292 
The following relation exists, therefore, between the electro-motive force 
applied to the plates expressed in volts and the force in grams exerted between 
the plates. 
/P 
Bie 4209S ta 
VY A 
If the plates are made of very great area, it may be assumed that the 
electrostatic field at their center is uniform provided that the plates are not 
far apart. 
In the apparatus constructed at Purdue University a cireular dise of very 
small area was cut from the center of the lower horizontal plate and this dise 
was mounted upon a float supported in a tank filled with oil in such a manner 
that its surface is horizontal and concentric with 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 dise is attracted by the upper plate and may be lifted into the 
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 be calculated 
from the above equation. If now an unknown high voltage be impressed 
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 be determined by the relation 
tlE 
E! = — 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 be considered standard 
for given dimensions of plates and further refinements are being made in 
its construction, method of reading, and calibration. 
