Effect of Electrical Convection. t 47 



in rotation about a vertical axis. A hole about 2 cms. in 

 diameter was cut through the upper ebonite plate C x dia- 

 metrically opposite the tinfoil segment, and so that its centre 

 came 1 cm. over the edge o£the disk. This hole was covered 

 on the side next to the disk with a thin sheet o£ mica. Fitting 

 loosely into the hole so as not to touch the sides or the mica 

 plate at the bottom was a brass tube in which was suspended 

 a delicate astatic needle N. The two magnets forming the 

 needle were 5 cm. apart. The needle and attached mirror 

 weighed about 3 mgs., and was suspended by a fine quartz 

 fibre. With the control-magnet properly placed the needle 

 could be given a period of 25 seconds. The case in which 

 the needle was suspended was fixed to a frame built over the 

 disk apparatus, having an independent support, so that when 

 the disk was set in rotation there was no jarring of the 

 needle. The position of the needle was read by the reflected 

 image of an electric-light filament on a ground-glass scale 

 2 metres distant. 



When the disk was set in rotation and the brush making 

 contact with the studs was connected to the Voss machine, a 

 deflexion of the needle was obtained which was in the proper 

 direction and of the proper amount to be accounted for on 

 the assumption of the magnetic action of a moving charge. 

 The arrangement here adopted precludes any conduction of 

 charge in the condensing-plates or in the disk itself, two pos- 

 sibilities which have been suggested to account for the deflexion 

 observed with solid condensing-plates and a disk of uniform 

 metallic surface. The results of one set of observations will 

 suffice to give an idea of the magnitude of the quantities 

 involved. 



Mean distance between the two surfaces of the disk and 

 needle 1*61 cm. 



Distance between the two condensing segments (one on 

 each ebonite plate C l and C 2 ) 2*18 cms. 



Thickness of disk *356 cm. 



Potential of the disk 5000 volts. 



Speed of disk 69 7 revolutions per second. 



Observed deflexion 47*9. 



Calculated deflexion 56*0. 



The calculation was made in a manner similar to that 

 employed in the previous experiments, the principle of it 

 being a calibration of the disk apparatus such as above 

 described. Only a rough calculation was attempted, as it 

 would be a matter of some difficulty to calculate the exact 

 •distribution of the charge on the sectors. As a first approxi- 

 mation this distribution was assumed uniform. The agreement 



