434 The Hon. J. W. Strutt on an 'Electromagnetic 'Experiment. 



judge of the correctness of the view here taken, I subjoin an esti- 

 mate of the period of oscillation in the actual arrangement of the 

 experiment. 



The time of a complete oscillation of a current in simple con- 

 nexion with a condenser =2tt v'LS*, where L and S are mea- 

 sured in absolute measure. If r be the time-constant of the 

 circuit, Jj = Rr, B being the resistance in absolute measure. Now 

 the capacity called a Farad is 10 -13 absolute measure, and the 

 B.A. unit is 10 7 on the same system. 



If therefore we take as practical units the B.A. unit of resist- 

 ance, and the Farad as unit of capacity, 



77 



' = 500 ' Mt - 



t and t are here measured in seconds. The condenser employed 

 (made by Elliott Brothers) had a capacity of half a Farad, so 

 that S = J. For one wire of coil A or B, 



R = i, 



T=-004f. 



For one of these wires in simple connexion with the condenser 

 the time of oscillation would be 



V I X \ x ~~ ''' '' ''" 



* /l 1 4 

 500 V I X 2 X 1000 



Comparing this with the value of t, we see that the first ten or 

 so oscillations would be comparatively unaffected by resistance. 

 By what has been proved, the time, when L = 5 (in different unit 

 of course), must be just double of this, or •00028". 



The action of the condenser in the inductorium is very imper- 

 fectly explained in the text-books, and is no doubt in many cases 

 rather complicated. From the reasoning of this paper, it appears 

 that it is by no means a complete account of the matter to say 

 that the advantage derived from the use of the condenser depends 

 only on the increased suddenness with which the primary cur- 

 rent is stopped. In a complete investigation (which I do not 

 mean to enter on here) a distinction would probably have to be 

 made, according as the secondary circuit when open allows the 

 passage of a spark or not, or, as a third case, is completely closed. 

 I would, however, remark that a good deal of misapprehension 

 arises in this and similar cases from forgetting that a condenser 

 is powerless to make away with electrical energy. Such energy 

 may be disposed of in the form of a spark, or it may be converted 



* The effect of resistance being neglected. 



t The value of r previously given for coil A is erroneous. 



