8 The Hon. J. W. Strutt on some Electromagnetic Phenomena 



according to the connexions) ; and the value of y for which this 

 is a minimum is —x(\ or J). In the first case, the exterior 

 part of the induced circuit having no sensible self-induction, 

 takes away nothing from the initial current ; but in the second 

 there is a reduction to one-fifth. On the other hand, it makes 



no difference to the total current ( — -^-x ) , as measured by the 



deflection of the galvanometer-needle, which way the connexion 

 is made ; for the smaller initial current, in virtue of its greater 

 inertia, sustains itself proportionally longer against the damping 

 action of resistance, which is the same in the two cases. The 

 heating-power and the effect on the electrodynamometer, which 

 depend on the integral of the square of the current while it lasts 



/ M 2 \ 



( i tvo x<i )> w ^ ^e different ; but the easiest proof of the diver- 

 sity of the currents is to be had by comparing their powers of 

 magnetizing steel. 



Thus, if we include in the induced circuit a magnetizing spiral 

 in which is placed a new sewing-needle, we shall find an im- 

 mense difference in the magnetization produced by a break-in- 

 duced current, according as its direction is the same or otherwise 

 in the wires B x B 2 . In the actual experiment the diluted current 

 was unable, even after several repetitions, to give the needle any 

 considerable magnetization (the vibrations were only about three 

 per minute), while after one condensed current the needle gave 

 sixteen, raised by repetition to nineteen f. A new needle submit- 

 ted to the action of several condensed currents also gave nineteen 

 per minute. The magnetic moments, which are as the squares 

 of these numbers, show a still greater disproportion. 



The truth seems to be that the time required for the perma- 

 nent magnetization of steel is so small as compared even with 

 the duration of our induced currents, that the amount of acquired 

 magnetism depends essentially on the initial or maximum cur- 

 rent without regard to the time for which it lasts. 



The increased heating-effect when the two parts of the current 

 in B are opposed in direction is, of course, at the expense of the 

 spark in the mercury-cup. The mechanical value of the spark 

 is the difference between the values of the currents which exist 

 at the moments before and after the breaking of the contact, and 



= iL**-JNy« =i* 2 (L-^) =i*»(L-j£) nearly. 



* R, S are the resistances of the primary and secondary circuits respec- 

 tively. 



t These were complete vibrations. 



