262 HENEY A. EOWLAND 



did not follow the law assumed. The deviation can best be explained 

 in this way: The equations above show that for a fixed position of 



N~ D N 



the disks J oc a-, a-x. If then, N and /9 being constant, the con- 

 p e p 



denser plates are moved up to the disk, step by step, thus varying e, 

 and D be changed at the same time so as to keep D/e <xa, a constant, 

 the deflexions should be constant. 



Such was not found to be the case; the deflexions were directly 

 proportioned to e instead of being constant : that is, with greater differ- 

 ence of potential, the deflexions were greater, although the surface 

 density remained constant. Finally this was found to be due to a 

 charge on the back surface of the gold coating. The end of the axle 

 comes nearly up to the surface of the disk and taken with all the brass 

 work must form a condenser of a certain capacity with the inner face 

 of the gold foil. 



This made a change necessary in the method of working; the disks 

 had to be earthed and the glasses charged. This was done; but now 

 the deflexions were found always to be greater with positive rotation 

 (Zenith, North, Nadir, South) then with negative. 



It was considered possible that the brushes might have something 

 to do with this, so they were taken off. Earth connection with the disk 

 was made by drilling through to the surface of the disk in the line of 

 the axle and setting in a screw, which came flush with the surface and 

 also made contact with the axle; this, however, made no difference, the 

 deflexions for negative rotation were always smaller. 



Table I gives the results of a number of observations. All were 

 taken with the plates charged and the disks earthed by means of the 

 axle. 



The meaning of the letters has been given; l//9is directly propor- 

 tional to the needle sensitiveness. 



The sudden variations in the values of 1//9 are due to changes pur- 

 posely made in the needle. 



The last column gives the values of V. This work is not intended 

 as a determination of V, but the calculation is made merely to show to 

 what degree of approximation the effect follows the assumed law. 



The deflexions are about the same as those obtained in the Berlin 

 experiments 5 to 8 mm. on reversing. The force measured then 

 was 1/50000 H; now it is 1/125000 H. The sensitiveness of the needle 

 in the two cases was almost the same. In the former experiment a 

 force of 3 X 10~ 7 deflected the needle 1' of arc; the corresponding num- 



