122 Prof. G. C. Foster on a Method of Determining 



at K, the galvanometer is traversed by a quantity Q', such that 



Q' = 7CE. 

 If the points A and D are found by trial so that the deflection 

 of the galvanometer is the same in both cases, we have 



M = CRr. 

 This mode of working, however, has the obvious defect that 

 the result, as stated, implies that the current in the battery- 

 circuit is of exactly the same strength during each part of the 

 experiment. As this cannot be looked for, it would be needful 

 to include a measuring galvanometer in the battery-circuit, so 

 as to take account of the variation of the current. The re- 

 quired coefficient is then obtained in the form 



M = ^ CEr. 



7i 

 But, instead of making two separate experiments, as above, 

 it is simpler to adopt an arrangement of apparatus which is 

 very nearly a combination of the two arrangements just 

 described. A single experiment then takes the place of two, 

 and, instead of having to reproduce a particular deflection of 

 the galvanometer, we have to adjust a resistance so as to 

 prevent deflection. The connections will be understood by 

 reference to fig. 3, where, so far as the reference-letters used 



Fig. 3. 



p s 



in the previous figures recur, they have the meanings already 

 given to them. The observation consists in adjusting a set 

 of resistance-coils at r, between the galvanometer and the 

 coil s, until there is no throw of the galvanometer on making 

 or breaking contact at K. 



Let the resistances A s f e, age, ad be represented by 

 p, q, r respectively, and the corresponding currents by x, y, z. 

 Further, let the current in the battery and primary coil be 



