Whitehead and Hill — Measurement of Self-Inductance. 149 



Art. IX. — Measurement of Self -Inductance / by J. B. 

 Whitehead and H. D. Hill. 



The absolute measurement and comparison of self-induct- 

 ance has within the last few years become a problem of 

 increasing importance. The best known method and that 

 most generally used is the Rayleigh method. However, this 

 necessitates very intelligent and careful work to obtain results 

 concordant to one per cent. What seem to be the most accu- 

 rate methods are various modifications of the Wheatstone 

 bridge using alternating currents with an electrodynamometer, 

 telephone, or optical telephone as the measuring instrument. 



In January, 1898, Professor Bowland* published a brief 

 description of some twenty-six methods for the measurements 

 and comparison of self-inductance capacity and mutual induct- 

 ance. These methods are mostly of the Wheatstone bridge 

 type and depend upon the use of an alternating current and an 

 electrodynamometer. As only a few of them have hitherto 

 been tried, it was deemed desirable to systematically test them 

 and determine their value, particularly in regard to the meas- 

 urement of self-inductance. Such is the aim of the present 

 investigation. 



Source of current and frequency determination. 



Since the value of the self-inductance or capacity as given 

 by many of the methods depends upon the square of the cur- 

 rent frequency, it was necessary to secure a current of harmonic 

 wave form whose period was as constant as possible, and to 

 devise a method of accurately determining that period at the 

 moment of adjustment. This was very satisfactorily accom- 

 plished. In the deduction of the formulae for all the methods 

 the assumption is made that we have a simple harmonic elec- 

 tromotive force. In general an alternating current supplied 

 by a dynamo has not only its fundamental period but also the 

 odd upper partials. However, a generator constructed with- 

 out too much iron and properly wound, if the field is not very 

 strongly excited, will give a very good sine wave ; especially if 

 the resistance in the circuit contains self -inductance and no 

 iron. These requirements were well satisfied by the dynamo 

 used, which* gave a very good sine wave. The alternator was 

 constructed at the University. Its armature consisted of 

 twelve coils fastened flat on a German silver plate revolving 

 between twelve field pieces producing six poles. This dynamo 

 was coupled directly to a direct current Crocker-Wheeler 

 *Phil. Mag. [51, xlv, 66-85, 



