The Hon. J. W. Strutt on an Electromagnetic Experiment. 431 



In the first two arrangements the initial currents would be 

 equal, while in the third they would be doubled. 



On the whole, I think these experiments confirm the view that 

 the acquired magnetism depends principally on the initial cur- 

 rent. The exact laws regulating the connexion between the 

 current and the magnetism produced by it are doubtless compli- 

 cated, and not of much interest. The facts here detailed should, 

 however, be borne in mind by any one who wishes to pursue 

 this subject, and they do something towards explaining the dis- 

 cordant results of previous experimenters. 



And now as to the effect of a condenser. Considering that, 

 in consequence of the length of wire in the quadruple coil, the 

 duration of a current in it, even under arrangement b, must 

 be much greater than the time occupied by the break, or, which 

 is the same thing, the duration of the spark, I did not anticipate 

 that a condenser (whose plates were connected with the breaking- 

 points) would have any influence. But to my surprise I found 

 that although, of course, the superiority of b was not disturbed, 

 the magnetic effects were all increased. The explanation is, I 

 believe, to be found by an examination of what takes place in the 

 two circuits after the primary current is thrown on the condenser 

 by the removal of the wire from the mercury-cup. 



The charge (or discharge) of a condenser through a circuit 

 endowed with sensible self-induction was first investigated by 

 Sir W. Thomson (see Phil. Mag. vol. v. p. 393, or Wiedemann's 

 Galvanismus, vol. ii. p. 1007). There are two cases according 

 to the comparative magnitudes of the three elements of the pro- 

 blem, which are (1) the capacity of the condenser, (2) the self- 

 induction of the circuit, (3) the resistance of the same. If these, 

 reckoned in absolute measure, be denoted by S, L, B, respectively, 

 the motion of electricity is of an oscillatory character if 



4L 



otherwise the charge is completed and equilibrium established 

 without a retrograde motion. The motion of a pendulum in a 

 viscous liquid is exactly analogous and may serve as an illustra- 

 tion. If the viscosity of the liquid exceed a certain limit, the 

 pendulum withdrawn from the vertical and then let go will sub- 

 side gradually back again into its position of equilibrium without 

 ever passing it ; but if the viscosity be small, rest is only at- 

 tained after a number (theoretically indefinite) of oscillations of 

 continually decreasing amplitude. To our case of currents 

 mutually influencing one another, Thomson's calculations are 

 not immediately applicable ; indeed the exact solution would be 



