ELEGTEOMEIERS. 239 



have to describe immediately, the portable electrometer, 

 but turned upside down, and used idiostatically. Above 

 this horizontal plate (the circular plate at the top of Fig. 

 6), connected with the suspension plate of the needle, there 

 is an arrangement resembling somewhat a cart-weighing 

 machine. G is a brass plate, with a square hole cut in it. 

 The hole is over the middle of the circular plate, connected 

 with the suspension plate, p is a square of light sheet alumi- 

 nium ; it is connected with the long arm h, the square p 

 and the long arm h being in one piece. This lever is 

 carried on a tightly-stretched platinum wire /, round 

 which as a fulcrum it is perfectly free to move ; and tor- 

 sion is given to the wire/, so as to tend to raise p upwards 

 and to depress the arm h. But when the little square p is 

 attracted by the plate connected with the needle and jar, the 



FIQ. 7. 



attraction draws p downward against the torsion of the 

 wire / to a fixed position. 



The extremity of h is cut out into a little fork, across 

 which a fine black hair is stretched, and behind the hair 

 there stands a minute white porcelain plate with two round 

 black dots upon it. The arrangement is looked at by the 

 magnifying lens I, and the plate p is in the proper posi- 

 tion when the hair is midway between the two dots. The 

 Leyden jar of the electrometer is then charged up until 

 this is the case. 



The little electric machine for increasing or diminishing 

 the charge of the jar is one of a kind first invented by 

 Nicholson, known by the name of Nicholson's Revolving 

 Doubler, and used by Yolta in some of his researches. 

 Recently several such machines have been constructed. 

 That of Holtz ; now well known, makes use of the " com- 



