Whitehead and Sill — Measurement of Self -Inductance. 163 



Then with alternating current W is adjusted until there is zero 

 current through the bridge arm, as shown by an electrodyna- 

 mometer or telephone. When this is the case the mutual 

 inductance completely counterbalances the self -inductance, so 

 that the currents and electromotive forces are in phase in each 

 arm of the bridge. Representing current through R' by F, 

 etc., 



R^rrRT + tpLI'-^M (I' + I, + I w ) 

 also R^RT 



r w 



and 



l w R + R 



/>M(I' + I,+I W ) 



I 



= ph 



L i R R' + R'' 

 Hence _ =1 + _ + ___ 



The method was set up so that by means of switches the resist- 

 ance W could be replaced by the battery, and the electrodyna- 

 mometer by the galvanometer. The method proved very 

 unsatisfactory. Using the electrodynamometer in the bridge 

 arm, its coils being connected in series, W could not be adjusted 

 closer than three per cent. Different 1 values of the resistance 

 were tried but the result was about the same. The electrodyna- 

 mometer was then replaced by a telephone. It was then possi- 

 ble to adjust to about two per cent but the noise of the dynamo 

 interfered somewhat with the use of the telephone. The con- 

 clusion was that the method is not sufficiently sensitive to be of 

 value. 



Method 19 is also intended for the measurement of — but 



M 



involves the use of a doubly wound coil. 



Methods 20 and 21 require a specially doubly wound induc- 

 tance coil. They were not thought sufficiently promising to 

 merit the construction of such a coil. 



Method 22 is Carey Foster's method adapted to alternating 

 currents. This method was rather thoroughly investigated by 

 Heydweiller.* 



Method 23 requires two simultaneous adjustments with alter- 

 nating currents, and Method 24 is only of special use in com- 

 paring two doubly wound coils. 



Summary. 



With the exception of Methods 14, 3 and 25 the Rowland 

 methods tested are of little value, their chief defect being a 

 *Wied. Ann., liii, p. 499, 1894. 



