128 Method of Determining Coefficients of Mutual Induction, 



Hence 



M=4-926 x 10- 15 x 2-415 x 10 23 

 = 1-1896 xlO 9 . 

 As a further test of the accuracy of the method, the 

 secondary wires of Apps's coil and of the small coil (A) were 

 connected in circuit with each other and with a galvanometer, 

 and the two primaries were connected so that the battery- 

 current was divided between them, as shown in fig. 5. The 



Fig. 5. 



connections being arranged so that the induced electromotive 

 forces opposed each other, it was possible, by the proper 

 adjustment of the resistances of the branches containing the 

 two primary coils, to prevent the galvanometer being deflected 

 when the battery-circuit was made or broken. When this adjust- 

 ment is made, it is evident that the ratio of the resistances of 

 the two primary circuits is the same as the ratio of the two co- 

 efficients of mutual induction. In this way, 39 - 45 was obtained 

 as the ratio of the coefficients ; whereas the condenser method 



described above gives » ( . = 39*13, giving a difference of 



about 1 per cent. As, however, the current in the primary 

 of the smaller coil has to be about forty times as strong as 

 that in the large coil in the comparison experiments, in order 

 that the induced electromotive forces may balance — and as, 

 moreover, the resistance of the copper wire of the former was 

 a comparatively large fraction, at least one sixth or one seventh, 

 of the whole resistance in circuit with it, while the copper 



