ELECTRIC CHARGE AND THE CONDENSER. 123 



said to become positively charged, and the plate out of which the 

 momentary current flows is said to become negatively charged. 

 The two metal plates A and B together with the layer of 

 insulating material between them constitute what is called a 

 condenser. A condenser is usually made of sheets of tin foil 

 separated by sheets of waxed paper. Thus the heavy horizontal 

 lines in Fig. 82 represent sheets of tin foil, and the finely dotted 

 regions represent insulating material. 



B 



Fig. 82. 



The flow of current in an open circuit may be shown by con- 

 necting a small incandescent lamp and a condenser in series to 

 alternating-current supply mains. With each reversal of the 

 alternating supply voltage a momentary current flows through 

 the lamp, and the repeated pulses of current heat the lamp 

 filament to incandescence. 



The dielectric. The insulating material between the metal 

 plates A and B in Fig. 81 is called the dielectric. In general 

 the insulating material surrounding a charged body or between 

 two charged bodies is called a dielectric. 



Electrostatic attraction. The charged metal plates A and B 

 in Fig. 8 1 have an evident attraction for each other when the 

 dielectric is a fluid like air or oil. This electrostatic attraction of 

 two charged metal bodies may be shown as follows : A gold leaf 

 is hung along side of a vertical brass strip. When the gold 

 leaf is connected to one terminal of a battery and the brass 

 strip to the other terminal of the battery the gold leaf is 

 attracted by the brass. It is necessary in this arrangement to 

 cover the face of the brass plate with a layer of paper to prevent 

 short circuiting the battery by the gold leaf. An electromotive 



