
 
 Plate, to clean ........ 36 
 
 Sulphate in batteries ...... 26, 39 
 
 Core (Hooper's), for cables and office wires . . .211, 212 
 
 Correction of measured values . . . 301,311-314,328 
 
 Corrosion of battery zincs . . . . -35 
 
 Covered wires ...... 211,212 
 
 Criterion : 
 
 Of efficiency of earths ....... 355 
 
 ,, galvanometers .... 148, 153, 155 
 
 relays ... . . 71 
 
 ,, battery cells . . . . 46 
 
 Of exhaustion of battery cells . . . . . 27, 28 
 
 Of magnetisation of galvanometer needles . . . . 238 d 
 
 Of preparation of battery cells ... . . 40 
 
 Of uniform insulation of line . . . . . . 312 
 
 Of state of insulation of line . . . . . 311 
 
 * Cross Leakage,' between battery cells . . . 37> 220 
 
312 MANUAL OF TELEGRAPHY. 
 
 Current : PARAGRAPH 
 
 Arriving, through a telegraph line . , . . 289-291 
 
 Electric or galvanic (see Introductory Remarks) . (Def. 4, Sec. A) 
 
 Heating effect of . . . . . . 161, 236 
 
 Intensity or strength . . . . (Def. 3, Sec. A) 
 
 Result of chemical action . . . . . i 
 
 Reverser . . . . . . .138, 208 
 
 Strength (effective) varies with resistance in circuit . 32, 38, 208, 253 
 
 Strength at all points of a circuit . *. . . .176 
 
 Unit of ....... (Def. 2, Sec. A) 
 
 Currents : 
 
 Action of, on magnets or soft iron (App. A, Law 13) 49, 50, 51, 52, 53, 54 
 Action on one another ..... (App. A, Law 4) 
 
 Amperian (Def. 49, Sec. A), (App. A, Law 25) . . 118 
 
 Earth or natural (Def. 29, Sec. A) . . 242, 308-310 
 
 Extra (Def. 23, Sec. A), (App. A, Law 21) . . 119, 120, 121 
 
 Induced, by currents (App. A, Law 20) . . . .118 
 
 Induced, by magnets ...... 166, 167 
 
 Magneto-electric . . . . . . .170 
 
 Maximum from a battery ..... (App. A, Law 5) 
 
 Primary . . . . . . . .118 
 
 Received, to measure . . . . . 289-291 
 
 Return (Def. 24, Sec. A) ..... 112, 119 
 
 Reverse . . . ... . . 65 
 
 Thermo-electric (on line) . 242 
 
 Transient . . . . . . . 321, 334 
 
 D working circuit . . . . . . . .185 
 
 Daniell's battery . . . . . . Sfc 
 
 D'Arlincourt relay ...... 130-132 
 
 Dead earth, to localise . . . . . 324, 326 
 
 Defects and their remedy : in 
 
 Batteries . . . . . . . . 216-224 
 
 Earths ......... 240 
 
 Galvanometers . . . . . . .235, 238 
 
 Transmitting and receiving apparatus . . . 225-234, 238 
 Definitions. (See Sec. A.) 
 Deflections : 
 
 Best with standard cell and tangent galvanometer . . .46 
 Controlled by adjustment of E.M.F. and galvanometer shunts . . 257 
 
 Inversely proportional to resistance . . . . 32, 253 
 
 Measurement of resistances by . . . . 250, 257-266 
 
 Vary with different galvanometers . . ... 144, 262 
 
 Vary with work done by battery . . . . 32 
 
 Demagnetisation of permanent magnets . . . . 226, 238 
 
 Deposit : 
 
 Copper in batteries . . . . . . 39, 40 
 
 Zinc sulphate in batteries . . . . . -35 
 
INDEX. 313 
 
 PARAGRAPH 
 
 Derived circuit (Def. 21, 22, Sec. A) . . . . . . 163 
 
 Current strength in . . . . (App. A, Law 6) 
 
 Combined resistance of . . . . (App. A, Law 8) 
 
 Detector (insulator and joint) . . . . . 171 
 
 Dial instrument . . . . . . . 172, 197 
 
 Faults in ........ 237 
 
 Diaphragm : 
 
 in batteries ; form and use of . . . . . 17, 18 
 
 Defects in . . . . . . 23 
 
 Dielectric (Def. 17, Sec. A) . . . 115, 116, 117, 231 
 
 Difference : 
 
 between line and local cells ..... 26, 46 
 
 of potential (see Introductory Remarks) . . . (Def. 5, Sec. A) 
 
 Differential (Def. 30, Sec. A) : 
 
 Duplex . . .... . . 205-208 
 
 Galvanometer . . . . . . 165 
 
 ,, To measure resistance with .... 248 
 
 ,, To test batteries with . . . . . 274 
 
 , , To test earths with . . . . 340, 342 
 
 ,, To test efficiency of . . . . . 249 
 
 Directive action of a current on a magnet . . . (App. A, Law 13) 
 
 Dirt or dust, faults caused by . . . . . . 215 
 
 Discharge . . . . . . . . 109-121 
 
 Discharging arrangements . . . . . 125, 130, 132 
 
 For translation working . . . . . .184 
 
 Discharging instruments : 
 
