474 Royal Society : — 



produce great effects on the electric conductivity of copper; the 

 following being the results of an assay by Messrs. Matthey and 

 Johnson, made on one of the specimens of copper wire which I 

 had found to be of low conducting power : — 



Copper 9975 



Lead -21 



Iron -03 



Tin or antimony -01 



100-00 

 The whole stock of wire from which the samples experimented on 

 were taken, has been supplied by the different manufacturers as 

 remarkably pure ; and being found satisfactory in mechanical quali- 

 ties, had never been suspected to present any want of uniformity as 

 to value for telegraphic purposes, when I first discovered the differ- 

 ence in conductivity referred to above. That even the worst of them 

 are superior in conducting power to some other qualities of commer- 

 cial copper, although not superior to all ordinary copper wire, appears 

 from the following set of comparisons which I have had made between 

 specimens of the No. 22 A wire, ordinary copper wire purchased in 

 Glasgow, fine sheet-copper used in blocks for calico-printers, and 

 common sheet-copper. 



To test whether or not the mechanical quality of the metal as to 

 hardness or temper had any influence on the electrical conducting 

 power, the following comparison was made between a piece of soft 

 No. 18 wire, and another piece of the same pulled out and hardened 

 by weights applied up to breaking. 



Soft No. 18 copper wire, 



No. 18 copper wire. stretched to breaking. 



Weight per foot, 57-5 grs. 

 Length used, 30'8 inches. 



Weight per foot, 44-8 grs. 

 Equivalent length, if of 



equal conductivity, 24-0 



inches. 



Length found equivalent 

 by experiment. 



24-0 inches. 



The result shows that the greatest degree of brittleness produced by 

 tension does not alter the conductivity of the metal by as much as 

 one half per cent. A similar experiment showed no more sensible 

 effect on tbe conductivity of copper wire to be produced by hammering 

 it flat. There are, no doubt, slight effects on the conductivity of 



