MEASUREMENT OF A QUANTITY OF ELECTRICITY. 555 



In the experiment 



0-07234; 

 r = 0-5768 cm., R 



the ratio of the capacity c of the small ball to that of the sphere 

 is thus 



= 0-007875. 



It follows that 



c C c i 



C C C' 3876 



this is the fraction of the charge q which the small ball takes on 

 contact. 



As the movable ball of the balance has sensibly the same radius 

 as the first (0*5798 cm.) the common charge, after contact, is equally 

 divided between them, and each of them has the quantity of 

 electricity 



After this preliminary determination, two observers are necessary 

 for the course of the experiments. 



The jar being charged, one of the observers makes the contacts 

 mentioned, puts the small ball in the Coulomb's balance, and 

 measures the repulsion ; while the other observer discharges the 

 residue q in a ballistic galvanometer, and makes the corresponding 

 readings. 



The balls of the balance were hollow, carefully turned, gilt, and 

 polished ; the torsion necessary to keep them at 90 was measured 

 or, more exactly, after having given slightly too weak a torsion, the 

 period is noted at which the angle of deflection passes through 90, 

 owing to leakage, and the necessary corrections are then made. 



The distance of the fixed ball from the axis was 9*353 cm., that of 

 the movable one 6*17 cm., hence for a deflection of 90 the distance 

 of the centres is 1 1*205; but this distance should be increased by 

 0-0124 cm., to allow for the distribution. The repulsion of the balls 

 is then 



ii-2i74 



