ELECTRICAL APPARATUS. 157 



modifications in the details of the arrangements employed, but so- 

 far no arrangement based upon them has been made as efficient 

 for yielding a great amount as well as a high degree of electrifica- 

 tion, or, as it is commonly expressed, a large quantity of electricity 

 of high potential, as those founded upon friction or on electrical 

 induction ; on the other hand, they are in general more efficient 

 than the latter for the purpose of maintaining the process of elec- 

 trical interchange on which electro-dynamical phenomena depend. 



Electroscopes and Electrometers. 



Nearly all the instruments hitherto devised for detecting electri- 

 fication depend upon the principle that the direction of electric 

 force at any point is the direction of most rapid variation of 

 potential, whence it follows that if an electrified body is at an 

 equal distance from two unequally electrified bodies it will tend to 

 move away from the one whose electrical condition (potential) 

 differs least from its own, and towards the one whose electrical 

 condition differs most from its own : while an electrified body, at 

 unequal distances /rom two equally electrified bodies, tends to 

 move towards the one to which it is nearest. Bennet's Gold- 

 leaf Electroscope (1787) and Bohnenberger's Dry-pile Electro- 

 scope are familiar examples of instruments founded upon this 

 principle. It has been recently shown by G. Lippmann (1873) 

 that the alteration of the tension of the surface separating 

 mercury from dilute sulphuric acid, which accompanies changes 

 in the difference of potential between the metal and the acid, 

 can be made to indicate degrees of electrification at least as 

 slight as those which can be detected by instruments that act 

 by electrical attraction and repulsion. 



In Electrometers, the thing that is measured is the difference 

 of potential between two bodies, one of which is often the earth ; 

 and the fundamental principle upon which all (except Lippmann's 

 capillary electrometer) depend is that the electric force at any 

 point is equal to the rate of variation of potential in space at that 



