CHAP, iv.] ELECTRICITY AND MAGNETISM. 213 



however, the force is measured not by being pitted against 

 the torsion of an elastic fibre, or against gravitation, but 

 against the directive magnetic force of the earth acting 

 on the small needle. Now this depends on the intensity 

 of the horizontal component of the earth's magnetism at 

 the place, on the magnetic moment of the needle, and 

 on the sine of the angle of its deviation. Moreover, the 

 repulsion here is not between two charges collected on 

 small spheres, but between the fixed arm and the mov- 

 able one. Hence, to obtain quantitative values for the 

 readings of this electrometer, it is necessary to make 

 preliminary experiments and to " calibrate " the degree- 

 readings of the angular deviation to an exact scale. 



261. Attracted - Disc Electrometers. Snow 

 Harris was the first to construct an electrometer for 

 measuring the attraction between an electrified and a 

 non-electrified disc ; and the instrument he devised may 

 be roughly described as a balance for weighing a charge 

 of electricity. More accurately speaking, it was an 

 instrument resembling a balance in form, carrying at one 

 end a light scale pan ; at the other a disc was hung 

 above a fixed insulated disc, to which the charge to be 

 measured was imparted. The disadvantages of this 

 instrument were manifold, the chief objection being due 

 to the irregular distribution of the charge on the disc. 

 The force exerted by an electrified point falls off inversely 

 as the square of the distance, since the lines of force 

 emanate in radial lines. But in the case of a uniformly 

 electrified plane surface, the lines of force are normal to 

 the surface, and parallel to one another ; and the force 

 is independent of the distance. The distribution over 

 a small sphere nearly fulfils the first of these conditions. 

 The distribution over a flat disc would nearly fulfil the 

 latter condition, were it not for the perturbing effect of 

 the edges of the disc where the surface-density is much 

 greater (see Art. 35); for this reason Snow Harris's 

 electrometer was very imperfect. 



