16 ELEMENTARY LESSONS ON [CHAP, i, 



m to ;/, and they repel one another because they are 

 -then similarly electrified. The ball #, therefore, is driven 

 round and twists the wire up to a certain extent. The 

 force of repulsion becomes less and less as n gets 

 farther and farther from m; but the force of the twist 

 gets greater and greater the more the string is twisted. 

 Hence these two forces will balance one another when 

 the balls are separated by a certain distance, and it is 

 clear that a large charge of electricity will repel the ball 

 n with a greater force than a lesser charge would. 

 The distance through which the ball is repelled is read 

 off not in inches but in angular degrees of the scale. 

 When a wire is twisted, the force with which it tends to 

 untwist is precisely proportional to the amount of the 

 twist. The force required to twist the wire ten degrees 

 is just ten times as great as the force required to twist 

 it one degree. In other words, the force of torsion is 

 Proportional to the angle of torsion. The angular 

 distance between the two balls is, when they are not 

 very widely separated, very nearly proportional to the 

 actual straight distance between them, and represents 

 the force exerted between electrified balls at that 

 distance apart. The student must, however, carefully 

 distinguish between the measurement of the force and 

 the measurement of the actual quantity of electricity 

 with which the instrument is charged. For the force 

 exerted between the electrified balls will vary at different 

 distances according to a particular law known as the 

 " law of inverse squares," which requires to be carefully 

 explained. 



16. The Law of Inverse Squares. Coulomb 

 proved, by means of the Torsion Balance, that the force 

 exerted between two small ' electrified bodies varies 

 1 inversely as the square of the distance between them 

 ' when the distance is varied. Thus, suppose two electri- 

 fied bodies one inch apart repel one another with a 

 certain force, at a distance of two inches the force will 



