514 PROFESSOR W. K AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



Denoting by (M a ' M,') the new value of (M a M^, we get 



(M/-M/) = (M a -M,) + 0-892 = 12980-562 + 0-892 



= 12981-454, 

 and therefore 



M L = (M 2 -M 1 ) + (M/-M/) 



= 25962-016 (22). 



For the right-hand set of coils the increments are 



dA." = 0-00111, dA.'" = 0-00052, 



. 



for the lower and upper fixed coils respectively, and for the current sheet a 2 a 2 ', 



da = 0-00027. 

 Hence 



(M/'-M/') = 12979728, (M/'-M/") = 12980726, 

 therefore 



I M H = (M/'-M/O+CM/'-M/") 



= 25960-454 (23), 



and 



ML + MR = 51922-47 (24). 



The values obtained by the calculating machine were as follows : 



M L = 25962-04 (22'), 



M R = 25960-43 (23'), 



and 



M L + M U = 51922-47 (24'). 



Thus the two methods give the same result for the sum M L + M K , although the 

 constituent values differ by nearly 1 in 1,000,000. It should, however, be pointed 

 out that one of us calculated the mutual inductions from the arithmetical mean 

 dimensions of the helices concerned, and the other from the calibrated mean dimen- 

 sions as obtained from the curves shown in figs. 13, 14, 15, and 16. The agreement 

 is, nevertheless, very close. 



Mr. G. F. C. SEARLE has calculated the force between the current in one set of 

 fixed coils and that in the suspended coils of the system not coaxial with it. The 

 distance between the axes of the coils is a most important factor in the calculation, 

 the accuracy of the calculation being approximately that with which the 5th power 

 of this distance is known. The distance was determined as 50*8 centims. approxi- 

 mately, and for a current of 1*018 amperes a balancing mass of 0'0427 6 gramme was 

 calculated by Mr. SEARLE'S formula. In practice the balancing mass for this current 

 is G'0424 gramme. The agreement is satisfactory. 



