PHYSICS, EIGHTEENTH CENT. ELECTRICITY. 335 



were extremely small, and were at. the same time liable to gradually 

 dimin'fsh even during the progress of an experiment. Coulomb never- 

 theless succeeded in making accurate determinations by availing him- 

 self of a principle with which the reader should here become ac- 

 quainted. Let A c, Fig. 1 6 1, be a wire or very slender metallic rod 

 suspended from A, and hanging vertically under the influence of the 

 small weight D. A B and c E are index fingers solidly attached to the 

 wire, and their points move round fixed graduated circles. When such 

 an arrangement is left to itself, the wire will come to rest in a position 

 in which it will have no twist (or torsion). Suppose that its upper end 

 A is then fixed by all movement of A B being prevented ; if we turn 

 c E round so that E shall point to one division to the right or left, a 

 certain amount of force must be applied, because in so doing we twist 

 the wire. Now, the thing to be here clearly understood is that to 



FIG. 162. 



keep it two divisions away from its normal position, twice as much 

 force will be required ; to maintain it ten divisions distant ten times 

 the force must be applied, and so on, the force always being exactly 

 proportional to the angle through which the wire is twisted. To put the 

 case in a slightly different manner. Suppose in the arrangement re- 

 presented in the figure we placed a peg at E, which would prevent c E 

 from turning round ; then if we turned A B, the wire between A and c 

 would be twisted, and the pressure of E against the peg would always 

 be proportional to the angles through which A B was turned ; that is, 

 the force is proportional to the angle of torsion. Now, Coulomb 

 balanced the forces of electrical attractions and repulsions against the 

 force of torsion, and the former could therefore be compared with each 

 other by simply reading off the angle of torsion. The apparatus he 

 employed, which is aptly enough termed a Torsion Balance, is repre- 

 sented in Fig. 162, where a cubical case of plate glass, A B c, is seen, 



