EXAMPLES OF THE FOUR METHODS. 489 



upon a conductor in its vicinity, so a voltaic current 

 running along a wire would induce an opposite current 

 upon another wire laid parallel to it at a short dis- 

 tance. Now this case is similar to the cases previously 

 examined, in every circumstance except the one to 

 which we have ascrihed 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 neighbouring body ; and 

 the interpretation of this, in the language of cause 

 and effect, is, that all causes which can excite the one 

 kind of electricity, have the property of simultaneously 

 exciting an equal amount of the other. But in Fara- 

 day's experiment this indispensable opposition exists 

 within the wire itself. From the nature of a voltaic 

 charge, the two opposite currents necessary to the 

 existence 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 Ley den jar must have a positive 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 Faraday's experiment with the second 

 wire, was that no opposite 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 Ley den 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. 



