292 



The following relation exists, therefore, between the electro-motive force 

 applied to the plates expressed in volts and the force In grams exerted bel 

 the pla tes. 



E = 47098 t 7 - 



*/ A 



If the plates are marie of very great area, it may be assumed thai the 

 electrostatic field at their center is uniform provided thai the plates are uoi 



far apart. 



In the apparatus constructed at Purdue University a circular disc of very 

 small area was cut from the center of the lower horizontal plate and this disc 

 was mounted upon a float supported in a tank filled with oil in such a manner 

 that its surface is horizontal and concentric Avith the stationary plate but 

 with its plane a small fraction of an inch below that of the stationary plate. 



When an electromotive force is impressed upon the two stationary plates 

 the movable disc is attracted by the upper plate and may lie lifted into tin- 

 plane of the lower plate by raising the voltage to the proper value. This 

 condition can be readily detected by means of a telescope sighted along the 

 surface of the lower stationary plate. 



With the plates very near together, and a voltage sufficiently low to be 

 readily standardized, the force necessary to raise the disc may he calculated 

 from the above equation. If now an unknown high voltage be impn 

 upon the plates which have in the meantime been sufficiently separated to 

 bring again the disc into alignment with the lower plate, the force will of course 

 be the same as before and the new voltage may lie determined by the relation 

 t'E 



E 1 = the voltages being directly proportional to the distances between 



t 

 plates. 



Such a voltmeter has been constructed and the ratio of impressed voltages 

 to distance between plates required for a balance has been found to follow 

 surprisingly close to a straight-line law when a previously determined and 

 constant value of force is used. Further studies are now being made to 

 determine the range within which this apparatus may lie considered standard 

 for given dimensions of plates and further refinements are being made in 

 its construction, method of reading, and calibration. 



