154 Whitehead and Hill — Measurement of Self Inductance ~ 



deflection method depending upon adjusting the resistances 

 until there is a ninety degree difference in phase between the 

 currents flowing in the flxed and hanging coils of the electro- 

 dynamometer. The method was devised and first used by 

 Oberbeck,* later by Troje.f However, the frequency deter- 

 A urination of both 



these investiga- 

 tors was quite 

 crude and, as will 

 be seen from the 

 formula, the 

 value of L or C 

 varies as the 

 square of the fre- 

 q u e n c y . Dia- 

 grams of the 

 method and con- 

 n ections are 

 given in figs. 4 and 5. As before, F and H represent the flxed 

 and hanging coils of the electrodynamometer ; R„ R //? R', R /A 

 and r are the entire resistance of the several branches, L is the 

 self-inductance of the coil to be measured, and I that of the 



hanging coil of the elec- 

 5 trodynamometer, "W is 



a commutator for re- 

 versing the current 

 through the hanging 

 coil, S is a switch by 

 means of which the 

 current could be sent 

 through a resistance X 

 in value closely equal 

 to the impedance of 

 the network. The coil 

 L, to avoid any possible 

 effect on the rest of the 

 network o r hanging 

 coil of the electrodynamometer, was placed some distance away 

 usually on a level with the hanging and fixed coils and perpen- 

 dicular to them. 



If now an alternating electromotive force be applied to the 

 terminals A and B, we may express the currents in the fixed 

 and hanging coils of the electrodynamometer by G a e %pt and 



C l€ 



i(pt + (t>) 



p being equal to 2tt times the frequency. By 



the application of Kirchhoff's laws to the Wheatstone bridge 



*Wied. Ann., xvii, p. 816, 1882. 



flbid., xvii, p. 501, 1892. 



