174 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



F-fq (66) 



in which f is the proportionality factor. This quantity f is 

 called the intensity of the electric field at the point. 



Comparing equations (i) and (66), it is evident that the intensity 

 of the electric field between the parallel plates AB in Fig. 106 is 



(67) 



That is, the intensity of the electric field between the plates is 

 equal to the electromotive force between the plates divided by 

 the distance between the plates. When E is expressed in volts 

 and x in centimeters, the field intensity is expressed in volts per 

 centimeter. The intensity of an electric field may also be expressed 

 in abvolts per centimeter. 



Direction of electric field at a point. The direction of an elec- 

 tric field at a point is the direction in which the field pulls on a 

 positively charged body placed at that point. A line of force in 

 an electric field is a line drawn so as to be in the direction of the 

 field at each point. 



95. Dielectric strength. The ability of a dielectric to with- 

 stand electrical stress or electric field is called the strength of the 

 dielectric. The strength of a dielectric is measured by the inten- 

 sity of the electric field in volts per centimeter which is just suffi- 



Flg. 108. 



Fig. 109. 



