Unit of Electrical Resistance, 171 



Exhibition of 1862, to force into Tables columns headed " Sie- 

 mens, Berlin'" and "Siemens, London," throws doubt on the 

 accuracy of my observation in 1862, and says that, even if I 

 made no mistake, the coils were adjusted when the art of copying 

 resistances was scarcely known. 



First, as to the accuracy of my observation, I may state that 

 the whole of my Report on Dr. Siemens' s instruments was sub- 

 mitted in manuscript long before publication to his Firm, in 

 order that any mistakes might be corrected. The discrepancy 

 between the coils was specially pointed out by me verbally, with 

 regret that it should have been found to exist. Messrs. Siemens 

 very kindly sent me a number of valuable corrections, but were 

 unable to explain to my satisfaction the discrepancy between the 

 coils. 



I received more than one verbal explanation of that discre- 

 pancy. Dr. Esselbach said the standard had been altered ; an- 

 other gentleman said the coils had been touched on their return 

 from the Red Sea ; and it was suggested they might have altered 

 with time. 



In Mr. Loeffler's written reply on behalf of the firm, a sugges- 

 tion was made that I might have observed the coils at different 

 temperatures. I pointed out that this would have required 45° 

 Fahrenheit as the difference between the coils, and that repeated 

 experiments were made, all with the same result. Messrs. Siemens 

 had the coils returned to them long before the publication of the 

 Report, and did not then deny that a discrepancy existed. 



I confess that I believed that they had been made from dif- 

 ferent standards ; nor do I now know the true cause of their 

 difference. In my belief I was strengthened by Mr. Siemens' s 

 statement (when the report was read at the Royal Society) that 

 much trouble had been caused by the premature issue of the 

 coils. But I entirely abandon that opinion since I understand 

 that Dr. Siemens states that the mercury standard has never 

 been sensibly changed. 



Dr. Siemens throws doubt on the accuracy of my observations 

 by saying that the difference given by me between the sets of 

 coils as 1*2 per cent, was really 1*8 per cent.; but on examina- 

 tion he will see that the difference reported between the two 

 1862 coils was really 1*2 per cent., though 1*8 per cent, is 

 nearly the difference between one of those coils and the 1864 

 issue; errors of 0*1 per cent, certainly did not exist in the coils 

 themselves, and therefore the discrepancy could not have re- 

 sulted from the rudeness of the methods employed to adjust 

 them. 



Next, as to the time at which those coils were made, Messrs. 

 Siemens in 1862 stated that both sets exhibited were made at 



