ELECTRIC CHARGE. THE CONDENSER. 163 



is q. Electrically charged bodies always occur thus in pairs, 

 the positive charge on one body being always associated with an 

 equal negative charge on some other body or bodies. 



Example. Two large sheets of tin foil separated from each 

 other by waxed paper are connected through an incandescent 

 lamp to supply mains. If this arrangement is connected to 

 direct-current supply mains a single pulse of current flows 

 through the lamp at the moment of connection, and the lamp 

 filament is not perceptibly heated. If the arrangement is con- 

 nected to alternating-current supply mains a pulse of current 

 flows through the wire at every reversal of the alternating elec- 

 tromotive force and the lamp filament may be heated to incan- 

 descence. 



The electric field. The dielectric, The region between the 

 two bodies A and B, Fig. iO2a, is understood to be filled with 

 some electrical insulator such as air, oil or glass. An insulator 

 between two charged bodies is called a dielectric. This dielectric 

 is the seat of a peculiar stress which is called the electric field. 

 The lines of force * of this electric field trend somewhat as shown 

 in the figure, touching the surfaces of the metal bodies A and 

 B at right angles. These lines of force are thought of as going 

 out from the positively charged body and coming in towards the 

 negatively charged body. 



Electrostatic attraction. The charged bodies A and B, Fig. 

 IO2, attract each other. This attraction, which is called elec- 

 trostatic attraction, shows that the lines of force of an electric 

 field are in a state of tension and have a tendency to shorten very 

 much as the lines of force in a magnetic field. This tension of 

 the lines of force pulls outwards on the surface of A and on the 

 surface of B at each point. 



The electrostatic attraction of two metal bodies which are con- 

 nected to a battery or dynamo may be shown as follows : A gold 

 leaf is hung alongside of a vertical brass strip. When the gold 



* The electric field is similar in many respects to the magnetic field, having a defi- 

 nite intensity and a definite direction at each point. 



