CLASSIFICATION OF ELECTROMETERS. 169 



constant distance from a horizontal conducting plane in connection 

 with the earth. He estimated in weight the attraction exerted at this 

 fixed distance between the electrified disc and the plane, and, on the 

 other hand, observed the deflection of the electrometer put in con- 

 nection with the disc and with the knob of a Leyden jar. 



These various methods are only approximate ; they might be 

 made more exact, but they involve numerous sources of error, 

 arising in particular from the loss by the air and by the supports. 

 The graduation may now be made in a far simpler and more accurate 

 method, by means of voltaic elements Daniell's, for instance well 

 insulated and arranged in series. The difference of potential of the 

 two poles is proportional to the number of elements, and we may 

 determine the absolute value of this difference by means of an 

 absolute electrometer. 



We may, however, add that such instruments as we have de- 

 scribed are most frequently used as simple electroscopes, to ascertain 

 if a body is electrified, and what is the sign of its electrification. 



799. CLASSIFICATION OF ELECTROMETERS. A great number of 

 different forms of electrometers have been devised.* We shall 

 restrict ourselves to a few only among them those which lend 

 themselves best to accurate measurements, and of which a natural 

 selection has brought into almost exclusive use. 



Sir W. Thomsonf divides the various electrometers in three 

 principal groups. 



I. This group comprises, for instance, the electrometer of Cavallo 

 and of Saussure, the straw electrometer of Volta, Bennet's gold-leaf 

 electroscope, Coulomb's balance, Peltier's electrometer, Riess' sine 

 electrometer, Dellmann's electrometer, etc. 



II. Symmetrical Electrometers. In these a movable part, such as 

 a needle or a gold-leaf, is arranged between two systems of con- 

 ductors arranged symmetrically, but insulated from each other, and 

 at different potentials. The deflection of the mobile part depends 

 on its own potential, and on the difference of potential of two fixed 

 conductors. Such are the electrometers of Bohnenberger, of Hankel, 

 and the quadrant electrometer of Thomson. 



III. Balance Electrometers. In this category are included all 

 the instruments in which the attraction between two conductors is 

 counterbalanced by a weight. The first idea of instruments of this 



* MASCART. Traite d" 1 Electricite Statique, Vol. I., pp. 344 and 499. 1876. 

 t SIR W. THOMSON. Report on Electrometers and Electrostatic Measure- 

 ments : British Association Report for 1867. Reprint of Papers, p. 260. 



