Polytechnic Association Proceedings. 635 



the atlantic cable. 



An article in The North British Review on submarine telegraph, 

 contains some interesting statements regarding the action of elec- 

 tricity, and the mode of using it in the Atlantic cable. Electricity 

 seems to have no proper speed, in the usual sense of the word. 

 The speed depends in each case on the condition of the conductor, 

 and may, on certain conceivable conditions, be regarded as infinite, 

 though we have no proof that the laws now known hold good up 

 to, or nearly up to, this limit. When the Valentia end of the 

 Atlantic cable is joined to the signaling battery, a current rushes 

 into the cable without any perceptible loss of time, but no effect 

 whatever can be perceived in America for at least one-tenth of a 

 second; after, say fifteen hundredths of a second, the received cur- 

 rent begins rapidly to increase, according to a definite law, 

 and if the battery contact at Valentia is continued, the current 

 enterinof th^Q cable there, and flowing out of the cable at Valentia, 

 will be sensibly equal, after say two and a quarter seconds. After 

 this the currents would remain equal so long as the battery con- 

 tinued in action. When the battery contact is broken at Valentia, 

 and the cable put to earth, the current flows on at Newfoundland 

 for say one-tenth of a second, as if nothing had happened; it then 

 begins rapidly to decrease, and it sensibly ends say two and a quar- 

 ter seconds after the contact was broken. Thus the current ai'iives 

 in gradually increasing waves, and dies out in a precisely similar 

 manner. 



In order to produce a succession of distinct and legible signals, 

 it is not necessary that the wave should reach its maximum and fall 

 to its minimum at each signal. If the sending battery contacts are 

 changed or reversed before the full height of the wave is reached, 

 the wave is not obliterated, it is simply diminished; if battery con- 

 tacts, alternately with one and the other pole of the battery, suc- 

 ceed one another with considerable rapidity, saj' three reversals 

 every second, or ninety dot signals per minute, the waves will be 

 reduced to say ten per cent of their maximum; but if w^e can ren- 

 der these little waves visible, they may be interpreted as legible 

 signals. The old Morse system, which simply indicated a blunt 

 yes or no, could not show these little waves, or follow them in any 

 way. Sir William Thompson's reflecting galvanometer does render 

 these little waves legible, even when they are no larger than one 

 per cent of tne maximum current. The received current deflects 

 a tiny magnet to and fro. A little mirror, swinging with the mag- 



