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LXXIX. On Pirani's Method of Measuring the Self- 

 Inductance of a Coil. By E. C. Snow, B.A* 



IN" Piranr's method of measuring the self-inductance of a 

 coil, it is assumed that the self-inductance of the galva- 

 nometer used may be neglected. That this assumption is 

 legitimate in the case in which the discharge of the condenser 

 employed (which, it will be shown, is of the same nature as 

 the discharge through the galvanometer) is continuous has 

 been proved |. 



The object of the present analysis is to investigate the 

 case in which the discharge of the condenser is oscillatory. 



A condenser of capacity C, the " internal " resistance of 

 which can be neglected, is placed in series with the coil whose 

 self-inductance is to be measured. The condenser is shunted 

 by a non-inductive resistance r. This combination forms one 

 arm of a Wheatstone's bridge, the resistances of the other 

 arms of which are adjusted so that a steady balance holds 

 between the lour arms. 



One or other of two methods can now be used: (i.) keeping 

 r constant, C can be varied until there is no throw on closing 

 the galvanometer-key before the battery-key; or (ii.) keeping 

 C constant r can be varied, the total resistance of the arm 

 being kept constant, until the same result is arrived at. 



The following symbols will be used: — 



I, the current in the battery. 



,r, the current in the galvanometer or (in the case of an 



alternating current) the telephone. 

 a, the current in the coil whose self-inductance is required. 



a, the current in the shunt resistance r. 

 e, the current flowing into the condenser. 



b, the current in the resistance B. 

 a\ the current in the resistance kA. 

 //, the current in the resistance 7tB. 



* ( Jommunicated by the Physical Society : read March 12, 1909. 

 t 0. de A. Silva, L'Uchtiruyt' Electrique, vol. 1. pp. 113-116. 