 (Key) . . ... . . . ' . . 128 
 
 (Relay) . . . . . . .126, 127 
 
 Faults in . ... . . . . 230 
 
 Disconnection : 
 
 of line circuit ....... . . . . 241 
 
 to localise . ... . . . . . 331-333 
 
 Dismantled battery cells . . . . . . 39, 40 
 
 Displacement of resultant fault . . . . . . 316 
 
 Divided circuit. (See Derived Circuit. ) 
 
 Double current : 
 
 Differential duplex . . ..... . . 208 
 
 Key ........ 106, 107 
 
 Working ...... 65, 191-194, 208 
 
 Double needle instrument . . . . . 97 
 
 Douglas sounder . . . . . . 80, 8 1 
 
 Dubern sounder ..... 82, 83, 84 
 
 Duplex : 
 
 Telegraphy ...... 198-210 
 
 Working, bridge method ...... 199-204 
 
 Differential ...... 205-208 
 
 Split battery ..... . 209, 210 
 
 Duration of battery cells . ..... . . .26 
 
3 14 MANUAL OF TELEGRAPHY. 
 
 Circuit of . . . . . . . . . 177, 240 
 
 Electrical standard required . . . . -335 
 
 Insulation, where necessary . . . . 178 
 
 Earth plates . . . . . . . 178 
 
 Not to be in metallic contact . . . . 335 
 
 Polarisation of . . . . . . . . 240 
 
 To lay (precaution) . . . . . 178 
 
 Earth currents, or natural currents (Def. 29, Sec. A) . 242, 308-310 
 
 Earth faults : 
 
 ' Dead * or * partial ' . . . . . . 241 
 
 Dead earth on a single line, to localise . . . 324 
 
 Earth on a multiple line, to localise . . . 326 
 
 Partial earth on a single line, to localise . . . .325 
 
 Earth testing : 
 
 with bridge . . . . . . . 339, 341 
 
 with calibrated galvanoscope ...... 338 
 
 with differential galvanometer ..... 340, 342 
 
 with tangent galvanometer ..... 336, 337 
 
 Effective strength of current varies with resistance in circuit . . 32, 253 
 
 Efficiency of : 
 
 Discharging instruments . . . . . 127, 134 
 
 Earths . . . . . . . . . 335 
 
 Electric bells . . . . . . 100, 103 
 
 Electro-magnetic shunts . . . . 124 
 
 Galvanometers ..... 148, 153, 155 
 
 Relays . . , . . . . 71 
 
 Sounders . . . . . . 77, 81, 84, 87 
 
 Electric : 
 
 Current . . . . (See Introductory Remarks, Sec. A) 
 
 Potential . . (See Introductory Remarks, Sec. A & Def. 3} 
 
 Quantity . . . . . . . (Def. i, Sec. A) 
 
 Resistance . . . . . (Def. 10, Sec. A) 
 
 Electric bells . . . . . . . .98-103 
 
 Electricity (see Introductory Remarks, Sec. A) : 
 
 Atmospheric, effect of, on lines ..... 242 
 
 Electrification. ...... (Def. 14, Sec. A) 
 
 Electrolyte. ...... (Def. 26, Sec. A) 
 
 Electrolytic action (Def. 26, Sec. A), (App. A, Law 7) 194, 195, 232, 239, 242 
 
 Electrostatic : 
 
 Capacity (Def. 15, 16, Sec. A) . . . 332, 334 
 
 Induction . . . . . . . . 114, 121 
 
 Electro-magnetic : 
 
 Capacity . . . . . . . 121, 122 
 
 Shunt ...... 65, 122, 123, 124 
 
 ,, Faults in . . . . . . 230 
 
 Electro-magnetism and its application to telegraphy . . . 49-57 
 
 Electro-magnets : 
 
 Best resistance for . . . . (App. A, Law 15) 
 

 INDEX. 315 
 
 Electro-magnets continued. PARAGRAPH 
 
 Effect of convolutions of . . . . . . . 54 
 
 Extra current in (App. A, Law 22) .... no, 121 
 
 Magnetic intensity of (App. A, Law 16) . . . 57 
 
 Polarity of . . . . (App. A, Law 14) 
 
 Faults in . . . . . . . . 230 
 
 Elements (galvanic) . . . . . . .5-12 
 
 Electromotive force : 
 
 (Def. 6, Sec. A) . . 2, 14, 30, 31, 43, 251, 253, 257, 279, 310 
 
 Constant for all similar elements . . . . 30, 31 
 
 Dependent on chemical combinations . . . 2 
 
 Of batteries, to measure by various meth ods . . . 279-287 
 
 Embossing recorders . . . . . . . . 94 
 
 Equating battery, for P work . . . . . .186 
 
 Equator, magnetic ...... (Def. 39, Sec. A) 
 
 Exhausted batteries . . . . . 27, 28, 39, 40 
 
 Extra current (Def. 23, Sec. A): . . . . 119, 120, 121, 162 
 
 Laws of ...... (App. A, Law 21) 
 
 In electro-magnets (App. A, Law 22) . . . . 119, 121 
 
 Farad, unit of capacity ..... (Def. 16, Sec. A) 
 
 Fault : 
 
