172 



Rev. T. R. Robinson on increasing the [May 31, 



Secondly, the secondary helix may be made of longer or thicker wire. 

 It will be shown immediately that the length does not increase the quan- 

 tity of the current at all, and that the effect of the thickness is limited. 



Thirdly, the analogy of the voltaic battery suggests the plan of combi- 

 ning several helices collaterally, as is done when cells are arranged for 

 quantity ; and this I think may avail to a very great extent. 



In spectral work, as in most other applications of the inductorium (as 

 the Germans have named it), the breaking current alone is of importance : 

 the other, though equal in quantity, is so much inferior in tension that it 

 is stopped by a thin film of highly rarefied air*. This current proceeds 

 from two causes. "When the circuit is broken, the current in the primary 

 ceases ; not instantaneously f, but in a time which is very small ; according 

 to Edlund less than -g-J-Q of a second. During its decline it induces a cur- 

 rent in the secondary. But while it was passing it had magnetized the 

 iron core of the apparatus : this magnetism now passes away, and in doing 

 so it also induces a current in the secondary, which lasts longer and is more 

 powerful than the other. I compared the two by measuring those given 

 when the core was removed from a primary, and when it was in its place : 

 they were as 1 : 8*62 when the rheotome made 17 discharges in a second ; 

 so that in round numbers the electric induction was only a tenth of the 

 whole effect. I shall therefore in what follows confine myself to the mag- 

 netic induction. 



If y be the magnetism at any time t, M its maximum, P the potential of 

 the magnet on the secondary helix, IT the potential of that helix on itself, 

 r the resistance of the secondary circuit, the secondary current at the 

 time ti we have, as is well known, 



, V dy Jl dd) /X 



.^=7^-J-7x^^' • • • • « 



the last term being the counter current produced by the reaction of ^ on 

 the helix. If the inductive coefficient of electric actiqn be different from 

 that of magnetic, 11 should be multiplied by a factor which in (6) will 

 multiply ju in the exponent and denominator. I see no reason why they 

 should differ unless some work be lost by molecular changes in the core 

 when excited. The great difference between the two currents which I 

 have just mentioned arises most probably from the different values of t in 

 the exponentials. 



To integrate {a) we must assume some relation between dy and dt. The 



^" This is not quite correct. Mr. Gassiot showed from the reversed curvature of the 

 strata in an exhausted tube that some of the closing current does pass when the action 

 is powerful. The same conclusion follows from a fact which I observed last year with 

 Mr. Atkinson's magnificent Euhmkorff. In general when a discharge is made between 

 platinum points, the negative one only is ignited ; in this case the positive one was so 

 also, though to a far less degree and for half the length. 



t As the tension at the opening of the rheotome decreases, so must also the velocity 

 of discharge there ; and time is required for the passage of the electricity from the 

 centre of the circuit to its extremities. 



