EXAMPLES OF THE FOUR METHODS. " 241 



From all these instancesj treated by the Method of Agreement, a - 

 general law appears to result. The instances -embrace all tlie known 

 modes in whicli a body can become charged with electricity ; and in 

 all of" thi'm tlicre is tbund, as a concx)mitant or consequent, the excite- 

 ment ot* the opposite electric state in some other body or bodies. It 

 seems to follow that the two facts are invariably connected, and that 

 the excitement of electricity in any body has for one of its necessary 

 conditions the possibility of a simultaneous excitement of the opposite 

 electricity in some neighboring body. 



As the two contrary electricities can only be produced together, so 

 they can only cease together. Tliis may be sho\vn by an application 

 of the Method of Difference to the example of the Leyden jar. It 

 needs scai'cely be here remarked that in the Leyden jar, electricity 

 can be accumulated and n^taincd in considerable quantity, by the con- 

 trivance of having two conducting surfaces of equal extent, and parallel 

 to each other through the whole of that extent, with a non-conducting 

 substance such as glass between them. When one side of the jar is 

 charged positively, the other is charged negatively, and it was by virtue 

 of this fact that the Leyden jar served just now as an instance in our 

 employment 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 the 

 other. 



The law thus strongly indicated admits of corroboration by the 

 Method of Concomitant Variations. 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 elec- 

 tricity, as the opposite surface of receiving and retaining the other : 

 but in the machine, the neighboring 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 elcctrifiiMl, to the conductor itself, their lim- 

 ited 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 pos- 

 sible : and to this appears to be owing the great superiority of the 

 Leyden jar. 



A further aiul most decisive confirmation by the Method of Differ- 

 ence, is to be found in out; of Faraday's experiments in the course of 

 his researches on the subject f)f induced electricity. 



Since common or machine electricity, and voltaic electricity, may 

 be considered for the present pui-pose to be identical, Faraday wished 

 to know whether, as the prime conductor develops opposite electri- 

 city upon a conductor in its \icinity, so a voltaic cuiTent miming 

 along a wire would induce 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 cii'cumstance except the one to which 

 Hh 



