1 68 ELECTROMETRY. 



In certain instruments this double determination may be made 

 d priori from the form and dimensions of the conductors in presence 

 of each other. These are what are called absolute instruments. 

 Coulomb's balance and Sir William Thomson's absolute electro- 

 meter, which we shall afterwards describe, satisfy this condition ; but 

 most frequently the law of the deflection would be determined 

 experimentally, and the value of the constant by comparison with 

 an absolute electrometer. 



Let us consider, for instance, the gold-leaf electrometer with a 

 metal shade, and suppose that the shade is connected to earth- 

 that is to say, kept at zero potential. If the capacity of the system 

 formed by the rod and the leaves is .sensibly independent of the 

 angle of deflection, the potential V is simply proportional to the 

 charge, and the instrument might be graduated by varying the charge 

 in a known ratio. 



De Saussure* used two identical electrometers. One of them 

 being originally electrified, the angular deflection is observed, and it 

 is then connected with a second one, originally in the neutral state. 

 The new angle of deflection corresponds to half the original charge, 

 and, on the supposition of a constant capacity, to a potential one- 

 half as great In all cases the apparatus is thus graduated as a 

 measurer of the charges. 



We should obtain the same result more simply by connecting the 

 gold leaves with a Faraday's cylinder (15), in which a succession of 

 equal charges are introduced. 



The method used by Voltaf gave a graduation as a function of 

 the potentials, the electrometer being connected with the knob of a 

 Leyden jar charged by a variable number of sparks from the plate of 

 an electrophorus. The sparks of an electrophorus rapidly decrease 

 from the outset, but they soon become sensibly equal. On the other 

 hand, the Leyden jar, in consequence of its great capacity, may be 

 considered as always taking from the plate the same fraction of its 

 charge, and consequently as acquiring a potential proportional to the 

 number of sparks. Volta observed in this way that up to 20 or 25 

 degrees the divergence of the leaves of his electrometer was propor- 

 tional to the number of sparks. 



In order to determine the constant of the instrument, Volta 

 suspended to the beam of a balance a metal disc, which he kept at a 



* DE SAUSSURE. Voyage dans les Alpcs, Vol. n., p. 165-175. Neufchatel, 

 1804. 



t VOLTA. Delia Meteorologia Elettrica. Second Letter. 



