62 STATIC ELECTRICITY 



Let a charge Q be placed at a point A, Fi.ir. 55, Let tin- 

 force on a small charged body at P, due to Q t , be F, and Id tin- 

 strain be PC. Both *\ and PC are along AP, produced, 

 charge Qj is removed and a charge Q 2 is placed at B, let the force 

 at P be F 2 and let the strain be PD, both along BP produced, 

 If the two charges coexist, the direction of the force coincides stil 

 with that of the strain. 



Now we know that the resulting strain represented l>v PI 

 the resultant according to the vector law of the two strains repre- 





sented by PC and PI), and in order that the resultant <>t I I -Mould 

 also be in the direction PK we must h 



FI PC 



or the force on a given small charged body due to an cl< i 

 another charged body is proportional to charge -f (distance) 1 . 



Further, assuming that action and reaction are equal and 

 opposite, the force is proportional to the charge on the body at P. 

 For if that charge be Q the force exerted on the body at A by the 



body at P is proportional to -, /.<. is proportional to Q. II- 



the reaction of the body at A on that at P which U ecjual and 

 opposite is also proportional to Q. Then the force exerted by the 



body at A on the body at P is proportional to 



Coulomb's direct measurement of the forces between 

 electrified bodies. Having regard to the importance of the 

 law just stated, it is worth while to consider the direct method of 

 obtaining it employed by its discoverer. Coulomb. To measure 

 the force, he used the torsion balance of which the general 

 arrangement is represented in Fig. 56. be represents a " tor- 

 rod" of shellac with a pith ball b at one extremity, c and d being 

 small loads to keep the rod horizontal. It is suspended by a 

 fine metal wire from the torsion head /. This can be rotated 

 and the angle through which the upper end of the wire is thus 

 turned can be read by means of a pointer p moving over a scale * 



