Effects of Moving Charged Spheres. 295 



Hence 



y _ A-B Ny tanfl 

 0-D I tan</>* 



Let E be the scale-deflexion on reversing the current I in 

 the ealibrating-coil, and A the scale-deflexion on reversing 

 the charges of the spheres. Then 



v _ A—B Ng 8 

 O-D I A* 



d and d\ the distances of the centres of the lower and 

 upper needles respectively from the centre of the axle, were 

 determined by means of a cathetometer, the distance of tho 

 mirror from the centre of the axle being directly measured, 

 from which d and d' were obtained. 



The current sent through the cali bra ting-coil for deter- 

 mining the needle-constant was measured by a Weston 

 milliammeter. The value found was accurate to within at 

 least one-half of one per cent., which is sufficient for this 

 purpose. 



The charge of the spheres is the most uncertain element in 

 the quantitative determination, and it is this uncertainty 

 especially which makes the method of revolving spheres far 

 less suitable for quantitative work than the method of rotating 

 disks, particularly as employed by Rowland in his second 

 experiment. If a single set of spheres had been used, 

 charged to the same potential, an equal opposite charge 

 would have been induced on neighbouring conductors which 

 would have travelled with the charges on the spheres. It 

 would have been difficult to determine just what the resultant 

 effect should be. For this reason two distinct sets of spheres 

 were used, charged oppositely, the spheres always keeping 

 the same relative positions. It was then assumed that the 

 only moving charges were those carried on the moving 

 spheres. The capacity of the spheres was calculated on the 

 assumption that they were the only conductors present. The 

 charge on any one sphere was calculated by the method of 

 images, the charges on all the other spheres being regarded 

 as concentrated at their centres. We then have (Maxwell, 

 vol. i. section 159) 



+ q — charge of each sphere. 

 + P = its potential. 

 a = its radius. 

 /]? /2> &c. = the distances of the centres of the spheres from 

 one another. 



X2 



