﻿M. W. von Bezold's Investigations on the Electrophorus. 215 



once occured to me to use this mixture in testing the electro- 

 phorus-cake. I was thereby convinced, not only that this powder 

 furnishes an excellent means of investigating the working of 

 the apparatus, but also that this working is far from being so 

 completely understood as is generally assumed. 



The only theory of the electrophorus corresponding to the 

 present state of science is that of Riess* ; and it is this which we 

 find in all the text-books of physics. The essential part of this 

 theory is that during the rubbing of the cake three layers are 

 formed — -two [of the same kind on the two surfaces, and one 

 electrically opposite in the interior. Of these, one is supposed to 

 pass to the form, so that only the two opposite electricities re- 

 main upon the cake ; by their cooperation all the phenomena may 

 be explained on known laws. 



This theory contains two very doubtful points. First, the 

 experiment upon which Riess bases his assumption of three layers 

 may just as well be explained in a different and, indeed, simpler 

 manner. Secondly, it is not very easy to understand why elec- 

 tricity should pass from the cake to the form, and not between 

 the cover and the cake. 



In the sequel it will first of all be shown that the first-men- 

 tioned experiment must indeed be differently explained. A new 

 theory, prompted by the experiments, will be proposed, by which 

 the second point will also be readily explained. 



I must, however, first of all briefly discuss the methods which 

 may be used in such investigations, in order to explain the signi- 

 ficance of the powder mixture for this purpose. 



§ 1. To investigate the deportment of the individual parts of 

 an electrified insulator two means have heretofore been principally 

 in use. Either the body has been laid directly upon an electro- 

 scope, or, when this was not possible, a proof-plane was used for 

 transference. The results thus obtained must be used with great 

 caution if they are not to lead to false conclusions. 



Assuming that, after having laid an electrified body (for in- 

 stance, an electrophorus-cake) on the cap of an electroscope, a 

 positive deflection has been obtained, we can by no means con- 

 clude that there is positive electricity at the place in the body 

 investigated. Such a deflection only teaches that at the place 

 in question negative electricity is attracted and positive repelled. 

 If the divergence continues even after the removal of the body, 

 either positive electricity has passed to the electroscope or nega- 

 tive to the body. 



Hence by the electroscope we only ascertain the direction of 

 the force-components acting at the point in question. The use 

 of the proof-plane may lead to still greater errors. It may be 

 * Die Lehre von der Reibungselektricitiit, vol. i. pp. 291-305. 



