ELECTRIC FIELD. 



139 



wire 



silk thread 



O 



machine is represented in Fig. 91 as a battery). A small metal 

 ball b is suspended between the plates A and B by a silk 

 thread, and if the ball b is started it continues to vibrate back 

 and forth from plate to plate. 

 Regarding the behavior of the 

 ball the following statements 

 may be made : 



(a) Work evidently is done to 

 keep the ball b oscillating 

 back and forth, and this work 

 is evidently done by the bat- 

 tery. 



(b) The only way the bat- 

 tery can do work is by con- 

 tinuing to draw charge out of 

 one plate and push it into the 

 other plate. It is evident 



therefore that the ball carries charge back and forth between the 

 plates. 



(c) The successive movements of the ball are similar, and there- 

 fore if the ball carries charge at all it must carry a definite 

 amount each time it moves across. Let this definite amount of 

 charge be represented by g; this charge is positive when the ball 

 moves from A to B, and negative when it moves from B to 

 A. At each movement of the ball the battery supplies the defi- 

 nite amount of charge q, drawing it out of plate B and pushing 

 it into plate A. Therefore, at each movement of the ball, the 

 battery does an amount of work Eq according to equation (i) 

 of Art. 80. 



(d) Let F be the average mechanical force acting on the ball 

 b while it is being pulled across from plate to plate. If the ball is 

 very small this force F does not vary as the ball moves across, 

 and if the ball is very small it moves the distance x in travelling 

 from plate to plate. Therefore Fx is the amount of work done 

 on the ball while it moves from plate to plate. 



