.'2 Prof. J. C. Poggendorff on the Extra Current 



citing current, that the tension observed only belongs to the 

 current when it is opened. 



These phenomena are certainly surprising when it is considered 

 that the poles of the inductorium lose their tension completely 

 when they are joined by a simple, by no means short, thick 

 metallic wire. It is indeed stated that they retain even then a 

 trace of tension ; but with my apparatus I could not observe it, 

 either by the gold-leaf electrometer or with the tongue. Even 

 when I had approached the poles of a powerful apparatus till 

 they produced sparks abundantly, I could, by means of 100 feet 

 of a platinum wire 0*1 millim. in diameter, which was bent back- 

 wards and forwards in the air, and was equal in resistance at 

 least to 1000 feet of the induction wire, entirely take away not 

 merely the sparks, but every perceptible trace of tension at the 

 poles, when I connected them with the two ends of this wire. 

 But these phenomena of tension were at once distinctly observed, 

 though in a less degree, even with a copper wire of # 66 millim. 

 in thickness and not more than 400 feet long which was rolled 

 in the form of coil. 



Non-metallic, relatively bad conductors alone exhibited a dif- 

 ferent deportment in this respect. A hemp string, for instance, 

 moistened with feebly acid water, showed not merely tension in 

 the electrometer, but gave just as delicate sparks as a large coil 

 of wire, even when its length was only an inch or less. 



It differed remarkably from metallic wires (in which, as is well 

 known, only very feeble heating is perceptible) by the fact that 

 it became considerably heated — so much so, that a thermometer 

 round whose cylinder it was coiled once, rose in a minute from 

 40° to 50°, that it smoked visibly, gradually dried, then car- 

 bonized, and finally disappeared with sparks. 



Although it was not impossible that even straight wires of 

 great thickness and length, simply in virtue of their resistance, 

 might exhibit a certain tension when united with the poles of 

 the inductorium, I was yet convinced that the phenomena 

 mentioned were not phenomena of resistance, but arose from an 

 induction which might indeed occur in a feeble degree in straight 

 wires, and I designated it even then as the result of such a 

 process, without expressing an opinion as to the particular manner. 



Continued occupation with the subject has confirmed me in 

 the original view, and leads me to consider it as not doubtful 

 that the tension observed proceeds from an induction current, 

 which the current of the inductorium, the current of opening, 

 produces in the spires of the accessory coil in the opposite direction 

 to its own. 



As it is usual to designate the current of induction, which the 

 voltaic current can produce in its own wire on opening and 



