in Electric Induction. 



407 



two discs, but also upon the form and dimensions of the inter- 

 mediate plate itself. By changing the dimensions of the interme- 

 diate plate, its action upon the induced disc will be changed in a 

 twofold manner; the two electricities upon it obtain another 

 distribution and quantity, and the distances at which these strata 

 act upon the induced disc are changed. We can determine two 

 strata, such that their total action upon the induced disc shall 

 be inappreciable, or like in kind to the action of the directly 

 charged disc, or lastly, unlike to this action. The question, 

 therefore, in what manner will a conducting intermediate plate, 

 change the induced electricity which a charged disc produces on 

 a neutral one, is completely indeterminate. The question will be 

 a determinate one if the conducting plate is not insulated, for then 

 to the action of a charged disc that of an unlike stratum of elec- 

 tricity is added, which necessarily opposes that action. But to 

 attempt to compare the action of conducting and non-conducting 

 intermediate plates under these circumstances, would be to com- 

 mit an experimental mistake not less grave, but less excusable, 

 than the mistake of those earliest electricians, who, without in-? 

 sulating the conductors, compared conductors with non-con- 

 ductors with respect to their capacities of becoming electric by 

 friction. 



The simplest form was now given to the induction experiment, 

 and the action of intermediate non-conducting plates examined. 

 Two metallic spheres, 0-87 inch in diameter, were fixed to thin 

 glass rods, and so placed that their central line lay horizontal, 

 and the distance between their nearest points amounted to a little 

 more than 0*6 inch. One sphere was charged with electricity, 

 the other was connected, by means of a wire 18 inches long, with 

 a dry-pile electroscope, and observations were conducted in the 

 manner already described. Every intermediate plate was care- 

 fully placed in the space between the two spheres, so that the 

 line joining the centres of the spheres passed perpendicularly 

 through the centre of the plate. 



* A disc of paraffine, of similar dimensions to 0, gave a decided decrease 

 of the induced electricity ; I have not recorded the observations with this 

 disc, because it was considerably thinner in the middle than at the edges. 



