Dr. Matthiessen on the Unit of Electrical Resistance. 379 



further, that the set of coils adjusted by this improved system 

 differ 0*5 per cent, from the standards of 1864. He concludes this 

 from measurements of a copper wire which Mr. F. Jenkin, 

 during the Exhibition of 1862, compared with the sets of coils. 

 What temperature the copper wire had in the two measurements, 

 with an interval of some years between them, is not given. A 

 difference of 1^ degree Centigrade would, however, account for 

 the apparent discrepancy." 



The passage referring to the above is, I suppose (Phil. Mag. 

 May 1865, p. 365), where I speak of a coil whose resistance equals 

 a mile of pure copper wire (the copper wire quoted by Dr. Sie- 

 mens) equalling in resistance 13*95 Siemens {London), and 14*12 

 [Berlin) units. The next sentence is as follows : — 



"Now, when I adjusted this mile coil for Messrs. Elliots 

 Brothers, I made another coil of the same resistance of different 

 German-silver wire, &c. ; M so that the " copper wire " was an 

 adjusted coil made of German-silver wire, and therefore of the 

 same material as the coils with which the comparisons were made. 

 The whole passage is too long to be quoted in extenso ; but the 

 above will show that Dr. Siemens has somewhat misunder- 

 stood it. 



With regard to the 08 per cent, difference existing between 

 the reproduction of the mercury unit by Dr. Siemens and Mr. 

 Sabine on the one side, and Mr. Hockin and myself on the other, 

 Dr. Siemens states (p. 334), " I do not think that Dr. Mat- 

 thiessen should have compared the results of his determina- 

 tions with mine, as he departed in several material points from 

 the method I laid down." He enumerates the following 

 points : — 



(1) As to the formula used for the correction for conicalness, 

 he asserts that that used by Mr. Hockin and myself is not so 

 correct a one as that used by himself. On referring the ques- 

 tion to a mathematician, he informed me that our formula was 

 quite as correct as that used by Dr. Siemens (both being only 

 approximate formulas). 



(2) Dr. Siemens takes objection to our method of determining 

 the weight of the tube full of mercury. In Dr. Siemens' s own 

 weighings (Phil. Mag. January 1861, p. 38) the maximum dis- 

 crepancy between the weighings of mercury from the same tube 

 is equal to 003 per cent., between Mr. Sabine's (Phil. Mag. 

 March 1863, p. 161) 003 per cent., and between ours (Report, 

 1864, p. 364) 0*07 per cent. Had the soft skin of the points 

 of the fingers been pressed into the openings of the tube when 

 taking it out of the trough, as suggested by Dr. Siemens, we 

 should scarcely have obtained such concordant weighings as we 

 did ; for with the first tube the maximum difference between 



