152 ADVANCED ELECTRICITY AND MAGNETISM. 



square centimeters, x in centimeters and E in volts; k being 

 the inductivity of the fluid dielectric, and B being equal to 

 1.131 X io 13 . 



It is to be noted that the force of attraction of two parallel 

 plates is inversely proportional to the square of the distance 

 between the plates for a given value of E. 



The absolute electrometer. If an electromotive force E 

 to be measured is connected to two parallel metal plates in air, 

 the value of E in volts can be calculated with the help of 

 equation (7), above, if a and x are known and if the force F is 

 measured ; k being equal to unity and B being a known con- 

 stant. A device arranged for measuring an electromotive force 

 in this way is called an absolute electrometer because only mechan- 

 ical and geometrical measurements need be made. 



balance beam 



B B 



Fig. 99. 



Figure 99 shows the essential features of an absolute electrom- 

 eter. The two attracting metal plates A and B are hori- 

 zontal, and the upper plate is in two parts, namely, a central 

 circular part of area a which is hung from one end of a balanced 

 beam, and a surrounding part gg which is supported rigidly and 

 which is called a guard ring. The force with which the suspended 

 portion a is attracted by the lower plate is measured by placing 

 weights on the pan pp. 



Equation (7) is accurately true only when the attracting plates 

 are large as compared with the distance x\ but equation (7) is 

 accurately true for the central portion a of the upper plate in 

 Fig. 99. 



