324 Royal Society :— 



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 

 as possible, and to allow it to be clearly seen from any external 

 position with as little embarrassment as possible, a cage made like a 

 bird's cage, with an extremely fine wire on a metal frame, inside the 

 glass shade used to protect the instrument from currents of air, &c, 

 may be substituted with advantage for the tinfoil network lining of 

 the glass. It appears therefore that a properly constructed electro- 

 meter is an instrument for measuring, by means of the motions of a 

 moveable conductor, the difference of potentials of two conducting 

 systems insulated from one another, of one of which the case or cage 

 of the apparatus forms part. It may be remarked in passing, that it 

 is sometimes convenient in special researches to insulate the case or 

 cage of the apparatus, and allow it to acquire a potential differing 

 from that of the earth, and that then, as always, the subject of 

 measurement is the difference of potentials between the principal 

 electrode and the case or cage, while in the ordinary use of the 

 instrument the potential of the latter is the same as that of the 

 earth. Hence we may regard the electrometer merely as an instru- 

 ment for measuring differences of potential between two conducting 

 systems mutually insulated ; and the object to be aimed at in per- 

 fecting any kind of electrometer (more or less sensitive as it may be, 

 according to the subjects of investigation for which it is to be used), 

 is, that accurate evaluations in absolute measure, of differences of 

 potential, may be immediately derivable from its indications. 



9. Relation between electrostatic force and variation of electric 

 potential. — § 7, otherwise stated, is equivalent to this : — The ave- 

 rage component electrostatic force in the straight line of air between 

 two points in the neighbourhood of an electrified body is equal to 

 their difference of potentials divided by their distance. In other 

 words, the rate of variation of electric potential per unit of length in 

 any direction, is equal to the component of the electrostatic force in 



