296 INDUCTION. 



ferior in their capacity of becoming electrified, to the conductor itself, theii 

 limited power imposes a corresponding limit to the capacity of the con- 

 ductor for being charged. As the capacity of the neighboring body for 

 supporting the opposition increases, a higher charge becomes possible : and 

 to this appears to be owing the great superiority of the Ley den jar. 



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 re- 

 searches on the subject of Induced Electricity. 



Since common or machine electricity, and voltaic electricity, may be con- 

 sidered for the present purpose to be identical, Faraday wished to know 

 whether, as the prime conductor develops opposite electricity upon a con- 

 ductor in its vicinity, so a voltaic current running along a wire would in- 

 duce an opposite current upon another wire laid parallel to it at a short 

 distance. Now this case is similar to the cases previously examined, 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 neighboring 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 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 Leyden 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 neighboring body. Now the result of the experi- 

 ment with the second wire was, that no opposite current was produced. 

 There was an instantaneous effect at the closing and breaking of the vol- 

 taic 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 neighboring 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 Agi*eement, 

 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 excitement of the other and opposite kind : that 

 both are effects of the same cause ; that the possibility of the one is a con- 

 dition of the possibility of the other, and the quantity of the one an im- 

 passable limit to the quantity of the other. A scientific result of consider- 

 able interest in itself, and illustrating those three methods in a manner 

 both characteristic and easily intelligible.* 



§ 3. Our third example shall be extracted from Sir John Herschel's Dis- 



* This view of the necessaiy co-existence of opposite excitements involves a great extension 

 of the original doctrine of two electricities. The early theorists assumed that, when amber 

 was rubbed, the amber was made positive and the rubber negative to the same degree ; but it 

 never occurred to them to suppose that the existence of the amber charge was dependent on 

 an opposite charge in the bodies with which the amber was contiguous, while the existence 

 of the negative charge on the rubber was equally dependent on a contrary state of the sur- 

 faces that might accidentally be confronted with it; that, in fiict, in a case of electrical ex- 

 citement by friction, four charges were the minimum that could exist. But this double elec- 

 trical action is essentially implied in the explanation now universally adopted in regard to 

 the phenomena of the common electric machine. 



