-y- .at. 

 dt 



424 Mr. T. H. Blakesley on the Differential 



[l 2 t- 2 m~\ = l^-n-k-hmj^m- 2 m^~\t 



S [Fi.,d][!=], 



Here the term on the right hand disappears necessarily, 



and the work expended, if an y, is equal to ^(-77) dt. Hence 



this work vanishes only if 1 = 0, i. e., if there is no component 

 field in quadrature with the induction ; a curious antithesis 

 to the electric problem. If there were a field induced in 

 phase with the induction, it would not result in the dissipa- 

 tion of energy. No argument for such a state of things can 

 be drawn on the score of loss of energy. 



If the phases of magnetism in any cycle coincided with the 

 phases of field, there could be no such thing as hysteresis ; 

 and, further, no radiation of energy from an alternating 

 magnet. 



But both these phenomena have been for many years recog- 

 nized. It follows, therefore, that when induction through 

 any space changes, a magnetic field is induced acting counter 

 to the change. 



Now it may be noticed that the tangents of the angles of 

 lag, whether electric or magnetic, involve a coefficient in 

 their numerator and the value of the period in their denomi- 

 nator. They therefore become larger as the period is made 

 less. It might therefore happen that extreme rapidity of 

 change would be necessary before a lag of current or of in- 

 duction could be detected. Electric lags, or, at all events, the 

 coefficients of self-induction can readily be measured. Mag- 

 netic lags have been measured by the author in certain cases 

 by special artifices, but when w r e deal with a medium of small 

 permeability, as air, the period must be extremely minute to 

 make the lag-angle sensible, and as yet no machines possess 

 a sufficient frequency to effect it. Recourse has been had to 

 the rapid oscillations which take place when a Leyden jar is 

 discharged. In these cases the radiation has been frequently 

 caught and approximately measured, and it is therefore in 

 these very cases that the rectification of the formula becomes 

 important. 



I propose to investigate by geometry and otherwise the 

 conditions under which a Leyden jar is discharged. Geo- 

 metry especially will afford an excellent and graphic insight 

 into the question of the oscillatory discharge. 



