GOLD LEAF ELECTROSCOPE. 



I6 7 



in the interior of the shade. This forms then, in reference to the 

 leaves, a conducting surface at constant potential, which protects the 

 gold leaves against the action of any external electrified body (57). 



Let us now suppose the leaves electrified; let V be their potential, 

 V that of the shade and terminals. The charge of the leaves is 

 proportional to the product of their capacity, which varies generally 

 with the angle of deflection, into the difference of potential V - V , 

 and is of the same sign as this difference. 



The divergence of the leaves, which is due to their reciprocal 

 repulsion, and to the attraction of each of them by the adjacent 

 terminal, only depends on the charge or on the difference of 

 potential ; it might then serve to measure either of these magni- 

 tudes. This would not be the same if the sides of the shade 



Fig. 149. 



electrified partially were not at a constant potential ; the terminals, 

 being insulated, would have a charge of their own. 



The advantage of a conducting cage is then manifest. Neverthe- 

 less, the projecting portion of the insulating rod might undergo an 

 extraneous influence. It would be sometimes advantageous to get 

 rid even of this source of error, by adopting Faraday's plan, and 

 completely surround the apparatus and the body which is to act, 

 by a metal cage in contact with the earth. 



798. GRADUATION OF ELECTROMETERS. It may be proposed to 

 determine the law of the deflection as a function of the charge, or of 

 the potential and the constant of the instrument that is to say, the 

 charge or the potential which corresponds to a given deflection. 



