Measurement of Mutual Inductance. 



495 



measured, its secondary coil being in series with an ad- 

 justable resistance in the branch AyF ; K is a condenser, 

 and G a ballistic galvanometer. When there is no throw on 

 G on reversing the battery then 



M = 10" 6 KRr, 



where M is in henries and K in microfarads, and r is the 

 resistance o£ the branch Ay¥. To increase the sensitivity a 

 secohmmeter arrangement was sometimes used. Knowing 

 the value of the vibration galvanometer in other cases, I 

 attempted to apply it to this one, but found that the method 

 had to be modified in order to make it applicable. I found 

 later that the modification I had introduced had been already 

 suggested by Rowland *. As the use of the vibration 

 galvanometer, however, is a great advantage (and novel in 

 this method, so far as I know), I think it will be of interest 

 to describe the complete method. 



2. Theory of Modified Carey Foster Method. 



In fig. 2 let Q be a source of alternating current and G a 

 vibration galvanometer or its equivalent. The necessary 

 modification consists in adding a resistance S in series with 

 the condenser K. Let the resistances of the other branches 



Fig. 2. 



be R and r, and let the instantaneous values of the currents 

 be i, ii, and i 2 as marked. Let the instantaneous potentials 

 on the terminals of the galvanometer G be and 0, so that 

 there is no deflexion (i. e. the condition of balance is to hold 

 at every instant). Let v be the instantaneous value of the 

 potential at D, and q the charge in the condenser. 

 * Phil Mag. [5] vol. xlv. pp. 65-85 (1898). 



2 L 2 



