INDUCTIVE ACTION OF A CURRENT UPON ITSELF. 47 



the wire, and that thus a long wire produces effects at the instant the current is 

 stopped, which a short wire cannot produce. Such an explanation is, however, at 

 once set aside by the fact, that the same length of wire produces the effects in very 

 different degrees, according as it is simply extended, or made into a helix, or forms 

 the circuit of an electro-magnet (1069.). The experiments to be adduced (1089.) 

 will still more strikingly show that the idea of momentum cannot apply. 



1078. The bright spark at the electromotor, and the shock in the arms, appeared 

 evidently to be due to one current in the long wire, divided into two parts by the 

 double channel afforded through the body and through the electromotor ; for that 

 the spark was evolved at the place of disjunction with the electromotor, not by any 

 direct action of the latter, but by a force immediately exerted in the wire of commu- 

 nication, seemed to be without doubt (IO7O.). It followed, therefore, that by using 

 a better conductor in place of the human body, the whole of this extra current might 

 be made to pass at that place ; and thus be separated from that which the electro- 

 motor could produce by its immediate action, and its direction be examined apart 

 from any interference of the original and originating current. This was found to be 

 true ; for on connecting the ends of the principal wire together by a cross wire two 

 or three feet in length, applied just where the hands had felt the shock, the whole of 

 the extra current passed by the new channel, and then no better spark than one 

 producible by a short wire was obtained on disjunction at the electromotor. 



1079. The current thus separated was examined by galvanometers and decom- 

 posing apparatus introduced into the course of this wire. I will always speak of it 

 as the current in the cross wire or wires, so that no mistake, as to its place or ori- 

 gin, may occur. In the wood-cut, Z and C represent the zinc and / \ 



copper plates of the electromotor ; G and E the cups of mercury V V 



where contact is made or broken (1052.) ; A and B the termina- 

 tions of D the long wire, the helix, or the electro-magnet, used to 

 complete the circuit ; N and P are the cross wires, which can 

 either be brought into contact at x, or else have a galvanometer 

 (1058.) or an electrolyzing apparatus (312. 316.) interposed there. 



The production of the shock from the current in the cross wire, 

 whether D was a long extended wire, or a helix, or an electro-magnet, has been 

 already described (1064. 1061. 1060.). 



1080. The spark of the cross-wire current could be produced at .2? in the following 

 manner : D was made an electro-magnet ; the metallic extremities at a; were held 

 close together, or rubbed lightly against each other, whilst contact was broken at 

 G or E. When the communication was perfect at x, little or no spark appeared at 

 G or E. When the condition of vicinity at x was favourable for the result required, 

 a bright spark would pass there at the moment of disjunction, none occurring at G 

 and E : this spark was the luminous passage of the extra current through the cross- 

 wires. When there was no contact or passage of current at x, then the spark ap- 



