ELECTROMETERS. m 165 



The second part of the electrometer consists of the 

 apparatus for expressing differences of potential, be- 

 tween conductors, in absolute measure. The metal 

 plate A, called the guard plate, has, at its center, a 

 circular opening about If inches in diameter, to which 

 is fitted the disc (7; which just fills it without touching 

 the edges; and is made of thin aluminium, flat and 

 smooth on its under side, but strengthened by a rim, 

 and radial arms, on its upper side. It is supported by 

 three light steel springs, shaped somewhat like tuning- 

 forks, and placed horizontally, at equal distances apart ; 

 one of which is shown at S. The lower end of each is 

 attached to the center of (7, and the upper end to a 

 brass socket, which is cemented to the lower end of a 

 glass rod, shown at /; which insulates it from the 

 metal rod above ; to the lower end of which the glass 

 rod is attached. And the metal rod is moved vertically 

 in guides by the micrometer screw M ; the movements 

 being registered by the scale 6r, and the graduated disc 

 D. 



To the center of the disc is attached a fine hair: 

 in front of which a lens, II, is so placed as to form, at 

 its conjugate focus, near the surface of the jar, an 

 image of the hair ; which may be viewed through the 

 eye-piece at L. This image is seen exactly between 

 the points of two screws K, when the lower surfaces 

 of the disc (7, and guard plate A, are in the same plane : 

 which is called the sighted position. 



On a support below A, is the metal disc B, known 

 as the attracting disc ; insulated from the jar, and mov- 

 able vertically by the micrometer screw M' ; the move- 

 ments being registered by the scale R, arid the grad- 

 uated disc T. It is connected with the electrode -ZV", by 



