ELECTROMETERS. 



2 4' 



(Fig. 7), only that it is inverted. This plate is kept 

 electrically connected with the interior coating of the jar ; 

 and the jar is kept at a high potential. There is a hori- 

 zontal brass plate A, which is borne on a glass column at its 

 middle, and insulated from everything but the spiral spring 

 r. This spiral spring is connected to an electrode which 

 passes, insulated, out through the top of the case of the 

 instrument, under the " umbrella " D ; and by means of this 

 electrode the plate A may be connected with any conductor 

 whose potential it is desired to measure. The plate A can 



FIG. 11. 



be raised and lowered. For this purpose the glass column 

 by which it is suspended is attached to a hollow nut m t 

 which works up and down on a vertical micrometer screw 

 turned by the milled head c. A vertical scale [not shown 

 in the diagram] and the horizontal graduated circle con- 

 nected with c, serve to read off changes of distance of A 

 relatively to the attracting plate G ; p is a mass of pumice, 

 on which drops of the strongest sulphuric acid are poured, 

 for the purpose of keeping the interior of the case dry. 



The portable electrometer is used in a way very similar 

 to that in which the absolute electrometer is used. The 



