552 



covered with gutta-percha to a diameter of a third of an inch, to 

 show how important it is to shareholders in submarine telegraph 

 companies that only the best copper wire should be admitted for 

 their use. When the importance of the object is recognized, there 

 can be little difficulty in finding how the best, or nearly the best, 

 wire is to be uniformly obtained, seeing that all the specimens of two 

 of the manufactories which have as yet been examined have proved 

 to be of the best, or little short of the best quality, while those of 

 the others have been found inferior in nearly constant proportion. 



What is the cause of these differences in electrical quality is a 

 question not only of much practical importance, but of high scientific 

 interest. If chemical composition is to be looked to for the explana- 

 tion, very slight deviations from perfect purity must be sufficient to 

 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 99'75 



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. 



