160 ELEMENTS OF STATIC ELECTRICITY. 



When motion begins, at 5, these forces act at right 

 angles to . each other but as the ball moves round, the 

 angle of repulsion constantly decreases; being repre- 

 sented at , by the angle a b c; and at d, by the 

 angle a b d; and vanishing at w, where the two forces 

 are in direct opposition. 



In this position the force of repulsion opposes further 

 movement : for, as it radiates equally in every direction 

 from the two balls, its force on opposite sides of n is 

 equal. But since, in the experiment given, the greatest 

 angle was 36, which is only one-fifth of the semi-circle, 

 the error is not sufficient to affect the result seriously. 



Another inaccuracy results from lack of rigidity in 

 the fine wire, which causes it to deviate slightly from a 

 true vertical, under the influence of repulsion ; moving 

 its lower extremity out of the center. 



There is also a slight inaccuracy resulting from the 

 force of repulsion being estimated from the centers of 

 the balls, instead of from their nearest points. 



It is also assumed that electric repulsion remains 

 constant during the experiment: which would not be 

 strictly true; since there is a continual reduction of 

 energy, from causes already explained, which would 

 produce serious error, if the experiment were of long 

 duration. 



Since each ball becomes a center of electric radiation, 

 it is evident that the lines of force cut by each repre- 

 sent but a very small part of the entire repulsive energy. 

 But since the balls are of equal size and equal poten- 

 tial, it may be assumed that the proportion between 

 the energy actually measured, and the entire energy, is 

 the same in each ball. But an instrument embracing 

 all the lines of force would evidently be more reliable. 



