116 



KEPOKTS ON THE STATE OF SCIENCE. 



very "reat in one of the observations, and the sensitiveness is, therefore, 

 very "small (see expression (D), p. 108). The method is unsuited for 

 accui'ate work. 



Fiu, 7. 



(b) Method suggested hy Lord Rayleujh (fig. 8). — As an alternative to 

 the previous method, the Jfollowing process was suggested by Lord Rayleigh 

 in 1881:.' P is the low resistance whose value is required. Q is a one- 

 or tenth-ohm standard which is shunted by the resistances h and c, the 

 ratio of c to 6 being approximately equal to Q/P if the resistance of 

 the calvauometer is comparatively great. When the galvanometer is con- 

 nected across h, c is adjusted until the combination gives the same effect 

 upon it as P does. Then, supposing the resistance of the galvanometer 

 branch to remain constant, 



P= 



Q6 



The method may be made a null one by using a differential galvanometer 

 knd an additional resistance S (approximately equal to P) in the main 

 circuit. One coil of the galvanometer is connected across ]? and the other 



Fig. 8. 



across S, the lesistance of the galvanometer arm of S being adjusted until 

 there is no deflection. The P galvanometer coil is then joined across h, 

 and c adjusted to obtain a balance. Small variations in current strength 

 have no effect, but the current must be reversed and the combination 



' Camb. Phil. Soc. Proc, 1884, v., p. 133. 



