200 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



mains unaltered, so that the electromotive force between the 

 plates may be increased to several thousands of volts. Thus, the 

 very fine lines in Figs. 124 and 125 represent the electric field 



copper 



Fig. 124. 



zinc 



copper 4 



+ 

 4 

 4 



zinc 



Fie. 125. 



between the copper and zinc plates when they are close together 

 and after they have been separated to a considerable distance. 



In order to produce an intense electric field by separating two 

 metal plates, the plates must be very flat, and they must be 

 separated in such a way as to avoid a lingering contact between 

 them. When both of the substances are good insulators, how- 

 ever, they always retain their charges (one positive and the other 

 negative) when they are moved apart, and the intervening region 

 becomes an intense electric field. This phenomenon is called 

 charging by contact and separation. In order to bring sealing 

 wax and fur, or glass and silk into intimate contact, vigorous 

 rubbing is necessary, and therefore charging by contact and 

 separation is frequently spoken of as charging by friction. 



To understand the phenomenon of charging by contact and 

 separation it is important to keep in mind that the charging is 

 done by contact (no one knows exactly how), and that the crea- 

 tion of an intense electrical field throughout a large region is ac- 

 complished by separation. In this case the electrical field is 

 wound up,* as it were, by pulling the charged surfaces apart, and 

 the work done in pulling the charged surfaces apart against their 

 force of attraction (tension of the lines of force) is the work that 

 goes to establish the field in the larger and', larger region between 

 the receding surfaces. 



* In the sense of winding up a spring so as to put it under stress. 



