11-15] The Fundamental Conceptions of Electrostatics 1 1 



particles will be retained. The positively electrified body will therefore 

 assume a yellow colour on being dusted with the powder, and similarly a 

 negatively electrified body would become red. It is often convenient to use 

 this method of determining whether the electrification of a body is positive 

 or negative. 



14. The attraction and repulsion of two charged bodies is in many 

 respects different from the force between one charged and one uncharged 

 body. The latter force, as we have explained, was known to the Greeks : it 

 must be attributed, as we shall see, to what is known as " electric induction," 

 and is invaribly attractive. The forces between two bodies both of which are 

 charged, forces which may be either attractive or repulsive, seem hardly to 

 have been noticed until the eighteenth century. 



The observations of Robert Symmer (1759) on the attractions and 

 repulsions of charged bodies are at least amusing. He was in the habit 

 of wearing two pairs of stockings simultaneously, a worsted pair for comfort 

 and a silk pair for appearance. In pulling off his stockings he noticed that 

 they gave a crackling noise, and sometimes that they even emitted sparks 

 when taken off in the dark. On taking the two stockings off together from 

 the foot and then drawing the one from inside the other, he found that both 

 became inflated so as to reproduce the shape of the foot, and exhibited 

 attractions and repulsions at a distance of as much as a foot and a half. 



" When this experiment is performed with two black stockings in one 

 hand, and two white in the other, it exhibits a very curious spectacle ; the 

 repulsion of those of the same colour, and the attraction of those of different 

 colours, throws them into an agitation that is not unentertaining, and 

 makes them catch each at that of its opposite colour, and at a greater 

 distance than one would expect. When allowed to come together they all 

 unite in one mass. When separated, they resume their former appearance, 

 and admit of the repetition of the experiment as often as you please, till 

 their electricity, gradually wasting, stands in need of being recruited." 



The Law of Force between charged Particles. 



15. The Torsion Balance. Coulomb (1785) devised an instrument known 

 as the Torsion Balance, which enabled him not only to verify the laws of 

 attraction and repulsion qualitatively, but also to form an estimate of the 

 actual magnitude of these forces. 



The apparatus consists essentially of two light balls A t C, fixed at the two 

 ends of a rod which is suspended at its middle point B by a very fine thread 

 of silver quartz or other material. The upper end of the thread is fastened 

 to a movable head D, so that the thread and the rod can be made to 

 rotate by screwing the head. If the rod is acted on only by its weight, the 



