456 INDUCTION. 



A further and most decisive confirmation by the Method 

 of Difference, is to be found in one of Faraday's experiments 

 in the course of his researches on the subject of induced 

 electricity. 



Since common or machine electricity, and voltaic electri- 

 city, may be considered for the present purpose to be identical, 

 Faraday wished to know whether, as the prime conductor de- 

 velopes opposite electricity upon a conductor in its vicinity, so 

 a voltaic current running along a wire would induce an oppo- 

 site current upon another wire laid parallel to it at a short 

 distance. Now this case is similar to the cases previously ex- 

 amined, in every circumstance except the one to which we 

 have ascribed the effect. We found in the former instances 

 that whenever electricity of one kind was excited in one body, 

 electricity of the opposite kind must be excited in a neigh- 

 bouring body. But in Faraday's experiment this indispensable 

 opposition exists within the wire itself. From the nature of a 

 voltaic charge, the two opposite currents necessary to the ex- 

 istence of each other are both accommodated in one wire ; and 

 there is no need of another wire placed beside it to contain one 

 of them, in the same way as the Leyden jar must have a posi- 

 tive and a negative surface. The exciting cause can and does 

 produce all the effect which its laws require, independently of 

 any electric excitement of a neighbouring body. Now the 

 result of the experiment with the second wire was, that no op- 

 posite current was produced. There was an instantaneous 

 effect at the closing and breaking of the voltaic circuit ; electric 

 inductions appeared when the two wires were moved to and 

 from one another ; but these are phenomena of a different class. 

 There was no induced electricity in the sense in which this is 

 predicated of the Leyden jar; there was no sustained current 

 running up the one wire while an opposite current ran down 

 the neighbouring wire ; and this alone would have been a true 

 parallel case to the other. 



It thus appears by the combined evidence of the Method of 

 Agreement, the Method of Concomitant Variations, and the 

 most rigorous form of the Method of Difference, that neither 

 of the two kinds of electricity can be excited without an equal 



