on STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 431 



between the temperature of the coils tluring these experiments and that 

 at which they were adjasted, we get for the nominal values 



pi = 73*16 ohms and p^ = 80"18 ohms. 



Hence, according to the first series of experiments, 



1 ohm ^ ^|p- X Kfi = 1-003 X ] O'J ; 



according to second series 



1 ohm = ^P^ X 100 ^ 0-999 x 10^ 



I do not attach any particular importance to these values, or to the 

 close agreement of their mean with the intended value of the ohm, as the 

 experiments, so far, have only been undertaken with the view of 

 ascertaining how far the method that has been described is capable, 

 when employed under favourable circumstances, of giving good results. 

 In this respect I think tlie experiments may be considered fairly satis- 

 factory, but the numbers obtained for the value of the ohm are subject 

 to several corrections, the most important of which are probably that for 

 errors of the clock, which I had no means of rating more accurately 

 than by comparison with a good watch ; that due to slight uncertainty 

 as to the value in ohms of the resistances measured; that due to self- 

 induction in the revolving coil, which, however, I believe must be very 

 small ; and perhaps errors due to unobserved disturbances of the magnetic 

 field during the experiments. 



I wish, in conclusion, to acknowledge with warm thanks the obliga- 

 tions I am under to Mr. Charles Hockin for most valuable aid of various 

 kinds, — important practical suggestions as to the construction of the 

 apparatus, information as to the conditions required in order to ensure 

 sensitiveness, and the loan, for a long time, of a very delicate zero- 

 galvanometer and a set of resistance coils. I am also greatly indebted 

 to Mr. Grant and Mr. G. W. von Tunzelraann, B.Sc, by whom conjointly 

 the actual observations were almost entirely made. 



Appendix II, 



On the Causes of the Variation in the Temperature-Coefficient of the 

 Alloys of Flatinum and Silver. By Herbert Taylor, Esq. 



In his report to the Committee of the British Association in 1862 

 Dr. Matthiessen proposed for the construction of standard resistance 

 coils the now widely-used alloj^ of silver and platinum, consisting of 

 two parts of silver and one part of platinum by weight. In the same 

 paper he gave the specific conducting power of the material and also its 

 percentage-variation in resistance due to a change of temperature of 1° 

 centigrade. 



The latter value, called the temperature-coefiicient in what follows, 

 he stated to be 0-031% per degree. 



It was, however, found, after the alloy came into general use, that the 

 temperature-coefiicient varied within moderately wide limits. 



And it was noticed, by the writer amongst others, that having 

 determined by experiment the coefficient of a particular wire, it was 

 necessary to make a fresh determination for the same wire when drawn 

 down to a finer gauge. 



