307 



especially in Germany, and were in reality all based on Professor 

 Wheatstone's invention of a beautiful method for comparing re- 

 sistances, to which I have frequently referred as Professor Wheat- 

 stone's electric balance. 



246 1 . What were the results at which you arrived ? That dif- 

 ferent specimens chosen at random from the stock supplied for 

 manufacture differed immensely in conducting power. 



2462. Although nominally the same quality of copper? Yes, 

 although nominally the same quality of copper. All those specimens 

 of wire were supposed to be of the very best quality, the only copper 

 supposed to be good being that which admitted of being drawn into 

 wire suitably for the purpose. A good mechanical quality was ne- 

 cessary to prevent frequent fractures in the wire-drawing ; and to un- 

 derstand that, I should say that hanks in unbroken lengths amount- 

 ing to a large mass were always required, the worse metal being 

 found to break before it could be drawn into a hank of a certain size. 

 The mechanical qualities seem to have been satisfactory, but no sus- 

 picion whatever was entertained that there were also large differences 

 in electric conducting power. W. Weber had many years before 

 pointed out considerable differences in different specimens of copper 

 wire which he had tested. I found differences much exceeding 

 those, and I did not, as I expected, find any approximation to a 

 uniform average among the different specimens tested ; some spe- 

 cimens I found nearly double in their conducting power, compared 

 with others, reckoned according to the weight and length, allowing 

 for the variations of gauge. Calling the best specimen which I had 

 in the summer of 1857, 100, I found many specimens standing at 

 60 in specific conductivity, many standing at 50, many standing at 

 80, a few above 90 ; and so far as I can recollect, the average of a 

 large number of specimens that I then examined may have stood 

 between 60 and 70, but I consider the statement of such an average 

 to be of no value, it is so much a matter of chance. If I had re- 

 ceived a dozen specimens of a low quality below the average, or if I 

 had chanced to receive a dozen specimens of a higher quality, the 

 average would have been so much the lower or the higher. I never 

 had an opportunity of measuring the conductivity of 200 or 300 

 miles of submarine cable ; such alone would have given me exact in- 

 formation as to the average for that portion of cable. I may men- 



