270 



INDUCTION. 



magnet, in neither of which it is 

 possible to produce one kind of elec- 

 tricity by itself, or to charge one pole 

 without charging an opposite pole with 

 the contrary electricity at the same 

 time. We cannot have a magnet 

 with one pole : if we break a natural 

 loadstone into a thousand pieces, each 

 piece will have its two oppositely elec- 

 trified poles complete within itself. In 

 the voltaic circuit, again, we cannot 

 have one current without its opposite. 

 In the ordinary electric machine, the 

 glass cylinder or plate, and the rubber, 

 acquire opposite electricities. 



From all these instances, treated 

 by the Method of Agreement, a gene- 

 ral law appears to result. The in- 

 stances embrace all the known modes 

 in which a body can become charged 

 with electricity ; and in all of them 

 there is found, as a concomitant or 

 consequent, the excitement of the 

 opposite electric state in some other 

 body or bodies. It seems to follow 

 that the two facts are invariably con- 

 nected, and that the excitement of 

 electricity in any body has for one of 

 its necessar}' conditions the possibility 

 of a simultaneous excitement of the 

 opposite electricity in some neigh- 

 bouring body. 



As the two contrary electricities 

 can only be produced together, so 

 they can only cease together. This 

 may be shown by an application of 

 the Method of Difference to the ex- 

 ample of the Leyden jar. It needs 

 scarcely be here remarked that in 

 the Leyden jar electricity can be ac- 

 cumulated and retained in consider- 

 able quantity, by the contrivance of 

 having two conducting surfaces of 

 equal extent, and parallel to each 

 other through the whole of that ex- 

 tent, with a non-conducting substance 

 such as glass between them. When 

 one side of the jar is charged posi- 

 tively, the other is charged nega- 

 tively, and it was by virtue of this 

 ^ fact that the Leyden jar served just 

 now as an instance in our employ- 

 ment of the Method of Agreement. 

 Now it is impossible to discharge one 



of the coatings unless the other can 

 be discharged at the same time. A 

 conductor held to the positive side 

 cannot convey 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 dissipation 

 of one must proceed pari passu with 

 that of the other. 



The law thus strongly indicated ad- 

 mits of corroboration by the Method 

 of Concomitant Variations. The Ley- 

 den 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 re- 

 ceiving and retaining the one elec- 

 tricity as the opposite surface of re- 

 ceiving and retaining the other ; but 

 in the machine, the neighbouring 

 body which is to be oppositely elec- 

 trified is the surrounding atmosphere, 

 or any body casually brought near to 

 the conductor ; and as these are gene- 

 rally much inferior in their capacity 

 of becoming electrified to the conduc- 

 tor 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 confir- 

 mation tby 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 electri- 

 city and voltaic electricity may be 

 considered for the present purpose to 

 be identical, Faraday wished to know 

 whether, as the prime conductor de- 

 velops opposite electricity upon a con- 

 ductor in its vicinity, so a vpltaiq 



