TELEGRAPH, ELECTRIC. 



TELEGRAPH, ELECTRIC. 



611 



turn away the current from its work. Wet and foggy weather always 

 has the effect of diminishing the current and requiring greater 

 battery power; unless, indeed, the plan should be adopted of mitching 

 the line, that is, instead of sending a message along a direct line, 

 where the wires are enveloped in rain or damp fog, or are otherwise 

 " ,-ick" as it is called, the line is switched on to another and more cir- 

 cuitous route, where the wires are in good working order. Spiders' 

 threads covered with dew sometimes divert the current, as does also 

 atmospheric electricity. Indeed, a thunder-storm was formerly a 

 source of danger to the telegraph clerks. Professor Loomis says that 

 the telegraphic wires are very sensitive to an approaching storm, and 

 often become highly charged, even when the storm is so distant that 

 the thunder cannot be heard nor the lightning seen. Under such cir- 

 cumstances, if one stand in the room of a telegraphic station, and 

 place one band upon a telejzraphic wire, and rest the other on the wire 

 which communicates with the earth, a sharp shock is felt in the arms, 

 and sometimes across the breast. This shock is very painful ; although 

 when the two wires are brought within striking distance of each other, 

 only a faint spark is to be seen. But when the thunder-cloud is near, 

 such experiments ^are dangerous. In such case, a thunder-cloud pass- 

 ing over the wires may charge them to such an extent that the 

 electricity may fuse the thin wire of the 'electro-magnet, and render 

 the magnet itself unserviceable. On some occasions an explosion 

 takes place in the telegraph-room sufficient to fuse thick wires? and 

 to expose the clerks to considerable danger. A weak charge of 

 atmospheric electricity has the same effect on the wires as the cur- 

 rent of a voltaic battery ; it makes a point in the telegraphic 

 register. If, however, a storm pass over the wires, these points 

 become numerous ; and as they show themselves between the points 

 of a telegraphic message, they make the writing indistinct, and often 

 illegible, so that on such occasions the clerks usually suspend their 

 labours. Various contrivances have been made for drawing off 

 electricity from the wires, without disturbing the current, advan- 

 tage being taken of the tension of the former, which gives it a striking 

 distance not possessed by the latter. In addition to atmospheric 

 electricity the line is liable to be disturbed by what are called " earth 

 currents." If both ends of a long wire be connected with the earth, 

 currents will pass through tha line apparently in consequence of 

 variations in the electrical condition of the earth in different places. 

 Mr. Varley has observed that these currents are continually flowing 

 about the earth in one direction or another throughout the day, and 

 reach a maximum about 2' 40 P.M. During magnetic storms or the 

 aurora borealis, currents are sufficiently strong to interrupt the working 

 of the lines : they flow sometimes in one direction, sometimes in 

 another, and often change their direction in a few seconds. 



But if in land lines the insulation of the wires is attended with 

 difficulty, the insulation is very much more difficult and uncertain in 

 carrying a cable through great lengths of ocean. In transmitting a 

 current along an insulated land line connected with the earth at one 

 end, and with a battery communicating with the earth at the other, 

 the strength of the current diminishes in an inverse ratio to the 

 length f the wire. When, however, the conducting wire is wrapped 

 up in an insulating material and immersed in water, we have the three 

 parU of a Leyden jar, namely, the wire takes the place of the inner 

 metallic coating, the gutta percha that of the insulating glass, and the 

 iron wire covering or the water that of the outer coating, so that not 

 only doea the strength of the current diminish inversely as the 

 length, but the arrangement must be discharged before a menage 

 can be sent, and the rate of signalling must depend ultimately on 

 the rapidity with which the charge and discharge can be effected. 



But there are mechanical difficulties arising from the materials 

 employed. The copper conducting wire is deficient in strength, and a 

 moderate strain put upon it may produce a permanent elongation : while 

 the materials which envelope it, being more elastic, return to their 

 original bulk when the strain is removed, the elongated copper does not 

 do so, but will thus cut its way through the gutta percha cover. The 

 copper wire is usually made into a strand for the sake of strength, and 

 it has even been proposed to cover the copper with fine steel wires 

 before putting on the gutta percha, so that the chief strength of the 

 cable might be in its core. Copper wire is preferred on account of its 

 uperior conducting power for electricity, which is seven times greater 

 than that of iron. But this superiority is greatly dependent on the 

 purity of the copper. No substance added to copper increases its con- 

 ducting power ; but the presence of other metals may reduce its con- 

 ducting power below tliat of iron. The conducting power of copper 

 also varies with the temperature. 



Gutta percha is also subject to various changes which interfere with 

 its insulating power ; although usually considered as a good Insulator 

 it is by no means perfect, especially when it is exposed to a tolerably 

 high temperature. At 32 there is a very small leakage of electricity 

 thrngh it; at 52 the leakage is three times as great; at 72 it is 

 nearly nix times ; and at 92 ten times as great as at 32. While laying 

 thn K' 'I Sea cable, the temperature in the hold of the ship was 92, 

 and the insulation was so bad that the engineers could not apeak 

 h the cable ; but when in the water at the depth of 300 fathoms 

 and the temperature 73, the insulation became much better. Gutta 

 percha also varies greatly In its quality : it may contain foreign matter, 

 or be porous, or air bubbles may become entangled with it during the 



laying on, Jail of which may tend to destroy its insulating power. 

