ADVANCED ELECTRICITY AND MAGNETISM. 



The behavior of the tooth pick shows that the whole region 

 surrounding the charged metal balls in Fig. 89 is in a peculiar 

 condition, and this region is called an electric field. The direction 

 of the electric field at each point is indicated by the suspended 

 tooth pick, and lines drawn through the electric field so as to be 

 at each point in the direction of the field at that point are called 

 the lines of force of the electric field. 



Fig. 90. 



Figure 90 shows the lines of force of the electric field between 

 two charged flat metal plates. The short black lines represent 

 the positions of a suspended tooth pick. The lines of force in 

 the region between the plates are straight lines, and the electric 

 field is said to be uniform. The electric field between the charged 

 balls in Fig. 89 is non-uniform. 



84. Intensity of electric field. It is permissible to adopt 

 arbitrarily the ratio E/x as a measure of the intensity of the 

 uniform electric field between parallel flat metal plates, where E 

 is the electromotive force between the plates and x is the distance 

 between the plates. It is desirable, however, to base the defini- 

 tion of electric field intensity upon some observable effect of the 

 field as in the following discussion. 



Two metal plates A and B, Fig. 91, are connected to an 

 electric machine giving a high electromotive force E (the electric 



