112 Professors Ayrton and, Perry on 



increase of resistance and y x the coefficient of increase of vefo 

 sistance of German silver per degree, 



or ^CB? 



r! = B-y- say, 



where B is a constant. Hence, if e is the error due to the 

 proportional change of resistance, 



r + if 



e — 



R + Ri' 



AR + 



BC 2 R 5 

 V 



R+Rx * 

 Let R t 



R = ^ 

 then A B 9 



^=-r-~- (20) 



To determine the value of the constants A and B we may 

 proceed as follows : — Wind a voltmeter according to the law 

 which is to be adopted with any or no resistance-coil outside ; 

 find the potential-differences P x and P 2 required to produce 

 the maximum reading of the instrument, 



1st, immediately on making connection ; 



2nd, after connection has been made for some time ; 

 then 



e- ^ 



Let e be experimentally determined in two cases, — one where 

 there is no resistance-coil outside, and the other where there 

 is such an auxiliary resistance-coil, obtaining values e x and e 2 

 respectively. Measure y, that is the ratio of R x to R, and 

 also V. Then from the equations 



*i=A, 



* 2= -rrr 



the values of the constants A and B can be determined ; and 

 when determined, the error arising with the employment of 



