250 PROCEEDINGS OF THE AMERICAN ACADEMY. 



^ The periodicity was measured by taking the speed of the alternator 

 continuously during the test, it being necessary that the speed be 

 constant, or nearly so. 



The value of the coefficient of self-induction as measured by a large 

 number of trials with Rayleigh's Bridge and the heat method com- 

 pared as follows : — 



Rayleigh's Bridge. Heat Method. 



.0157 henrys. .0159 - .0164 henrys. 



We then tested the effect of permeability and periodicity by putting 

 an iron core into this same coil. The results were : — 



p = 629 Li = .066 henrys. 



p = 1332 Zi = .029 " 



These last results have to be considered with caution, as the im- 

 pressed electromotive force is changed by changing the periodicity. 



This gives an easy practical method of determining the coefficient 

 of self-induction under the ordinary conditions of use, which, so far as 

 we know, none of the various methods attempt to do. The method 

 was suggested by Professor Trowbridge. 



Jefferson Physical Laboratory, 



March 1, 1894. 



