ELECTRIC CHARGE. THE CONDENSER. 183 



Attraction for a given electromotive force. The charge q in 

 the above discussion is equal to the capacity of the condenser 

 times the electromotive force between the plates, according to 



equation (61), that is, 



I kaE 



according to equations (61) and (62). Substituting this value of 



q in equation (73), we have 



I kaE? 

 F= ^S ~f (74) 



in which F is the force in joule-units with which two metal plates 

 attract each other in air or oil, E is the electromotive force be- 

 tween the plates, a is the area of each plate, x is the distance be- 

 tween the plates in centimeters, and B is equal to 1.131 X io 13 . 

 It is worthy of note that the force of attraction of parallel plates 

 is inversely proportional to the square of the distance between 

 them and directly proportional to the inductivity of the dielectric 

 for given electromotive force. 



102. The absolute electrometer is an arrangement for determining 

 the value of an electromotive force by measuring the force of at- 

 traction of parallel metal plates. BALANCE 



The value of the electromotive 

 force is calculated from equa- 

 tion (74) when k (k = i, for 

 air), a, x, and B are known, r~ 



I /* 



and F is observed in joule- __^ _ 

 units. Figure 112 shows the 

 essential features of the abso- 



lute electrometer. A portion of area a of the upper plate is hung 

 from one end of a balance beam so that the force with which this 

 portion is attracted by the lower plate may be counterpoised by 

 weights placed upon the scale pan and thus determined. The 

 stationary portion gg of the upper plate completely surrounds 

 the portion a and is called the guard ring. Equation (74) is 