 Resultant (bzL 36, Sec. A) ..... 316,317 
 
 Testing ....... 3 * 9-333 
 
 Faults : 
 
 In apparatus generally . . . . . . 214, 215 
 
 In batteries ....... 216-224 
 
 In earth circuit . . . . . . . 240 
 
 Line circuit ...... 241, 242, 321-333 
 
 Office circuit, including instruments .... 225-239 
 
 ,, to discover ...... 320 
 
 Field, magnetic ..... (Def. 40, Sec. A) 
 
 Fuller's bichromate battery . . . . . .n 
 
 Formulas and their solutions ...... (App. B) 
 
 G working circuit . . . . . . . .185 
 
 Galvanic : 
 
 Current . . . (See Introductory Remarks, Sec. A and Def. 4) 
 
 Elements ........ 5-12 
 
 Polarisation (Def. 19, Sec. A) . . .13, 15, 23, 222 
 
 Galvanometers: ... 55, 143, 144, 145 
 
 Astatic (Def. 32, Sec. A) . . . . . . 154 
 
 Constant of (Def. 27, Sec. A) . . . . . . 259 
 
 Defects in . . . . . . . . 235 
 
 Departmental tangent . . .152, 260-264, 269, 280, 336, 337 
 
 Differential . . 165, 274, 248, 249, 340, 342 
 
 Effect of convolutions of wire in ... 54 
 
316 MANUAL OF TELEGRAPHY. 
 
 Galvanometers continued. PARAGRAPH 
 
 Inaccuracies in . . . . . . . -149 
 
 Needle deflection of ... . . (App. A, Law 18) 
 
 ,, demagnetisation of, causes and remedy . . 226, 238 
 
 Reflecting . . . , . . . 156, 250, 258, 275, 287 
 
 Sine (App. A, Law 19) .... 150, 265, 272 
 
 Tangent (App. A, Law 17) . . -IS 1 } 273, 280, 340, 342 
 
 Thomson's reflecting . . . . .156, 258, 275, 287 
 
 Use of .two coils ....... 261-263 
 
 Galvanoscopes . ... . . . . .142 
 
 Calibrated ... . . .147, 278, 338 
 
 Gas in battery cells, effect of . . . . 27 
 
 Gauge : 
 
 of wire (App. A, Law 11) . . . . . 41, 322, 323 
 
 battery power with reference to . . . . 41 
 
 Gerrit Smith's battery reverser . ... . . . 208 
 
 Gravity battery, principle of . . . . .29 
 
 Grove's battery . . . ..... 9 
 
 Guttapercha for cables or office connections . . . 211,212 
 
 Heat: 
 
 Effect of, on insulation of covered wires . . . . . 211 
 
 ,, resistance of wire . . . . 161, 236, 307 
 
 Heating effect of current ...... 161,236 
 
 Helix, right and left handed . . . . . -5 
 
 High resistance galvanometer, use of . . . . . 261 
 
 Hooper's core, for cables and office connections . . .211, 212 
 
 Horizontal galvanoscope for G working . . . 142 
 
 Hydrogen (free) . . . . . - (Def. 19, Sec. A) 
 
 Inaccuracies in galvanometers . . . . . 149 
 
 Indiarubber for cable core and connections . . . .211 
 
 Induced currents, laws and principles of . . (App. A, Law 20) 
 
 Imperfectly insulated break . . . . -333 
 
 Induction : 
 
 Electrostatic and dynamic . . . . 113,114,118-121 
 
 Magnetic . . . ..... (Def. 41, Sec. A) 
 
 Magneto-electric . . . . . . . . . . . 166 
 
 Retardation caused by . . . . .121 
 
 Inductive : 
 
 Action between telegraph wires . ., . . 118 
 
 Action of current on itself (extra current) . . .119 
 
 Capacity ... . (Def. 15, Sec. A) 
 
 Inertia : 
 
 Magnetic (Def. 48, Sec. A) . . . 121, 226 
 
 Mechanical . . . . 121, 226 
 
INDEX. 
 
 317 
 
 Injurious : 
 
 Effects of salt in battery cells . 
 
 ,, zinc sulphate in battery cells 
 Ink-writers .... 
 
 Defects in 
 
 Instrument faults, nature of 
 Instruments : 
 
 (See Sec. c) 
 
 ABC 
 
 Adjustment of 
 
 Alarum (trembling) 
 
 Astatic galvanometer 
 
 Bells ..... 
 
 Commutators (battery and line) 
 
 Condenser .... 
 
 Constant resistance key 
 
 Current reversers 
 
 D'Arlincourt relay . 
 
 Departmental tangent galvanometer 
 
 Dial .... 
 
 Differential galvanometer 
 
 Discharging key 
 
 Discharging relay 
 
 Double current key 
 
 Double needle .... 
 
 Douglas sounder 
 
 Dubern sounder 
 
 Electro-magnetic shunt 
 
 Embossing recorder 
 
 Galvanometers 
 
 Galvanoscope, calibrated 
 
 ,, horizontal, for G working 
 
 Ink-writer .... 
 
 Insulator and joint detector 
 
 Keys or handles 
 
 Leyden jar . 
 
 Magneto-electric 
 
 Microphone 
 
 Needle. .... 
 
 P switch .... 
 
