MEASUREMENT OF POTENTIAL DIFFERENCE 245 



weighed by a calibrated spring balance. The guard ring and at- 

 tracted disc are supported from the sides of a large glass jar which 

 serves as a case for the instrument; the disc is made of aluminum 

 and is carried by three small springs, having the shape of coach 

 springs. These are supported from an insulating rod, which by 

 the use of a micrometer screw, B, can be raised or lowered through 

 known amounts. In order that the attracted disc may always be 

 returned to its proper zero position, with its lower surface 

 coplanar with that of the guard ring, a sighting arrangement is 

 provided. A fine hair is stretched between two small pillars at 

 the center .of the disc; this hair is at the focus of a lens which 

 forms an image between the points of two small screws carried by 

 the guard ring. The image of the hair and the points of the 

 screws are viewed through a lens. 

 When the disc is in its zero posi- 

 tion the hair appears to bisect 

 the distance between the points. 

 The attracted disc is shielded 



e ,. v Attracting Plate* 



from extraneous action by the ,-, 10 _ , . . 



FIG. 135. Pertaining to attracted- 

 removable box C. The attract- disc electrometer. 



ing plate is carried on an insulat- 



ing glass rod D and can be moved vertically through known 

 amounts by means of the micrometer screw E. T is the well- 

 insulated terminal connecting with the attracting plate. Con- 

 nection between the guard ring and the attracted disc is made by 

 a flexible wire. 



The relation between the difference of potential of the plates 

 and the force of attraction may be established thus: 



Referring to Fig. 135, the attracted plate has an area of A sq. 

 cm. and is distant S cm. from the attracting plate. Vz and Vi 

 are the potentials of the two plates. The arrangement forms an 

 electrical condenser and if S is small compared with the size of the 

 plates, the capacity will be 



The energy necessary to raise one plate to the potential V\ 

 and the other to the potential Vz will be 



E = 



