The Hon. J. W. Strutt on an Electromagnetic Experiment. 429 



electrical phenomena in the light of some dynamical theory. It 

 was for this reason that 1 considered the matter worthy of expe- 

 rimental investigation, the fruits of which were given in the paper 

 referred to. One point, however, still required a little clearing 

 up ; and it is this which I now propose to deal with. I mean 

 the mode of action of the condenser, which was employed, as in 

 the inductorium, for the purpose of rendering the break more 

 sudden, and which I had found necessary for the success of the 

 experiment as then arranged. At this necessity I was not sur- 

 prised ; for, according to the indications of theory, the effect was 

 only to be expected when the fall of the primary current is sud- 

 den compared to that of the secondary. Now the duration of 

 free transient currents in a circuit varies, ceteris paribus, as the 

 self-induction; so that when the number of turns in the secon- 

 dary is too much reduced, there is danger of the condition not 

 being fulfilled. If it be objected that as much would be gained 

 by improved conductivity as lost by diminished self-induction, 

 I answer this is not the fact, the resistance varying as the num- 

 ber of turns simply, while the self-induction varies as the square 

 of the same number. Besides, I had reasons for keeping the 

 resistance in all cases invariable. 



Wishing, however, to obtain the effect without the aid of a 

 condenser, I prepared a quadruple coil by bending into the form 

 of a compact ring a bundle consisting of four No. 16 copper 

 wires, each 70 feet long. Into one of these the current from a 

 Daniell cell was permitted to flow, subject to interruption at a 

 mercury-cup. The secondary circuit consisted of the other three 

 wires arranged consecutively, and of the magnetizing spiral, which 

 contained the needle destined to measure the effect of the mo- 

 mentary current. The three wires could be joined so that the cur- 

 rent circulated the same way round them all [a), or so that in one 

 of them the direction was the opposite to that in the two others (b). 

 It will be seen that the resistance was always the same, the only 

 change being in the coefficient of mutual induction (M), and of 

 self-induction of induced circuit (N). In the former paper it 

 was shown that the initial induced current, being proportional to 



M 



^-, is three times as great in (b) as in (a) (a sufficient sudden- 

 ness of break being assumed), while the total currents are in the 

 reciprocal proportion. In carrying out the experiments, I sub- 

 mitted the needle (a new one in each measurement) to the action 

 of six break-induced currents, always removing it from the spiral 

 when the battery contact was made. In this way a more con- 

 stant result is obtained than from one discharge only, which is 

 liable to vary from slight differences in the character of the break. 

 The needle w r as then swung by a silk fibre and the number of 



