GALVANISM. 



preferred, because it is less likely to break. Cover- 

 ing the core are three layers of gutta-percha, G, 

 with Chatterton's Compound between each. Over 

 the percha is a coating, H, of tarred yarn, to act 

 as a padding between the insulating material and 

 the external sheathing, I. This consists of eighteen 

 wires of galvanised iron, laid on spirally. It has 

 to bear the strain of ' paying out,' and to protect j 

 from roughness of sea-bottom. 



The two Atlantic cables, laid in 1865 and 1866, 

 do not differ in any essential from the one 

 described. Their external diameter is li inch, 

 and the sheathing consists of ten steel wires, each 

 of which is separately surrounded with five strands 

 of hemp. In water, they weigh about fourteen 

 hundredweight per nautical mile, and each cable 

 would bear about eleven knots of itself in water 

 without breaking. Thus the difficulties of obtain- 

 ing sufficient strength and insulation have been 

 wholly overcome. 



But there remains a more perplexing difficulty 

 still. It is seen on very long land-lines ; only it 

 can be easily met in them by retransmission. The 

 difficulty lies here. What is given as a momentary 

 signal at one end of a cable, oozes out as a 

 prolonged signal at the other. Time must thus 

 be given for each signal to ooze out before another 

 is sent ; and the result is a great delay in the 

 transmission of messages. Sir William Thomson 

 has found that the greatest speed of signalling 

 possible on an air-line, of iron wire i inch 

 diameter, and two thousand miles long, would be 

 twenty words a minute. Now, with the two 

 thousand miles of Atlantic cable, laid in 1858, the 

 greatest speed attained was only 2^ words a I 

 minute, though its core was a better conductor j 

 than the \ inch iron wire. The cause of this 

 extraordinary retardation of the current is usually 

 ascribed to an inductive action of the water sur- j 

 rounding the cable. In fact, the cable is just an ! 

 enormous Leyden jar, with its core for the inside 

 coating, the gutta-percha for the dielectric, and 

 the water and sheathing for its outer coating. 

 When a signal is sent, but a very small part of 

 the current instantly reaches the other end. The j 

 rest is absorbed on the way, bound as a statical 

 charge by the electricity it has induced on the 

 water. The gradual oozing out of the electricity 

 stored up in the cable, after cutting off the battery 

 connection at the signalling end, is like the re- 

 sidual charges of a Leyden jar. What should 

 come out at the other end as a short, sharp 

 discharge, comes drawling out as a series of them. 



No way of preventing this inductive action has 

 yet been found, except thickening the core and in- 

 sulation. But, of course, only a certain weight of 

 cable can practically be laid. Yet, though it 

 cannot be prevented, its effect on the signals has 

 been in a great measure obviated. The energy 

 and genius of such men as Sir W. Thomson, Mr 

 Varley, Mr Siemens, and others have wrought 

 wonders. 



First of all, the statical charge can be got rid of 

 to a great extent in this way. After sending a 

 short signal, the clerk sends a second of opposite 

 name by rapidly reversing the poles of the battery 

 which are connected with the cable. This sends a 

 wave of opposite kind to the first, which serves to 

 counteract its inductive action, and discharge the 

 cable. More than one may be required. For the 

 Atlantic cable, for instance, five signals were 



considered necessary to completely discharge it : 

 first, a wave from the copper pole, then one half 

 as long again from the zinc, then one from the 

 copper four-fifths of length of the first, and so on. 

 Next, and not less important, is the invention 

 of a sufficiently delicate indicator to be affected 

 by the extremely weak current which escapes 

 induction. This is found in Sir William Thom- 

 son's Reflecting Galvanometer, which was adopted 

 for the Atlantic cable. Fig. 39 will give an idea 

 of it. The current is passed through a small 

 vertical coil, D, of fine wire, in the centre of 



Fig. 39- 



which swings a short magnetic needle hung by a 

 silk fibre. The needle carries a small hollow 

 mirror, and, mirror and all, only weighs about a 

 grain and a half. At a distance of two or three 

 feet from the mirror is a solid wooden stand, with 

 a graduated scale, AB, forming part of a circle 

 facing the mirror. In the stand, just under the 

 centre of the scale, a hole, C, is drilled, and a fine 

 wire stretched upright across it A strong lamp, 

 E, stands behind the opening, so that its light 

 will fall on the mirror, and be reflected back on 

 the scale. An image of the wire will thus be con- 

 stantly thrown on the scale, and the slightest 

 motion of the needle and its mirror will produce a 

 much greater motion of the image index. As the 

 current flows one way or another, the index will 

 move to one side or another, and thus an alphabet 

 may be formed as with the common needle tele- 

 graph. The Morse system is adopted, a dash 

 corresponding to a right, and a dot to a left deflec- 

 tion of the image. To make it come quickly to 

 rest after a deflection, a magnet is placed near the 

 needle, and in this way a very delicate and quick 

 indicator is got. 



To the ingenuity of the same inventor is due an 

 ink-recorder, which marks the right and left 

 deflections on a paper ribbon. Here the coil is 

 movable, and the magnet fixed. The coil is 

 of very fine wire, and is delicately hung between 

 the poles of a strong horse-shoe magnet, and so 

 that the current from the cable can pass through 

 it. It will turn to one side or another, therefore, 

 according to the direction of the current througli 

 it. In front of the coil is placed an insulated 

 vessel filled with common ink. Into this dips 

 a very fine glass siphon, the leg of which is 

 attached to the fine coil, and is made to turn with 

 it. It is clear, then, that if ink issue from the 

 siphon, it will trace a straight line (on the paper 

 which passes under it) so long as no signals flow. 

 But a signal will bend the line to one side or 

 another according to its direction. By a very 

 ingenious idea, Sir W. Thomson avoids the rub- 

 bing of the siphon against the paper. He keeps 

 the vessel and ink constantly electrified, say 

 positively, and the paper which passes under the 

 end of the siphon also negatively. This produces 



