64 Royal Society : — Rev. T. R. Robinson on increasing 



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

 combining several helices collaterally, as is done when cells are ar- 

 ranged 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 im- 

 portance : 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 -^o °f a 

 second. During its decline it induces a current 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 dis- 

 charges in a second ; so that in round numbers the electric induc- 

 tion was only a tenth of the whole effect. I shall therefore in what 

 follows confine myself to the magnetic induction. 



If y be the magnetism at any time t, M its maximum, P the poten- 

 tial of the magnet on the secondary helix, II the potential of that 

 helix on itself, r the resistance of the secondary circuit, the secon- 

 dary current at the time t, we have, as is well known, 



, P^ dy II ■ efo , ' 



^-r X dt—r X ^ « 



the last term being the counter-current produced by the reaction of 

 (j) on the helix. If the inductive coefficient of electric action be dif- 

 ferent from that of magnetic, II should be multiplied by a factor e, 

 which in (b) will multiply fx 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 most probable is that the loss of magnetism is as the mag- 

 netism, which gives -^-= — /xe?£, whence y=Me~^, 

 V 



* This is not quite correct. Mr. G-assiot 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 elec- 

 tricity from the centre of the circuit to its extremities. 



