ELECTRO-TELEGRAPHY 245 



in a little less than 5 miles deep of that fluid. Its breaking strain was estimated to 

 be 3 tons 5 cwts. This enterprise failed from causes which were foretold. Before the 

 cable left this country it was imperfect ; it had been carelessly kept, and greatly 

 injured by the cutting-out of pieces while in Keyham yard, so that its power of 

 carrying an electrical current was reduced to a minimum. 



The unfortunate failure of this enterprise in which both Europe and America were 

 equally interested led to considerable discussion, and to a careful examination of all 

 the conditions which were supposed to influence such electric cords. All the laws 

 regulating the flow of an electrical wave were re-examined by some of our best elec- 

 tricians, and many previous conclusions received some correction. It had been supposed 

 that an insulated submarine wire conducts according to a different law from that of a 

 suspended circuit. This difference was thought to depend upon a system of induced 

 earth currents influencing the submerged wire, which would not influence a wire when 

 suspended in the air. A simple example to render this clear to the non-scientific 

 reader may be necessary. If a wire insulated by means of gutta-percha, or any other 

 non-conducting substance, is connected, so as to complete the circuit, with the two 

 ends of a voltaic battery, a current is said to flow through it. This means that an 

 electrical wave of motion is propagated along it in a given direction. If another 

 copper-wire is placed near it though not in contact with either it or the battery 

 another current, or wave, is generated, that is, induced, in it. This induced current 

 interferes, to a greater or less extent, with the primary current ; and where great 

 lengths of cable are concerned it becomes an element of serious consideration. If a 

 coated wire is placed on the earth, and an electrical current is established in it, the 

 earth acts the part of the second wire, and induction is developed on its surface. 

 Mr. S. A. Varley has examined this problem with much care, and has arrived at the 

 following conclusions. In a suspended wire the insulatory medium of the air takes the 

 place of the gutta-percha of the submarine circuit. The earth, which is the nearest 

 conductor, is a considerable distance off, and is only on one side of the wire ; therefore, 

 but little induction can take place between the wire and the earth. Nevertheless, in- 

 duction to a certain extent does take place, and it can be detected with delicate ap- 

 paratus in circuits of very moderate lengths. If the distance between the wire and 

 the earth is decreased, induction will be developed more strongly, and the wire could 

 be brought down step by step, until the condition of a submarine circuit would be ap- 

 proached, where the earth surrounds the wire on all sides, and is only separated from 

 it by the thickness of one-eighth or three-sixteenths of an inch of gutta-percha, a sub- 

 stance possessing, moreover, specifically a much greater inductive capacity than air. 

 It, therefore, appears that the conditions are precisely the same, only differing in 

 degree. It is, nevertheless, evident from this, that a submarine wire is placed under 

 circumstances of greater difficulty than those which surround an aerial wire. This 

 difficulty increases with the length of the wire, retardation becoming more and more 

 powerful as the distance through which the wave has to move is extended. A sub- 

 marine cable may be regarded as a Leyden jar, and the telegraphic indications are 

 analogous to the discharges of a coated glass. An impulse is given by making a con- 

 nection with a voltaic battery at one end of a wire suspended in air and it indicates 

 telegraphically by producing magnetic disturbance at the other and a succession of 

 impulses will rapidly give a succession of indications. Now, in an insulated submarine 

 wire this is not exactly what takes place. A distinction has to be drawn between the 

 simple arrival of a current which may be regarded as instantaneous and the pro- 

 duction of a telegraphic signal. After having charged the wires sufficiently to develope 

 an appreciable current, owing to the wire taking some time to empty itself, if currents 

 are sent in succession with any rapidity, they will blend into one another, and, instead 

 of getting a series of distinct impulses at the further extremity, a continuous undulating 

 wave will be obtained. This sluggishness has been obviated, to a great extent, in cir- 

 cuits of moderate length, by employing opposite currents of electricity in succession. 

 The effect of this is to absorb the preceding wave, and to neutralise it much more 

 quickly than would be the case if the wire were left to discharge itself in the usual 

 'way. In a suspended wire, as there is but little induction, there can be no accumula- 

 tion of static charge worth noticing ; whereas a submarine wire, unless attention be 

 paid to this, becomes useless for telegraphy. 



The electric conductor of the Atlantic cable is formed of 7 wires, 6 around 1 (Jig. 819). 

 3ven wires are used rather than 1 to give greater freedom to the electricity in passing 

 y presenting a more extensive surface to it. There are mechanical reasons also why 

 band of wires is preferable to one wire. The most serious accident that can happen 

 j a submerged wire is its severance. Copper-wire cannot always be depended upon 

 ^ 5 present the same amount of strength in every part. It frequently happens that an 

 inch, or even a less portion of a wire, is crystalline, and therefore liable to break in 

 v "ing coiled. By using 7 sections instead of 1, the probability of a fatal breach of 