 Portable sounder 
 
 Recording .... 
 
 Reflecting galvanometer 
 
 Relay, D'Arlincourt 
 
 Relay, Siemens 
 
 Reverse current key 
 
 S.T.D. switch 
 
 Shunts, electro-magnetic . 
 ,, (galvanometer). . 
 
 PARAGRAPH 
 
 20 
 
 35 
 9i 92, 93 
 
 . . 228 
 . 225 
 
 . 48-172 
 . 172 
 
 66, 76, 81, 83, 86, 99, 102 
 99, 100 
 
 154, 155 
 
 98-103 
 . 140, 141 
 
 . Il6, 117, 202, 231 
 
 . . 108 
 
 . 138 
 
 . 130-132 
 
 152, 153 
 
 . . 172 
 
 . 165 
 
 . . 128 
 
 126, 127 
 
 . 106, 107 
 
 . 97 
 
 . 80, 81 
 
 82, 84 
 
 . 122 
 
 94 
 
 143-165 
 
 . 147 
 
 . . 142 
 
 . 91, 92, 93 
 
 . . 171 
 
 . . 104-108 
 
 . . US 
 
 166-172 
 
 . 172 (*) 
 
 95-97 
 
 135 
 
 85-87 
 
 . 91-94 
 
 . 156 
 
 . 130-132 
 
 58-71 
 
 . 106, 107 
 
 . 136 
 
 . 122 
 157-159 
 
3l8 'MANUAL OF TELEGRAPHY. 
 
 Instruments continued. PARAGRAPH 
 
 Shunts, prolonging. . . . . . 126 
 
 Siemens' lightning discharger . . . . . 133 
 
 ,, ink-writer . . .", . . 92, 93 
 
 Morse .... 73-78 
 
 Sine galvanometer . . . . . 150 
 
 Single needle ........ 96 
 
 Single current key ,. . . . . . . . 105 
 
 Single stroke bell . . . . . . 101-103 
 
 Sounders ........ 72-87 
 
 State railway sounder . . . . . 80, 8 1 
 
 Switches ........ 135-141 
 
 Tangent galvanometer . . . . . . 151-153 
 
 Telephone . . . . . . . 172 (a} 
 
 Wheatstone's bridge or balance. . . . . 163, 164 
 
 Zinc sender ....... 129, 196 
 
 Insulated break, to localise . . . . .331, 332 
 
 Insulating material for cables . . . . . 211 
 
 Insulation : 
 
 of batteries . ..... 38 
 
 of earth wires, necessity for . . . . ..178 
 
 Insulation test of lines ..... 298,313,314,331 
 
 Insulator : 
 
 Definition of ..... (Def. n, Sec. A) 
 
 Detector . . . . . . . .171 
 
 Intensity (of current). (See Potential.) . . . (Def. 3, Sec. A) 
 
 Internal resistance : 
 
 of battery cells, how regulated . . . . . 17, 30 
 
 ,, to measure ..... 267-278 
 
 I.W.G. (Indian wire gauge) . . . . . 41 
 
 Ions . ' . . . . . (Def. 26, Sec. A) 
 
 Iron, to render soft ........ 238 
 
 Joint detector . . . . . . . . 171 
 
 Joints in office wires . . . . .- . .212 
 
 battery wires . . . . . . 25 
 
 Kathode ....... (Def. 26, Sec. A) 
 
 Key: 
 
 Constant resistance . . . . .108, 206 
 
 Discharging . . . . . . . .128 
 
 Faults in . . . . . . . 232 
 
 Morse . ..... 104, 105 
 
 Reversing ........ 106, 107 
 
 * Kick,' use of, in D'Arlincourt's relay ..... 132 
 
 Kirchhoff's laws ...... (App. A, Law 2) 
 
INDEX. QRH\^ X3I9 
 
 PARAGRAPH 
 
 Laws ..... (See App. A ; Laws and Principles) 
 
 Leading wires, best insulating covering for . . . . . 211 
 
 Leakage : 
 
 of batteries . . . . . . 37> 219, 220 
 
 of lines, corrections for . . . 301,311-314,328 
 
 Leclanche M s battery . . .7 
 
 Lever switch ... ... 139 
 
 Leydenjar. . . . 115 
 
 Lightning : 
 
 Conductors, to test ... . 343 
 
 Dischargers ...... -133 
 
 ,, faults in .... . . 233 
 
 Line : 
 
 and local cells, difference between ... 26, 46 
 
 Batteries, preparation of . . . . 21, 44 
 
 Lines : 
 
 Charge and discharge .... .109-117,121 
 
 How affected by earth currents (Def. 29, Sec. A) . . 242, 308-310 
 
 Length, gauge, battery power, and resistance . 41 (Law II, App. A) 
 
 Lines or cables, faults in . . 194, 195, 241, 242 
 
 Line testing : 
 
 By strength of received currents . . . .291 
 
 For faults .... . 3*9-333 
 
 Regular . . . 292-318 
 
 Line wire, resistance of (App. A, Law 1 1) . . . 307 
 
 Local : 
 
