220 LECTURES TO SCIENCE TEACHERS. 



The laws of torsion being established, the first experi- 

 ments of Coulomb were on the force of electric repulsion at 

 different distances. The balls being electrified as I have 

 explained, the torsion ball takes up a position in which the 

 repulsion couple is balanced by the couple of torsion. The 

 distance between the balls was determined from reading off 

 the position of the torsion ball by means of the scale round 

 the glass cylinder. By a simple trigonometrical calculation 

 the distance is deduced from this reading. The torsion 

 head was then turned, so as to reduce the distance, which 

 was again measured in the same way, and the torsion couple 

 at this new distance was again determined. The torsion 

 couple required to equilibrate the repulsion between the 

 balls at different distances, the charge being supposed to 

 remain unaltered, was thus obtained. By these experi- 

 ments, the results of which, however, required correction 

 for the inevitable loss of charge during the experiment, 

 Coulomb obtained his well-known law of the inverse square 

 of the distance. 



This law, I must remark, requires careful exactness in 

 its statement. Through a complete misunderstanding of it 

 Sir W. Snow Harris was led to experiments to disprove 

 it. A proper statement is, that if the two quantities of elec- 

 tricity are placed upon balls which are so small that the 

 diameters of the balls are insensible in comparison with 

 the distance between their centres, the force of repulsion is 

 inversely proportional to the square of the distance between 

 the centres of the balls. Even this statement must be 

 taken with limitations, because the effects of induction 

 prevent the experiments from showing precisely that law. 

 There are, however, other proofs, partly given by Coulomb, 

 partly by Cavendish, which establish the truth of the law 

 with minute exactness. 



The mathematical theory for a particular form of 

 torsion balance, taking induction into account, has been 

 given by Professor Clerk-Maxwell in his great work on 

 electricity and magnetism. 



The next part of Coulomb's investigation consisted in 

 determining the effect of electrifying the balls with different 

 quantities of electricity, and to do that he used an ingenious 

 device. Electrifying the carrier ball, and bringing it to 

 the electrometer, the movable or torsion ball received its 



