.S/A- //'//./.A/,/.!/ SIEMENS, /-.K.S. 



lines, the effects of the change produced by that surface are very 

 considerable. The amount of charge varies according to the 

 surface of the conductors, and is inversely proportionate to the 

 thickness of the insulating material. Therefore if we could by 

 some means or other double the conductivity of a conductor of a 

 given size, the rate of transmission through the same would exactly 

 be doubled ; from which it at once appears how important it is to 

 make a conductor of the least possible size for a given amount of 

 conductivity. The best conductor to answer this requirement 

 would be a single cylindrical wire of silver, which is known to be 

 the best conductor ; but a single wire would in the first place be 

 objectionable, because a break or flaw in it would destroy the 

 efficiency of the whole line, and the employment of silver would 

 not be warranted, because a cheaper metal, copper, in its pure 

 state, has very nearly the same conductivity. 



The following table shews the conductivities of various 

 metals : 



TABLE OP THE CONDUCTING POWERS OF METALS. 



Taking silver at 100, pure copper is 99' 9, or virtually the same. 

 But the best commercial copper we can get at present for the 

 construction of telegraphs has a conductivity of 85. The Burra 

 Burra copper, a good copper of commerce, has only a conductivity 

 equal to 14, owing to a small percentage of foreign matter. From 

 this it appears how important it is to get copper of the highest 

 conductive quality. Since it would not be safe to use a single 

 wire, owing to the risk of a break occurring, we must use a rope 

 of copper wires of the highest conductivity, and it is important 

 that this rope of wires should be as densely united together as 

 possible. Generally seven wires are twisted together, which 

 arrangement, however, might be improved in conductivity for the 