 Action in battery cells . . . . . 33, 218 
 
 and line cells, difference between . . .26, 46 
 
 batteries, arrangement of .... .42 
 
 Local circuit . . . . . . 42, 72, 181 
 
 Localisation of faults : 
 
 in offices ........ 320 
 
 lines ...... . 321-333 
 
 (Resultant fault) . . . . .. . 317 
 
 Loop test . . 326, 329, 330 
 
 Looped open circuit working . . . 1 88, 190 
 
 Low resistance galvanometer, use of . . . . 261 
 
 Low resistances, measurement of . . . . . . 255 
 
 Machines, magneto-electric ...... 166-172 
 
 Magnet, action of currents on (App. A, Law 13) . . . 49-54 
 Magnetic : 
 
 Attraction and repulsion .... (App. A, Law 24) 
 
 Equator , . (Def. 39, Sec. A) 
 
 Field . . . . . . (Def. 40, Sec. A) 
 
 Induction . . . (Def. 41, Sec. A) 
 
 Inertia (Def. 48, Sec. A) . . . . . . . 121 
 
320 MANUAL OF TELEGRAPHY. 
 
 Magnetic continued. PARAGRAPH 
 
 Meridian ...... (Def. 42, Sec. A) 
 
 Moment ...... (Def. 46, Sec. A) 
 
 Polarity ...... (Def. 39, Sec. A) 
 
 Magnetisation : 
 
 of astatic needles . . . . . . . 238 
 
 of galvanometer needles . ... . . . 238 
 
 Magneto-electric : 
 
 Currents . -. . ' . . . . . 166, 170 
 
 Instruments ....... 166-172 
 
 Machines . . . . . . . 167, 168 
 
 Magnets : 
 
 Action of, on one. another . . . . (App. A, Law 23) 
 
 Currents induced by . . . . .118, 166, 167 
 
 Portative force of (Def. 45, Sec. A) . . (App. A, Law 26) 
 
 Management of batteries, rules for . . . 45 
 
 Mance's methods of measuring battery resistance . . . 276, 277 
 
 Mathematical proofs of electrical formulae .... (App. B) 
 
 Mechanical inertia . . . . . . .121 
 
 Measurement : 
 
 of arriving currents . . . . . . . 289-291 
 
 battery resistance . . . . . 267-278 
 
 capacity . . ... 334 
 
 electromotive force ...... 279-287 
 
 resistance of ordinary conductors .... 243-266 
 
 ,, of earths and lightning conductors . . 335-343 
 
 Meridian, magnetic ...... (Def. 42, Sec. A) 
 
 Metrical units ...:.. (Def. 35, Sec. A) 
 Microfarad ...... (Defs. 16, 34, Sec. A) 
 
 Microphone . : . . . . . 172 (b) 
 
 Millioerstedt (Def. 2 and 34, Sec. A) . . : . . . 289 
 
 Minotti element, description and advantages of .6, 13, 14, 15, 16 
 
 Moisture, effect on lines . . . . . . 242 
 
 Moment, magnetic . . . . . (Def. 46, Sec. A) 
 
 Monsoon batteries, provision for . . . . . 22 
 
 Morse alphabet : . . . . . . . -89 
 
 Characters . . . . . . ... 88 
 
 Instrument ....... 73-79 
 
 Key . . . . * . . . 104, 105 
 
 Multiple arc. (See Derived Circuit. ) (Def. 21, 22, Sec. A) . . 163 
 
 Multiplying effect of convolutions in galvanometer coils . 54, 144, 261 
 
 power of shunts (App. A, Law 9) ' . . . .159 
 
 Natural currents, or earth currents (Def. 29, Sec. A) . 242, 267, 308-310 
 
 to measure strength of . . . . .310 
 
 Needle, astatic (Def. 32, Sec. A) . . . . . . 154 
 
 To remagnetise ...... 238 
 
INDEX. 321 
 
 Needle instrument : PARAGRAPH 
 
 Single ... 95, 96 
 
 Double. . . . . . . . '97 
 
 Faults in . . . . . . . 229 
 
 Needle, magnetic, deflection of (App. A, Law 18) 32, 144, 250, 253, 257-266 
 
 Negative signalling currents . . . . . 195 
 
 Null methods . ...... (Def. 31, Sec. A) 
 
 Obstacles to telegraphic communication ..... 24^ 
 speed . . . . . . . 121 
 
 Oerstedt's discovery (action of a current on a magnet) . . 52 
 
 Oerstedt (unit of current) (Defs. 2 and 34, Sec. A) . . . 289-291 
 
 Office connections : ...... 211-213 
 
 Faults in, to discover . . . . . . 320 
 
 Joints in ........ 212 
 
 Ohm, unit of resistance . . . (Defs. 12 and 34, Sec. A) 
 
 Ohm's law : 
 
 Illustrated by newly prepared battery cell . . . 30-32 
 
 Simple form of . . . . (App. A, Law i) 
 
 Open circuit working ..... 179,180,182 
 
 looped . .... 188, 190 
 
 Oscillation : 
 
 of magnetic needle ..... (Def. 50, Sec. A) 
 
 Test of magnetism in galvanometer needles . . . 153 (Hi) 
 
 ,, accurate suspension of needle . . . 153 (ii) 
 
 Osmose (Def. 28, Sec. A) . . . . . . 5 
 
 Parallel : 
 
