196 Dr. Matthiessen on Standards of Electrical Resistance, 



improbable that it differs by 0*2 per cent, from the truth, and it 

 is scarcely possible that it is wrong by 05 per cent.*" 



2. M. Siemens states, page 93 (2nd paper), "Because the differ- 

 ences found in the conducting powers of the gold-silver alloys 

 I had made in different places amount to 1*5 per cent., the alloy is 

 useless for the purpose proposed by me (Phil. Mag. Feb. 1861), 

 namely, the reproduction of a resistance by means of which the 

 observations of different experimenters may be compared with 

 each other, or the reproduction of a resistance in absolute mea- 

 sure. For if two alloys are made and their resistances deter- 

 mined, we should certainly come within one half per cent, of the 

 true value, six out of the eight alloys tested agreeing within 

 that limit." Let us now for a moment see what M. Siemens 

 says of his proposed mercury standard ; and on referring to his 

 first paper we find a Table, where he gives the resistances of six 



tubes filled with mercury. The values found by him for — , 



where w is the calculated and w l the observed resistances, are given 

 in the following Table, together with those found by myself for the 

 conducting power of the gold-silver alloy. 



Table I. 



No. of tube. 



Values found for 

 w 



Conducting power of hard- 

 drawn alloys. 



2 

 3 

 4 

 5 

 6 



1-008 



1-000 



1-0008 



0-992 



0-994 



1-005 



1 = 1003 



2 = 1-002 



3 = 0-988 



5 = 1004 



6 = 0-997 



7 = 1003 



8 = 1001 



M. Siemens, when speaking of the differences he found, says 

 they are not greater than were to be expected; and further on, he 

 continues, the temperatures of the etalon [copper) and the mercury 

 varied 2 — 3° C. during the experiments ; but does not state which 

 of the determinations were made at the higher or lower tempe- 

 rature, so that the differences he finds may be greater or smaller, 

 as the case may be. Now on comparing the above values it will 

 be seen that the maximum differences are in each case the same. 

 If, therefore, in the opinion of M. Siemens, the gold-silver alloy 

 is useless as a standard, how much more must his mercury 

 standard be so, when, according to his own determinations with 



* Prof. Weber, in a letter written a short time since to Prof. Thomson, 

 states, when speaking of some new determinations of resistance in absolute 

 measure he is about to undertake, that by some improvements in the me- 

 thod and apparatus he hopes to arrive at a still greater accuracy than that 

 which he formerly obtained. 



