﻿216 M. W. von Bezold's Investigations on the Eiectrophorus* 



used in two ways ; either first the body to be tested is touched 

 with it and then the electroscope, or when first in contact it is mo- 

 mentarily connected with the earth. It may happen in the first 

 case that the disk when raised has no electricity, even when at 

 the place touched there was some, or quantities otherwise distri- 

 buted have really exerted a decomposing action at the place in 

 question. Here the question is solely and simply to know 

 whether the force at work between the insulator and the proof- 

 plane is strong enough to permit electricity to pass between the 

 two. Only when this is the case can an electroscopic indica- 

 tion be obtained, which, however, gives nothing more than the 

 direction of the force which acted at right angles to the proof- 

 plane. 



The other mode of testing with the aid of the disk is princi- 

 pally applicable when the forces at work are too small to permit 

 a passage between the body and the disk. The repelled elec- 

 tricity is then removed by the earth- connexion, and only the 

 attracted remains, and then gives a deflection on the electroscope. 

 If, however, the action upon the disk was too powerful, the thin 

 layer between the body to be tested and the proof-plane is tra- 

 versed by sparks, and afterwards no indications at all, or at most 

 too feeble, are obtained. Of course in this case, too, we only 

 acquire information as to the direction of the force, without any 

 indication as to its locality. Conclusions as to the magnitude 

 of this force are entirely untrustworthy ; for we can never be cer- 

 tain whether in the present case the thin layer of air has acted 

 as a complete insulator, or whether it has been traversed by 

 sparks. 



But, apart from this uncertainty, testing with the earth-con- 

 nected proof-plane is accompanied by another great evil. On 

 such a disk the value of the potential function is always =0. If 

 now electricity is only diffused on non-conductors (that is, bound 

 to solid points), by approaching such a disk, not the distribution 

 of the force, but its direction is everywhere altered. On the 

 contrary, if electricity is also distributed upon conductors, the 

 arrangement of these quantities of electricity is materially altered 

 by the approach of the proof-plane. Hence all the data which 

 are obtainable, even in the most favourable cases, with the aid of 

 such earth-connected planes only refer to the greater or less mo- 

 dification of the system of forces by the presence of the plane. 

 More favourable are the circumstances when the sensitive mixed 

 powder is used as the test. 



It is true that primarily only the direction of the component 

 normal to the surface is given by its means ; that is, we know 

 that at the places covered with the yellow sulphur negative 

 electricity is attracted towards the surface, and positive at those 



