MAGNETISM AND ELECTRICITY 



197 



Storing a Charge : Condensers. If the finger or 

 some other conductor is brought near a charged body, 

 sparks will pass. The amount of the charge which may 

 be stored on an object is limited by the size and shape 

 of the object and its proximity to conductors. A charge 

 escapes more readily from points than from evenly curved 

 surfaces. Usually before the charge on an object is very 

 great it escapes by way of the air or some other conductor. 

 A greater charge can be stored on a body on a dry day 

 than on a damp day, because moisture in the air makes 

 it a better conductor of electricity. To store a large 

 amount of electricity on an object, it is simply necessary 

 to insulate it properly and bring it near another con- 

 ductor which is attached to the earth. This is the prin- 

 ciple of the condenser. 



Experiment 56. Place some tin foil on both sides of a common 

 windowpane as shown in Figure 166 and connect one side with the 

 earth. Now if the foil on the side A be connected with some source 

 of positive electricity, electrons will 

 be attracted from the earth to plate 

 B in sufficient numbers to balance 

 the positive charge on A. A very 

 large charge thus may be stored on 

 the insulated plates. The plates 

 may be discharged by touching the 

 end of a wire to one side, B, and 

 bringing the other end near A. 



Explain what would happen if 

 plate A be connected with a source 

 of negative charges instead of posi- 

 tive charges. FIG. 166. 



The Leyden Jar was one of the first forms of con- 

 densers. It was first used in 1745. It consists of a 

 glass jar, coated inside and outside to about two thirds 



