166 ELEMENTS OF STATIC ELECTRICITY. 



which it can be put in electric connection with bodies 

 whose potential is to be tested. 



The attracted plates P and C are really movable 

 centers of the guard plates Cr and A; and since loss of 

 charge, from radiation and otherwise, affects chiefly the 

 outer edges, the small centers are practically unaffected 

 by such loss. Hence the large discs Cr and A are 

 appropriately called guard plates. 



MODE OF USING THE ABSOLUTE ELECTEOMETEE. 

 The plates are first brought to zero potential, by put- 

 ting them, for an instant, in electric connection, by the 

 electrode -ZV, connecting with Z?, and a wire connecting 

 with A through the cover. The disc O is then brought to 

 its sighted position by the micrometer Jf, and the read- 

 ing noted. A known weight, w, is then placed upon it 

 so as to depress it below the level of the guard plate 

 A; and M is turned till C is again raised to its sighted 

 position : the reading is noted, and the weight removed. 



The Leyden jar is then charged to potential V, as 

 determined by the idiostatic gauge, and kept constant 

 by the replenisher, during the experiment. The disc 

 B is now put into connection with the outside coating 

 by the electrode N; and the micrometer M r turned till 

 the attraction of B on the disc C brings it again to its 

 sighted position. Hence the attraction of B is known to 

 be equal to the weight w. This reading being noted, B 

 is insulated, and the bodies, the difference of whose po- 

 tentials x and z is required, are successively put into 

 contact with B through N. The distances d and h 

 through which B has to be moved to bring the disc (7, 

 in each case, to its sighted position, are noted, and the 

 difference of potential can then be calculated. 



FOE CHAEGED SUEFACES. With a 



