335 



Cor. 2. The electric potential throughout the interior of a hollow 

 metallic body, electrified in any way by external influence, or, if in- 

 sulated, electrified either by influence or by communication of elec- 

 tricity to it, is constant, since there is no electric force in the interior 

 in such circumstances. 



[It is easily shown by mathematical investigation, that the electric 

 force experienced by an electric point containing an infinitely small 

 quantity of electricity, when placed anywhere in the neighbourhood 

 of a hollow electrified metallic shell, gradually diminishes to nothing 

 if the electric point be moved gradually from the exterior through 

 a small aperture in the shell into the interior. Hence the one value 

 of the potential close to the surface outside, mentioned in Cor. 1, is 

 equal to the constant value throughout the interior mentioned in 

 Cor. 2.] 



8. Interpretation of measurement by electrometer. Every kind 

 of electrometer consists of a cage or case containing a moveable and a 

 fixed conductor, of which one at least is insulated and put in metallic 

 communication, by what I shall call the principal electrode passing 

 through an aperture in the case or cage, with the conductor whose 

 electricity is to be tested. In every properly constructed electrometer, 

 the electric force experienced by the moveable part in a given position 

 cannot be electrically influenced except by changing the difference 

 of potentials between the principal electrode and the uninsulated 

 conductor or conducting system in the electrometer. Even the best 

 of ordinary electrometers hitherto constructed do not fulfil this con- 

 dition, as the inner surface of the glass of which the whole or part 

 of the enclosing case is generally made, is liable to become electrified, 

 and inevitably does become so when any very high electrification is 

 designedly or accidentally introduced, even for a very short time ; 

 the consequence of which is that the moving body will generally not 

 return to its zero position when the principal electrode is perfectly 

 disinsulated. Faraday long ago showed how to obviate this radical 

 defect by coating the interior of the glass case with a fine network 

 of tinfoil ; and it seems strange that even at the present day electro- 

 meters for scientific research, as for instance for the investigation 

 of atmospheric electricity, should be constructed with so bad and 

 obvious a defect uncured by so simple and perfect a remedy. When 

 it is desired to leave the interior of the electrometer as much light 

 VOL. x. 2 B 



