378 Dr. Matthiessen on the Unit of Electincal Resistance. 



been issued, I think this is sufficiently proved by Dr. Siemens's 

 statement (page 331) that the " standards will not represent the 

 unit at the temperature stated, but at 10 o, 5 higher"*. I merely 

 mention this to point out a probable error in this value (10 o, 5) ; 

 for to obtain it, Dr. Siemens uses a coefficient for the correc- 

 tion of resistance for temperature of the units which appears 

 very low, namely 0-0272 per cent, for each degree Centigrade — 

 in fact a much lower one than that sent with the units distri- 

 buted in 1863 or 1864 (the so-called 1864 unit), namely 0043 

 per cent, for each degree. 



Page 332, Dr. Siemens states, " The proof advanced by Dr. 

 Matthiessen f in support of his assertion that the units I have 

 determined do not represent the same resistance, is based upon 

 the measurement of two of my resistance-coils, of each from 1 to 

 10,000 units, by Mr. F. Jenkin, who was employed by the jurors 

 of the 1862 Exhibition to report upon the electrical instruments. 

 Mr. Jenkin found a difference of 1*2 per cent. between the values of 

 these two sets of resistances." Dr. Siemens then blames me for 

 using these values, as the coils were only adjusted for practical 

 use, &c. 



My reason for using these values in my paper will, I think, 

 be perfectly justified when it is considered that (1) in the Report 

 of 1862 (p. 147), these same values are brought forward when 

 discussing mercury as a means of reproducing standards. This 

 Report had been published for about two years, and during 

 that time no explanation of these differences has been pub- 

 lished. (2) In the Jurors' Report, Mr. Jenkin, when speak- 

 ing of these coils, states, " Each set of coils exhibited by the 

 London and Berlin firms is adjusted with an accuracy of one per 

 thousand, but those exhibited by the two firms differ from one 

 another by more than 1 per cent. Messrs. Siemens suggest 

 that this difference may be due to an alteration in the German- 

 silver wire of which the coils were made .... On the other hand, 

 Messrs. Siemens inform the writer that the two coils were really 

 made in Berlin about the same time. They suggest that when 

 compared by the writer, they may not have had the same tem- 

 perature; but precautions were taken to prevent mistakes of 

 this kind, and a difference of temperature of about 45° F. would 

 be required to account for the difference of resistance observed." 

 And (3) that Mr. Jenkin % considers that the values given in his 

 Table may possibly be 0*1 per cent, wrong, but does not think 

 any of them 0*25 per cent, wrong. 



Page 333, Dr. Siemens states, "Dr. Matthiessen asserts, 



* In PoggendorfF's Annalen (p. 337) this stands "at 10 o, 5 lower." 

 f The term I used was issued. 



