NATURAL PHILOSOPHY. 153 



OX THE ELECTRIC CONDUCTIVITY OF COMMERCIAL COPPERS. 



The following is an abstract of an important paper recently presented to 

 the Royal Society G. B., " on the electric conductivity of commercial cop- 

 per of various kinds." In measuring the resistances of wires manufactured 

 for submarine telegraphs, the author was greatly surprised to find differences 

 between different specimens, so great as most materially to affect their vahie 

 in the electrical operations for which they are designed. It seemed at first 

 that the process of twisting into Avirc rope, and covering with gutta percha 

 to which some of the specimens had been subjected, may be looked to, to 

 find the explanation of these differences. After, however, a careful exam- 

 ination of copper wire strands, some covered, some uncovered, some var- 

 nished with india-rubber, and some oxidized by ignition in a hot flame, it 

 was ascertained that none of these circumstances produced any influence on 

 the whole resistance ; and it was found that the wire rope prepared for the 

 Atlantic cable (No. 14, composed of seven No. 22 wires, and weighing al- 

 together from 109 to 125 grains per foot) conducted about as well on the 

 average as solid wire of the same mass, but in the larger collection of speci- 

 mens which thus came to be tested still greater differences in conducting 

 power were discovered than any previously observed. It appeared now 

 certain that these differences were owing to different qualities of the copper 

 wire itself, and it therefore became highly important to find how wire of the 

 best quality could be procured. Accordingly four samples of simple No. 22 

 wire, and of strand spun from it, distinguished according to the manufac- 

 tories from which they were supplied, were next tested, and the differences 

 of conducting power were found to be 100, 96'05, and 54 - 9. Two other 

 samples, chosen at random, about ten days later, out of large stocks of wire 

 supplied from the same manufactories, were tested with different instru- 

 ments, and exhibited as nearly as could be estimated the same relative quali- 

 ties. It seems, therefore, that there is some degree of constancy in the quality 

 of wire supplied from the same manufactory, while there is vast superiority 

 in the produce of some manufactories over that of others. The great im- 

 portance to shareholders in submarine telegraph companies, that only the 

 best copper wire should be admitted for their use, is at once rendered ap- 

 parent by the fact that a submarine telegraph constructed with copper wire 

 having the conducting power of 100, and only one twenty-first of an inch in 

 diameter, covered with gutta percha to a diameter of a quarter of an inch, 

 would, with the same electrical power, and the same instruments, do more 

 telegraphic work than one constructed Avith copper wire of one sixteenth of 

 an inch diameter, having the conducting power of 54 - 9, covered with gutta 

 percha to a diameter of a third of an inch. 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 speci- 

 mens of two of the manufactories Avhich have as yet been examined, have 

 proved to be of the best, or little short of the best; quality, Avhile those of 

 other manufactories have been found inferior in nearly constant proportions. 

 The cause of these differences in electrical quality is a question not only of 



