Dr. Matthiesscn on the Unit of Electrical Resistance. 377 



some of the mistakes which Dr. Siemens has made in his paper, 

 probably owing to his not having carefully read the Reports and 

 papers he criticises. 



Page 327, Dr. Siemens states, " I am rather surprised that 

 Dr. Matthiessen should have observed in German silver marked 

 changes within short periods of time, as I have always found 

 this alloy remarkably constant." In the Report of 1863 (p. 126) 

 I point out that, although the conducting-power of the German 

 silver experimented with altered, yet this is no proof that all 

 German silver will do so. Again, in Report for 1864 (p. 347), 

 it is remarked, "A somewhat capricious change has been ob- 

 served in certain annealed German-silver wires, while others have 

 been proved constant [referring to my experiments in Report of 

 the same year, p. 352J . This result has been independently ob- 

 served by other members of the Committee/' 



Page 327, Dr. Siemens states, "It is true Dr. Matthies- 

 sen brings forward an argument in favour of the electrical 

 permanency of an alloy of silver and gold — in which I can 

 scarcely suppose him to be serious : it is, that we have never 

 found a gold chain to become brittle." In the Report for 1862 

 (p. 149), the passage referring to this will be found in that part 

 of the Report where we discuss the effects of homogeneity and 

 molecular condition on the gold-silver alloy as a means of repro- 

 ducing standards, and is as follows : — ff It has been argued that 

 the molecular condition of all alloys is liable to undergo a change 

 by age, and that therefore alloys are not fit to be used as stand- 

 ards. Thus it is well known that brass and German silver 

 become brittle and crystalline by age, and that the same may 

 occur with the gold-silver alloy ; but on looking at the compo- 

 sition of the alloy, it will be found to have nearly the same as 

 that of the gold chains of commerce. Now we do not know of 

 a single instance where such a chain, even after years of use, 

 becomes brittle or crystalline by age ; so that we think it more 

 than possible that the alloy will not change its molecular condi- 

 tion by age." 



Page 328, Dr. Siemens states, " On this ground it is certainly 

 significant that the Committee should have made ten normal 

 measures instead of one single one, even supposing them to 

 agree with each other to within O03 per cent., as asserted," 

 Dr. Siemens should have added after the words within 0*03 per 

 cent., at 15°*5, and are equal to one another at some temperature 

 stated on each coil, lying behveen 14°-5 and 16 0, 5 C* 



With regard to my assertion f, that no true mercury unit has 



* Proceedings of the Royal Society, No. 74, p. 160. The translation of 

 this paper (Poggendorff's Annalen, vol. cxxvi. p. 369) is afterwards quoted 

 by I)r. Siemens. 



f Phil. Mag. May 1855, p. 364. Poggendorff 's Annalen, vol. cxxv. p. 91. 



