ELECTRIC CHARGE. THE CONDENSER. 185 



electromotive force which is to be measured is connected to the 

 two plates and the scale is numbered so as to indicate the value 

 of the electromotive force directly. 



The fixed and movable plates of the electrostatic voltmeter are 

 usually arranged as shown in Fig. 113*7, in which FF and 

 F F are the fixed plates, and MM is the movable plate. Figure 

 113^ shows an electrostatic voltmeter designed by Lord Kelvin 

 for measuring electromotive forces ranging from 80 to 140 volts. 

 The movable plate in this instrument consists of a large number 

 of vanes which are drawn into the spaces between a large num- 

 ber of stationary plates essentially as in Fig. 1 13*2. 



104. Energy and tension of the electric field in air. Consider 

 the charged metal plates A A and BB in Fig. 107 ; the capac- 

 ity of the plates considered as a condenser is 



c- l a 



~B x 



according to equation (62), where B = 1. 1 3 1 x io 13 . The energy 

 of the charged condenser A ABB, Fig. 107, is 



according to equation (65^). Therefore, using \IB-a\x for 

 we have 



(0 



This energy of the charged condenser resides in the region be- 

 tween the plates, that is, in the electric field. The volume of this 

 region is ax. Therefore, dividing both members of equation (i) 

 by ax, we find 



Energy of an electric field in ) I - 2 / \ 



joules per cubic centimeter j %B ' 



in which f, which is written for Ejx, is the intensity of the 

 electric field between the plates in volts per centimeter. 



The force of attraction of the two metal plates A A and BB, 

 Fig. 107, is given by equation (74), and this force is transmitted 



HEHTY OF ELECTRICAL LABORATORY, 1 