 Cells joined ....... 19, 254 
 
 Instrument coils joined . . . . . 120 
 
 P. switch . . . ..... . . 137 
 
 Relay working . . . . . . . . 186 
 
 Partial earth on a single wire, to localise ..... 325 
 
 Pedal key . . . . . . . . 107 
 
 Permanent balance . . . . . 201,207,210 
 
 Permanent state . . . . . . . . no 
 
 Pivots, to test efficiency of . . , . . . 153 (ii) 
 
 Plates : 
 
 Battery . . . . . . . . . I 
 
 ,, to clean ...... . 36 
 
 Earth . . . . . . . . . 178 
 
 Platinum points ..... 79, 226, 232, 233, 238 
 
 Play: 
 
 of keys ......... 232 
 
 of relay tongue ....... 64, 68, 226 
 
 of sounders. . . . . . . . . 76 
 
 Y 
 
322 
 
 MANUAL OF TELEGRAPHY. 
 
 PARAGRAPH 
 
 Poggendorffs method of testing battery resistance . . .270 
 
 E.M.F. . . . . 286 
 
 Polarisation by galvanic current (Def. 19, Sec. A) 13, 15, 23, 222, 325, 337 
 Polarised earth plates . . . . . 240, 337 
 
 Polarised relay : 
 
 Defects in ....... 63, 226 
 
 System of working . . . . . . 1 86 
 
 Polarity : 
 
 Magnetic ...... (Def. 39, Sec. A) 
 
 electro-magnets ..... (App. A, Law 14) 
 
 Poles, battery ....;...! 
 
 Portable battery, construction and action . . . . . 47 
 
 sounder, ,, ,,.... 85, 86, 87 
 
 Portative force (Def. 45, Sec. A) ... (App. A, Law 26) 
 
 Positive signalling currents . . . . . . 1 94 
 
 Potassium battery . . . . . .10,11 
 
 Potential : 
 
 Difference of . . (See Introductory Remarks, Sec. A and Def. 5) 
 
 Directly proportional to E.M.F. and inversely to resistance . . 3 
 
 Electric . . (See Introductory Remarks, Sec. A and Def. 3) 
 
 Preparation of batteries, rules for . . . 44 
 
 Pressure, effect on cables . . . . . . .211 
 
 Processes by which a battery cell comes into working order . 30 
 
 Prolonging effect of shunts . . . . . .126 
 
 Proofs, mathematical, of electrical formulae . . . (See App. B) 
 
 Quantity : 
 
 Batteries joined for ...... 19,254 
 
 Electric ....... (Def. i, Sec. A) 
 
 ,, unit of. . . . . (Def. 2 and 34, Sec. A) 
 
 Range of relays . . . 71 
 Received currents, to measure ..... 289-291 
 
 Reciprocals . . . (App. A, Law 8) 
 
 Recording instruments . . . . 91-94 
 
 Reduced length (Def. 38, Sec. A) . . 315 
 
 Reduction coefficient of coils . . . . . 262 
 
 Reduction of resistances : 
 
 To corresponding temperatures . . . . . 307 
 
 To corresponding units ...... 306 
 
 Reflecting galvanometer . . . 156 
 
 To measure resistance with . . 258, 275 
 
 To measure E. M. F. with . . . 287 
 
 Regular tests of batteries . . . . . .288 
 
 Regular tests of lines . . 292-318 
 
 Relays and sounders . 58-87 
 
 Defects in working of, and remedy . . . 226 
 
INDEX. 
 
 323 
 
 PARAGRAPH 
 
 Relay, D'Arlincourt ...... 130-132 
 
 Relay resistance, measured and corrected . . . 305, 311-314 
 
 Relay, Siemens ....... 59-71 
 
 Remagnetisation of galvanometer needles . . . . 238 
 
 Repulsion, magnetic ..... (App. A, Law 24) 
 
 Reserve batteries . . . . . . 22 
 
 Residual magnetism (Def. 44, Sec. A) . . 121, 131, 132, 226 
 
 Resistance : 
 
 Apparent (not real) . . . . . . 267 
 
 Coils ........ 160, 236 
 
 Containing E.M.F. . . . . . . 267-278 
 
 Electrical ...... (Def. 10, Sec. A) 
 
 Greater than one million tested with bridge . . . 266 
 
 ,, ,, with reflecting galvanometer . . 258 
 
 Of batteries . . . . .27, 28, 32, 46, 222, 223 
 
 , ,, To measure, by various methods . . . 267-278 
 
 ,, When taken into account .... 252, 253 
 
 Of battery cells, how regulated . . . . . 17, 30 
 
 derived circuit ...... (App. A, Law 8) 
 
 earths and lightning conductors, to measure by various methods 335-343 
 electro-magnets ..... (App. A, Law 15) 
 
 exhausted cells . . . . . . .28 
 
 line cells . . . . . . 46 
 
 line wire ...... (App. A, Law n) 
 
 local cells . . . . . . . . 46 
 
 shunts (App. A, Law 10) . . . . .158 
 
 standard cell . . . . . . 46 
 
 Ordinary, to measure by various methods . . . 243-266 
 
 Reciprocal, of conductivity . . . . (App. A, Law 8) 
 
