330 Mayer — Experiments with Pendulum-electrometer. 



corresponding distances of the centers of the two spheres ; the 

 fourth, the direct ratios of the deflections ; the fifth, the 

 inverse ratios of the squares of the distances between centers 

 of spheres; the sixth, the approximations to the law of the 



u ■"'■*- d (d D'\ 



inverse squares. By approximation we mean — -r I -=, — =r- ). 



The average approximation is quite close to the law, being T fa. 



One cause of this departure from the law of the inverse 

 squares is the mutual influence of the spheres causing a depar- 

 ture from uniform electrical density over their surfaces.* If we 

 replace the cork ball by one of pith, the approximation to the 

 law is much closer, as the distances of the spheres in the experi- 

 ments are much greater. A gilded pith ball of 1 cm. radius, 

 made of pieces of pith cemented together, weighs, with one 

 half of the suspending filaments, only -25grm., about \ of the 

 weight of cork ballf and filament ; so that, on the pith ball 

 pendulum, a dyne will produce a deflection of 13 3 mms. This 

 pendulum is not suited to class experiments, as it is too readily 

 moved by air currents, and has to be inclosed in a large cage 

 formed of silk fastened to a light wooden frame. When this 

 pith sphere is thus protected and suspended by an exceedingly 

 fine fibre, it gives as close an approximation to the law of 

 inverse squares as does the Coulomb torsion-balance. Another 

 cause of error is the continual loss of electricity during the 

 experiments. This error can be partly corrected by repeating 

 as nearly as possible the first experiment and properly combin- 

 ing this third experiment with the first. This was not done in 

 those given, our object having been to see how close an 

 approximation to the law is reached by methods suited to 

 experiments before a class. The error caused by loss of elec- 

 tricity can be allowed for in measurements of quantity and 

 potential by a determination of this loss by experiments similar 

 to those given below. 



Experiments to determine the law of the loss of electricity, per 

 minute, from a sphere of the radius of (1 an.) the unit of 

 capacity. 



The pendulum sphere was touched by the charged sphere 

 on the rod, and the latter remained in one position during the 

 experiments. The deflections of the pendulum and the dis- 

 tances of the centers of the spheres were read every three 

 minutes. 



* On this subject see Sir W. Thomson's papers on Electr. and Mag., p. 86; and 

 Mascart and Joubert's Electr. and Mag., vol. i, p. 153, and vol. ii, p. 173. 



f The specific gravity of elder pith, in air, is "04. This I determined by weigh- 

 ing a right parallelopiped cut out of this substance. The specific gravity of cork 

 is 24. 



