Unit of Electrical Resistance. 1 73 



1864 Report to the Royal Society : and even if it be conceded 

 that the old discrepancies gave no measure of the accuracy of 

 the plan of reproduction, they greatly weaken any argument 

 founded on the priority of Dr. Siemens's unit, and its extended 

 use. One reason for repeating these various values of all units, 

 besides that of showing the discrepancies in those hitherto issued, 

 is that results have been published expressed in each unit. 



Dr. Siemens is in error in supposing that Dr. Matthiessen's 

 calculation of the value of the 1862 mercury coils, as com- 

 pared with the others, rests on the measurement of a copper 

 wire. He had been led into this error, I suppose, by some am- 

 biguity of language ; but Dr. Matthiessen's mile of copper wire 

 exhibited in 1862 was a German-silver resistance-coil, as stated 

 in my Juror's Report, and in the Report to the Royal Society. 

 This German-silver coil Dr. Matthiessen still has, and I have 

 the set of German-silver coils called " Thomson's units" in 1862. 

 Both of these have remained constant in their ratio, and the 

 value of all Dr. Siemens' s coils has been determined by compa- 

 rison with them. 



A difference of 1J° C. would therefore by no means account 

 for the discrepancy of 0*5 per cent., which we believe exists 

 between Dr. Siemens' s 1862 coils and 1864 standards. 



I have now concluded all the remarks I have to make on 

 those parts of Dr. Siemens's paper which are strictly relevant 

 to the matter at issue ; but Dr. Siemens, at the end of his paper, 

 makes some criticisms on the historical sketch I gave in the 

 paper above referred to, of the various units of resistance which 

 have been proposed. 



I think none of these criticisms would have been made by 

 him if he had observed that I did not profess to give a history of 

 the " progresses in the field of resistance-measurements," as he 

 supposed, but only of the units proposed. A whole book would 

 have to be written to do justice to a history of resistance-mea- 

 surements, whereas my sketch occupies only three pages. 



First, Dr. Siemens complains that I did not mention the sets 

 of resistance-coils made since 1848 in Berlin. I presume these 

 are what I mistakenly called the German mile of No. 8 iron wire. 

 This mistake of mine was not corrected by Messrs. Siemens in 

 1862, and appears again in the historical sketch. Unfortunately 

 Dr. Siemens's letter to me, giving information for my historical 

 sketch in answer to mine of the 28th February, did not come 

 till the 21st April, while the Report was sent in on the 16th of 

 March, and read on the 6th of April. The value of the coils 

 he refers to is given in the Table annexed to the sketch, and I 

 am sorry that I did not add a note to the reprint giving the exact 

 date of their introduction and their true definition. Dr. Siemens 



