ELECTRIC CHARGE. THE CONDENSER. 



173 



silk 

 thread 



very small metal ball b is suspended between A and B by a 

 silk thread. If this ball is started it continues to vibrate back and 

 forth from plate to plate, and at each movement it carries across 

 a definite amount of charge q. Every 

 time the ball carries charge q across 

 from plate to plate an amount of 

 charge q flows through the battery, 

 the battery does an amount of work 

 equal to Eq and this work reappears 

 as mechanical work done on the ball 

 as it is pushed across from plate to 

 plate by the electric attraction or re- 

 pulsion. Let F be the force which 

 pushes on the ball, then Fx is the 

 work done by this force in pushing 

 the ball across from plate to plate,* 



so that Fx = Eg, or 



Fig. 106. 



B 



B 



- 



Flg. 107. 



X J 



Any region in which a charged body is 

 acted upon by a force f is called an electric 

 field ; thus the region between the plates 

 A and B, Fig. 1 06, is an electric field, as 

 indicated by the fine lines of force in Fig. 

 107. 



The force -Fwith which an electric field 

 pulls on a charged body (of small size) 

 placed at a given point in the field is pro- 

 portional to the charge q on the body so 

 that 



* The ball is supposed to be quite small so that the distance moved by it may be 

 taken to be equal to the distance x between the plates. Under these conditions the 

 force which acts upon the ball is constant throughout its movement from plate to plate. 



f That is, a force which depends upon the charge on the body and which does not 

 exist when the body has no charge. 