 Units of (Def. 12, 34, Sec. A) . . . . . . 306 
 
 Restitution, force of . . . . . . 62 
 
 Resultant fault (Def. 36, Sec. A) . . . . . 316, 317 
 
 * Rest force ' . . . . . . . 62 
 
 Retardation of signals . . . . . . .121 
 
 Return current (Def. 24, Sec. A) . . . . . 112, 119 
 
 Reverse current : 
 
 Working ...... 65, 191-194, 208 
 
 Key ........ 106, 107 
 
 Rules for : 
 
 Accurate signalling . . . . . . .90 
 
 Adjustment of battery power to circuit required . . . . 253 
 
 Conservancy of batteries . . . . 45 
 
 Preparation of batteries . . . . . . . 44 
 
 Regulating battery power for line circuits . . . 41 
 
 ,, ,, local circuits . -42, 181 
 
 Testing with the tangent galvanometer ..... 264 
 
324 MANUAL OF TELEGRAPHY. 
 
 PARAGRAPH 
 
 S. working . . . .. .. . , . 179, 180, 182 
 
 Salt in batteries, its use and abuse . . . . . .20 
 
 Saturation point of magnet . . ... . . (Def. 47, Sec. A) 
 
 Section tests ........ 318 
 
 Sensitiveness : 
 
 of galvanometers . . . . . . 144 
 
 of relays ........ 67 
 
 of sounders . . . . . . . . 76 
 
 Series, cells joined in . . .. .. . . 19 
 
 Shaking of cells, why avoided . . . . . 29,40,217 
 
 Short circuit, cells joined on . . . . . 19 
 
 Shunts : 
 
 Electro-magnetic ..... 65, 122, 123, 124 
 
 ,, Faults in ...... 230 
 
 Galvanometer, use and adaptation of . . 157, 257, 283 
 
 Multiplying power of (App. A, Law 9) . . . -159 
 
 Prolonging effect of . . . . . . . . . 126 
 
 Resistance of (App. A, Law 10) . . . . . . .158 
 
 To discharging relays "... . ... . . 126 
 
 Siemens : 
 
 Lightning discharger . . . . . . 133 
 
 Morse instrument {relay and sotmder combined] . . . . 78 
 
 Morse sounder . . . . ... . 73-77 
 
 Siemens relay ........ 59-71 
 
 Siemens unit of resistance . . . . (Def. 34, Sec. A) 
 
 Signalling, rules for . . . . . 90 
 
 Signals : 
 
 Retardation of . . . . . . . .121 
 
 Received measurement of . . . . . . 289-291 
 
 Signal room, battery testing from ...... 288 
 
 Sine galvanometer : 
 
 Principle of (App. A, Law 19) . . . . . . 150 
 
 To measure resistances with . . . . 265, 272 
 
 Single : 
 
 Line. To localise faults in . . 324, 325, 331-333 
 
 Needle instrument ...... 95, 96 
 
 Stroke bell . . ... . . . 101-103 
 
 Solenoid . . . . (Def. 25, Sec. A) 
 
 Solution of formulae ...... (App. B) 
 
 Sounder faults . . . 227 
 
 Sounders and relays ..... . 58-87 
 
 Specific inductive capacity . (Def. 18, Sec. A) 
 
 Speed of signalling (App. A, Law 12) . . . . . 121 
 
 Split battery, Duplex ...... 209, 210 
 
 Sponge for batteries .... . . . 34 
 
 Springs (instrument), defects in . . . 227, 228, 232, 238 
 
 Standard : 
 
 Cell ........ 43, 45, 46 
 
INDEX. 
 
 325 
 
 Standard continued. PARAGRAPH 
 
 Current . . . . . . . 289 
 
 Range of relay . . . . . . ..71 
 
 Resistance of earth plates . , . . . -335 
 
 State railway sounder . . . . . 80, 81 
 
 Sticking of relay tongue, cause and remedy . . 69, 70 
 
 Strength of battery current (Def. 3, Sec. A) . . 32, 176, 253 
 
 Switches ........ 135-145 
 
 Faults in . . . . . . . 234 
 
 S.T.D. switch . . . . . . . .136 
 
 Sulphate of copper in batteries . . . . 26, 39 
 
 Zinc ...... 21, 35 
 
 Symbols to represent telegraphic circuits . . . 175 
 
 Syringes or sponges for batteries . . . . . -34 
 
 Tangent galvanometer . . . . . . . 151 
 
 Best deflection . . . . . . . . 153 
 
 Departmental, construction and principle . . . . . 152 
 
 Inaccuracies in deflection. Remedy . . . . 151, 264 
 
 Principle of (App. A, Law 17) ... . . 151 
 
 Reduction coefficient of coils ...... 262 
 
 Testing earth with .... . 336, 337 
 
 ,, E.M.F. of batteries, with ..... 280 
 
 ,, Internal resistance of batteries with . . . 269, 273 
 
 To measure resistance with . . . 260, 263, 264 
 
 To control readings of . . . . . . 264 
 
 Use of two coils ...... 261,262 
 
 Tapper key . . . . . . . . 107 
 
 Telegraphic communication, natural causes obstructing . . . 242 
 
 Telephone ...... .172(0) 
 
 Temperature : 
 
