488 INDUCTION. 



away any electricity unless an equal quantity be 

 allowed to pass from the negative side : if one coating 

 be perfectly insulated, the charge is safe. The dissi- 

 pation of one must proceed pari passu with the other. 

 The law thus strongly indicated admits of cor- 

 roboration by the Method of Concomitant Varia- 

 tions. The Leyden jar is capable of receiving a 

 much higher charge than can ordinarily be given to 

 the conductor of an electrical machine. Now in the 

 case of the Leyden jar, the metallic surface which 

 receives the induced electricity is a conductor exactly 

 similar to that which receives the primary charge, and 

 is therefore as susceptible of receiving and retaining 

 the one electricity, as the opposite surface of receiving 

 and retaining the other : but in the machine, the 

 neighbouring body which is to be oppositely electrified 

 is the surrounding atmosphere, or any body casually 

 brought near to the conductor : and as these are 

 generally much inferior in their capacity of becoming 

 electrified, to the conductor itself, their limited power 

 imposes a corresponding limit to the capacity of the 

 conductor for being charged. As the capacity of the 

 neighbouring body for supporting the opposition 

 increases, a higher charge becomes possible: and to 

 this appears to be owing the great superiority of the 

 Leyden jar. 



A further and most decisive confirmation by the 

 Method of Difference, is to be found in one of Fara- 

 day's experiments in the course of his researches on 

 the subject of induced electricity. 



Since common or machine electricity, and voltaic 

 electricity, may be considered for the present purpose 

 to be identical, Faraday wished to know whether, as 

 the prime conductor developes opposite electricity 



