270 



THE ELECTRIC TELEGRAPH. 



The comparison being made with pure silver at 100 C., from 

 which it appears that the conducting power of copper in- 

 creases 2-5 per cent., and that of silver nearly 8 per cent., 

 by annealing. 



According to Dr. Siemens, the conducting powers of 

 copper, silver, and brass, hard and annealed, are as follows, 

 compared with pure mercury at C. : 



This property of copper is especially of advantage in the 

 manufacture of telegraph cables, galvanometers, &c., where 

 a great length of conductor is required with little resistance, 

 and where the metal must be as soft and as little liable to 

 change its molecular condition as possible. 



34. Metals fused. Although not strictly within the 

 domain of telegraphy, the conducting power of fused 

 metals is interesting, and brings us to a material most im- 

 portant in the reproduction of standards of resistance 

 mercury. 



Of the metals which have been subjected to electrical 

 measurement when in a state of fusion, all, as far as we 

 know, with the exception of bismuth, lose their high con- 

 ducting power, and at the point of solidification regain it 

 very rapidly. Tin may be taken as a fair example of 

 this behaviour of the metals. A determination, which we 

 made in 1862, with some pure tin, melted in a glass 

 spiral surrounded by hot oil, gave the following results 

 as compared with the conducting power of pure mercury 

 at C. : 



