Coefficients of Induction. 333 



be placed so as to bring the coil H into circuit with the 

 galvanometer. When the battery-circuit is completed or 

 broken, after a balance has been obtained as before, the 

 quantity of electricity which flows through the galvanometer 

 is 



M(V,-V,)/2R(.- + G), 



r being the resistance of the coil H, which must be previously 

 determined, and M the coefficient of mutual induction. If 

 a" denote the angle of throw in this case, a, a', and K signi- 

 fying the same as before, and the latter not being altered, we 

 have 



a" 



M = 2KR(V + G)sin^ /sin£, 

 and / 



M/L=(r + G) sin y (6 + 2G + R) sin |. 



Of course we may, in this case also, employ an ordinary high- 

 resistance galvanometer, but with somewhat more trouble and 

 perhaps less accuracy. 



In the following determination of a coefficient of mutual 

 induction, although the galvanometer used was not ballistic, 

 the throws of the needle are assumed to be, although they are 

 not, proportional to the quantities of electricity passing- 

 through the galvanometer-coils. The resistance of the coil E 

 was 1*003 ohm, that of H 157*7 ohms, and that of the 

 galvanometer 164*8 ohms. The throws a u and a! were 

 respectively 72 and 5 scale-divisions, and K, the capacity of 

 the condenser, was 1 microfarad. 



Hence 



M = 10 3 x 2*006 x 332*5 x 14*4 centim. 



= 9315864 centim. 



= •0093 henry. 



3. Null Method of measuring the Coefficient of Self- 

 induction of a Coil. 



In the method given in Maxwell (art. 778, vol. ii.) the cur- 

 rent through the galvanometer at any moment is zero, but 

 the double adjustment which is necessary is inconvenient. 

 Two methods are given by Mr. Rimington in the * Philo- 

 sophical Magazine ' for July 1887 ; but this condition is not 

 fulfilled, and Mr. Rimington shows this for one of his methods. 

 Prof. Niven's methods, given in the ' Philosophical Magazine ' 

 for September 1887, do not differ greatly from those given 

 by Mr. Rimington, 