 Effect on guttapercha . . . . . . .211 
 
 ,, wire ....... 161, 236 
 
 Formula for correcting resistance according to temperature . . 307 
 
 Tension : 
 
 Batteries joined for (Def. 3, Sec. A) . . 19, 254 
 
 of battery current. (See Potential) . . . (Def. 3, Sec. A) 
 
 Testing, object of . . . . . . . 243 
 
 Testing battery, adjustment of . 251-257, 294 
 
 Tests of batteries (various methods) ..... 267-287 
 
 For E.M.F. ....... 279-287 
 
 For resistance ....... 267-278 
 
 Regular, from signal room ...... 288 
 
 Tests of bridge for conduction and insulation .... 246, 247 
 
 Tests of differential galvanometer for conduction and insulation 248, 249 
 
 Tests of earths or lightning conductors (various methods) . . 335-343 
 
 Tests of instruments and connections ..... 344 
 
326 MANUAL OF TELEGRAPHY. 
 
 Tests Of lines : PARAGRAPH 
 
 By strength of received currents . . . . . 291 
 
 (Fault) with bridge or differential galvanometer . . 3!9~333 
 
 (Regular) with bridge or differential galvanometer . . . 292-318 
 
 Thermo-electric currents on lines ..... 242 
 
 Thomson's galvanometer : 
 
 To measure E. M. F. of batteries with . . . . . 287 
 
 , ,, resistance of batteries with . . . 258,275 
 
 Thomson's reflecting galvanometer . . . . 156 
 
 Thomson's, Sir W., method of testing resistance of battery . .271 
 
 Throw of galvanometer needle . . . . . 334 
 
 Transient : 
 
 Balance ...... 203, 207, 210 
 
 Current ...... . 321, 334 
 
 Translation work ....... 183, 184 
 
 Translation springs . . . . . . 75 
 
 Transmitting apparatus ...... 104-108 
 
 Trembling bell ....... 98-100 
 
 Uniformity of telegraph lines, test of .... 312-314 
 
 Units (Def. 34, Sec. A) . . . . . . 306 
 
 Absolute ...... (Def. 35, Sec. A) 
 
 Metrical ..... . (Def. 35, Sec. A) 
 
 Unit: 
 
 of capacity (Def. 16 and 34, Sec. A) . . . . 334 
 
 of current (Def. 2 and 34, Sec. A) . . . . . 289-291 
 
 of deflection (Def. 34, Sec. A) ... . . 290 
 
 of E.M.F. (Def. 7 and 34, Sec. A) . . . . . 43 
 
 of length ...... (Def. 34, Sec. A) 
 
 of quantity. ..... (Def. 2 and 34, Sec. A) 
 
 of resistance ..... (Def. 12 and 34, Sec. A) 
 
 of time ....... (Def. 34, Sec. A) 
 
 of weight ...... (Def. 34, Sec. A) 
 
 Variable state . . . . . . . . . 1 10 
 
 Variation : 
 
 of current in cable or land line (Def. 26 and 29, Sec. A) . 242, 308 
 
 of faults . . . . . . . . . 325 
 
 Vibration of magnetic needle .... (Def. 50, Sec. A) 
 
 Volt (Def. 7 and 34, Sec. A) . . . . . . . 43 
 
 Voltaic induction . . .118-121 
 
 Weber, unit of electric quantity . . .. (Def. 2 and 34, Sec. A) 
 
 Wheatstone's : 
 
 ABC instrument , . . . . . 172, 197 
 
 Bridge or balance ....... 163, 164 
 
INDEX. 
 
 327 
 
 Wheatstone's bridge : PARAGRAPH 
 
 Battery testing with .... 270, 271, 276, 281 
 
 Earth testing with ..... 339, 341 
 
 Line testing with . 293-298, 303, 322-333 
 
 Principle of, c. . . . . 163 
 
 Resistance greater than one million tested with . . 266 
 
 To test for conduction . . . . . 246 
 
 ,, insulation . . ... . 247 
 
 Wheatstone's method of testing E.M.F. of batteries . . . . 284 
 
 Wire: 
 
 B.W.G. and I.G. ...... 41 
 
 Effect of convolutions ..... 54, 144, 261 
 
 Resistance of, with reference to length and gauge . (Law n, App. A) 
 ,, affected by temperature . . . 161, 236, 307 
 
 Wires : 
 
 Connecting for batteries . . . 24, 25, 239 
 
 (office), joints in ...... 211,212,239 
 
 Effect of heat on ..... 161, 236, 307 
 
 Working force . . . . . . 61 
 
 Zinc sender ... 129, 196 
 
 Zinc sulphate, use and danger of . . 21, 35 
 Zincs : 
 
 Battery to clean ... ... 36 
 
 Composition for . . . . . 35 
 
 Corrosion of . . . . . . -35 
 
 Discoloration of . . . . . 29 
 
 PRINTED BY 
 
 SPOTTISWOODE AND CO., NEW-STREET SQUARE 
 LONDON 
 
ERSITY OF CALIFOENIA LIBBABY 
 BEBKELEY 
 
 TH IS BOOK IS DUE^ THE LAST DATE 
 STAMPED BELOW 
 
 SEP 7 1917 
 NOV221918 
 
 50rn-7,'16 
 
YG 19558