THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JUNE 1888. 



LVII. The Variation of the Coefficients of Induction. By W. 

 E. Sumpkek, B.Sc, Assistant in the Physical Department, 

 Central Institute, South Kensington* '. 

 [Plate III.] 



1. rTlHERE are three ways of defining the coefficient of self- 

 -B- induction, which lead to the same result if the magnetic 

 permeability of the medium is a constant quantity, but which 

 lead to three different results if it is a variable one, as in the 

 case of iron. 



The coefficient of self-induction L of a coil of wire through 

 which a current C is passing may be defined as the ratio 

 between the back electromotive force and the time-rate of 

 change of the current C to which it is due; or it may be 

 defined as the ratio between the flux of induction through 

 the coil and the current producing it; or it may be defined 

 with reference to the electrokinetic energy possessed by the 

 current C. 



The three definitions are expressed by the equations 



n\ t dG 



(2) N = L 2 C and e=^p> 



(3) T = !L 3 C*. 



In these equations e is the back electromotive force pro- 

 duced by varying the current C, N is the number of lines of 



* Communicated by the Physical Society: read 14th April, 1888. 

 Phil. Mag. S. 5. Vol. 25. No. 157. June 1888. 2 H 