 There may be bad joints or email punctures, or a strong battery 

 power may produce a chemical action injurious to its stability. Its 

 durability is also seriously affected by exposure to light and air, so 

 that in shallow water iudia rubber may be a preferable coating. Gutta 

 percha is also liable to injury from friction or from pressure, and both 

 it and the hempen packing are liable to the attacks of marine animals, 

 a species of Teredo devouring the hemp, and another Teredo penetrat- 

 ing the gutta percha. 



Hence it will be seen that the great problem of ocean telegraphy 

 cannot by any means be regarded as solved. The best form of cable 

 remains to be invented : it should be light and flexible for deep waters, 

 and sufficiently strong in shallow ones to resist the rude grip of an 

 anchor. Some cables weigh 3 or 4 tons per !mile ; others 8 or 9 or 

 even more, while light cables weight about 1J or 2 tons per mile. The 

 method of stowing them on board a ship, and the machinery for pay- 

 ing them out require to be reconsidered ; but, above all, the many 

 costly blunders which have been perpetrated with various cables 

 ought not to be again possible, such as the heating of the Rangoon 

 cable, and the error iu reckoning in paying out the Cagliari and 

 Algeria cable, so that there was not enough cable to reach the land : 

 the ship held on during five days to the cable and then broke off 

 in the midst of a storm. It is not our business here to do more than allude 

 to the failures of telegraphic cables consequent on the plan adopted by 

 our government of granting premiums to telegraphic companies. 



This part of our subject will not be complete without a few 

 details as to the mode of preparing the Atlantic cable. A strand 

 of seven wires of pure copper of the No. 22 gauge, was first pre- 

 pared, it being the sixteenth of an inch iu diameter when twisted. 

 The strand of seven wires was adopted in preference to a single 

 wire of the same practical capacity, because the probability of a 

 destruction of continuity was in this way greatly diminished. In 

 case of any accident occurring it was very unlikely that all the seven 

 wires would be broken in exactly the same place, and so long as only 

 one of them remained sound, the electrical transmission could be carried 

 on. The strand itself was subject to a strain which stretched it 

 twenty per cent, without any appreciable injury to its conducting 

 power being discovered. To show that no amount of attenuation which 

 could be produced by accident, could interfere to any important extent 

 with its utility as a telegraphic conductor, one mile of wire eleven 

 times smaller than the strand, was introduced into a gap made in ,1 

 >;<> miles length of the cable, and the effect produced ou the trans- 

 mitting power of the cable by the interpolation was tested. It proved 

 that the transmitting capacity of the cable was only diminished by 

 one thirty-seventh part. 



As the copper strand was prepared, it was rolled upon drums, and 

 then taken from the drums to have three separate coatings of gutta 

 percha applied, until the aggregate diameter was thus brought up to 

 about three-eighths of an inch. The gutta percha used for these 

 coatings was prepared with great care. It was first rasped into shreds, 

 and washed, and next pressed through several layers of fine wire gauze, 

 and kneaded for hours in the interior of iron cylinders by steam 

 machinery. It was then squeezed by powerful screws, through dies, 

 as the strand of copper was gradually drawn along between them, and 

 so made to adapt itself as a compact sheath to the strand. Three 

 several and successive coatings were given to the strand in order that 

 any imperfection left in the first might be compensated and remedied 

 by the next coat applied. The completed core was subjected to 

 pressure of five tons upon the square inch, by the use of hydrauli 

 power, without the insulating material being at all injured by the force 

 applied. 



During the process of the manufacture of this core it was submitted 

 to constant examination to prove both that the continuity of the 

 copper strand continued unimpaired, and also that the insulating 

 power of the gutta percha sheath was as complete as it was required to 

 be. The continuity was proved by passing a voltaic current of low 

 initially from a battery of a single pair of plates, through the strand, 

 and then causing it to record a signal after issuing from the wire. 

 A battery of low intensity was employed for this purpose, because it 

 made the test so much the more severe. A strong battery might have 

 thrown the current through a slight imperfection, which a. weak 

 battery might not be able to overcome. The due perfection of the 

 insulation was tried by turning up into the air the end of the length of 

 core about to be examined, and by then connecting one pole of a 

 voltaic battery of five hundred pairs of plates with the nearer end of 

 the length of wire, and the other pole with the earth, a magnetic 

 galvanometer being suspended within a coil continuous with the strand. 

 So long as the insulation of the strand was fairly perfect, the copper 

 wires became charged with the electricity of which but very little 

 could escape, and so no current was produced through the strand, and 

 no deflection of any consequence appeared in the magnetic needle. 

 When the insulating sheath, on the other hand, was imperfect, the 

 electrical charge leaked through the imperfections to the earth, and so 

 got back to the opposite pole of the battery. In this way a current 

 was set up in the wire to supply the leakage, and the magnetic needle 

 was deflected from its position of equilibrium, the deflection being in 

 proportion to the amount of the current. A strong battery of fiv 

 hundred pairs of plates was employed in detecting imperfect insula- 



