LIVING TORPEDOS IN BERLIN. 435 



8. On Iodide of Potassium Electrolysis by the Torpedo-shock. 



When, following 1 Faraday's procedure with the Gymnotus, I 

 made use of iodide of potassium electrolysis in order thereby to 

 determine the direction of the shock in Malapterurus, I came upon 

 the phenomenon of the secondary iodine spot (as already many 

 times related) ; that is to say, I saw a spot arise under the nega- 

 tive as well as under the positive platinum point. I at once per- 

 ceived that I had before me an occurrence with which I had already 

 become acquainted when I employed the same means for ascer- 

 taining the direction of the current in complicated induction-cir- 

 cuits 1 . The platinum points are polarised by the decomposition of 

 the iodide of potassium, and if the circuit is not instantly opened 

 again, whether in experiments with the induction coil or with 

 electrical fish, a secondary or polarisation- current follows on the 

 top of the primary-current in the opposite direction, and gives 

 rise to a spot on the formerly negative but now positive point. 

 I was induced by the experience of the Malapterurus experiments 

 to make the appearance of the secondary iodine spot the object of a 

 complete investigation, of the results of which I will recall only 

 one or two. In the first place, if the quantity of electricity in the 

 primary-current which is restored within the unit of time exceeds a 

 certain limit no secondary spot is seen. But there are circumstances 

 in which the secondary spot surpasses the primary, so that the 

 iodide of potassium electrolysis becomes a completely deceptive 

 indication of the original direction of the current. This is the 

 case if, besides the platinum points dipped in iodide of potassium 

 solution, another pair of platinum electrodes in say a weak solution 

 of sulphuric acid, are also in the circuit 2 . 



The possibility of the double iodine spot being caused by a 

 double shock of the fish was worth considering. I satisfied myself 

 by means of the frog-interrupter that this was not the case. The 

 secondary spot was absent if the circuit was opened either imme- 

 diately after the shock or towards the end of it. Besides this, in 

 Malapterurus it was sometimes possible to make the secondary spot 

 the stronger of the two by leading-off from the fish with platinum 

 electrodes. Thus the dependence of the secondary spot on polari- 

 sation was established in this case, and in this respect there remained 

 no obscurity as regards Malapterurus. 



It is, however, incomprehensible that none of the observers who 



1 Untersuchungen, vol. ii. Part i. 1849, p. 400. 

 a Gesammelte Abhandlungen, vol. ii. pp. 648, 666. 



if* 



