ELECTROMETERS. 161 



These inaccuracies doubtless account fWcJhe slight 

 error observed in Coulomb's experiment, am^ifrnd to 

 confirm the correctness of his results by showing suf- 

 ficient cause for the error. 



ATTRACTED-DlSC ELECTROMETERS. Sir W. SnOW 



Harris was the first to construct an electrometer on the 

 attracted-disc principle. His instrument consisted of a 

 scale beam, carrying at one end a pan for the weights, 

 balanced at the other end by an insulated metal disc, sus- 

 pended horizontally over a similar fixed, insulated disc. 



An electric charge being given to the lower disc, the 

 force of attraction between it and the upper disc was 

 measured by weights placed in the scale pan. 



The rapid loss of charge, from the edge of the elec- 

 trified disc, was the chief objection to this instrument. 

 But the principle has been adopted, and the construc- 

 tion improved by Sir William Thomson, whose instru- 

 ment, shown by Fig. 52, is described as follows: 



THOMSON'S ABSOLUTE ELECTROMETER. This in- 

 strument consists of two distinct parts; one for testing 

 and maintaining a certain constant auxiliary potential 

 V, and the other for determining, in absolute measure, 

 the difference between the potentials of any two given 

 conductors. The first of these parts embraced a Ley- 

 den jar, forming the case of the instrument, an idio- 

 static gauge, -and a replenisher E. 



The Leyden jar is a glass cylinder, closed at top and 

 bottom by metal plates; and coated, inside and out, 

 with tin-foil, in which openings are left for viewing the 

 internal parts; and an uncoated surface, for insulation, 

 left at the top and bottom, between the inner coating 

 and the metal plates. 



The idiostatic gauge will be understood from Fig. 



