166 ADVANCED ELECTRICITY AND MAGNETISM. 



slope at each point is equal to the intensity of the electric field 

 at that point. The height of this imagined hill at a point is called 

 the electric potential at that point. Thus the fine curved lines 

 with arrow heads in Fig. 112 represent the lines of force of the 



Fig. 112. 



electric field between two parallel oppositely charged metal 

 cylinders, the axes of the cylinders being perpendicular to the 

 plane of the paper. The lines of force are the slope lines of the 

 potential hill, and the heavy lines are the contour lines or lines 

 of equal level on the potential hill. The slope lines are every- 

 where at right angles to the contour lines. The height of the 

 potential hill at each point is expressed in volts. 



Eguipotential surfaces. Each of the contour lines in Fig. 112 

 represents a surface perpendicular to the plane of the paper, 

 the lines of force of the electric field cross these surfaces 

 everywhere at right angles, and each surface is everywhere at 

 the same potential, that is to say, the value of the potential in 

 volts or abvolts is the same at every point on each surface. Such 

 a surface is therefore called an equipotential surface. 



A clear idea of what is represented in Fig. 112 may be obtained 

 by imagining that the positively charged cylinder is at a high 



