.s/A' WILLIAM SIEMENS, F.R.S. 41 



is, if we consider that the charge, say of the Atlantic cable, is a 

 fixed quantity which has to be furnished before the current can 

 make its appearance at the further end ; and that, in filling this 

 large jar, we are limited only by the conductivity of the insulated 

 wire, which allows the electric current to flow in. If the conduc- 

 tivity of that wire could be doubled from what it is at present, we 

 should, in exactly half the time, accomplish our end, or, in other 

 words, we should double our rate of speaking. If, on the other 

 hand, we should allow ordinary copper of commerce to be employed 

 which had a conductivity under twenty-five times that of mercury, 

 whereas we can practically obtain wire conducting fifty times 

 better, we should speak with only half the velocity which we 

 ought to have attained to. Therefore, as the conductivity in- 

 creases, so the velocity of transmission will also increase, hence 

 the importance of using the very best copper for the purpose. 



Having satisfied ourselves upon that point, the wire is handed 

 over to the manufacturer to be covered. It is thereupon tested 

 for insulation. But the electrical resistance of gutta-percha varies 

 with temperature, and in order to get proper comparative tests, 

 we must know the exact temperature of the wire. For this pur- 

 pose cisterns were arranged, at my suggestion, in testing the 

 Malta- Alexandria core, in which each coil of wire, as it is covered, 

 is soaked for twenty-four hours, the water being maintained at a 

 standard temperature of 75. The resistance of the insulator, 

 which we obtain at the standard temperature, may be directly 

 compared with the resistance which we have observed in another 

 coil to determine their relative perfection. The coil is next put 

 under great hydrostatic pressure in Reid's pressure-tank. Under 

 this pressure a curious phenomenon appears, namely, the electrical 

 resistance of gutta-percha increases in a fixed ratio. Experiments 

 which I had occasion to make, and which went up to a pressure 

 of 300 atmospheres, showed that the increase due to that amount 

 of pressure, three-folds the electrical resistance. That is to say, 

 if we had electrical resistance of one hundred million of units at 

 atmospheric pressure, by increasing the pressure to 300 atmos- 

 pheres, we increase the resistance to three hundred millions of 

 units. The moment the pressure is released, the resistance falls 

 rapidly to its original amount. At the bottom of the Atlantic 

 there is a pressure of nearly 300 atmospheres ; it will therefore be 



