130 HENRY A. EOWLAND 



The direction of the action may be thus defined. Calling the motion 

 of the disc -\- when it moved like the hands of a watch laid on the 

 table with its face up, we have the following, the needles being over 

 one side of the disc with the north pole pointing in the direction of 

 positive motion. The motion being -f> on electrifying the disc -)- the 

 north pole moved toward the axis, and on changing the electrification, 

 the north pole moved away from the axis. With motion and -(- 

 electrification, the north pole moved away from the axis, and with 

 electrification, it moved toward the axis. The direction is therefore 

 that in which we should expect it to be. 



To prevent any suspicion of currents in the gilded surfaces, the 

 latter, in many experiments, were divided into small portions by radial 

 scratches, so that no tangential currents could take place without suffi- 

 cient difference of potential to produce sparks. But to be perfectly 

 certain, the gilded disc was replaced by a plane thin glass plate which 

 could be electrified by points on one side, a gilder induction plate at 

 zero potential being on the other. With this arrangement, effects in 

 the same direction as before were obtained, but smaller in quantity, 

 seeing that only one side of the plate could be electrified. 



The inductor plates were now removed, leaving the disc perfectly 

 free, and the latter was once more gilded with a continuous gold sur- 

 face, having only an opening around the axis of 3-5 cm. The gilding of 

 the disc was connected with the axis and so was at a potential of zero. 

 On one side of the plate, two small inductors formed of pieces of tin- 

 foil on glass plates, were supported, having the disc between them. On 

 electrifying these, the disc at the points opposite them was electrified 

 by induction but there could be no electrification except at points near 

 the inductors. On now revolving the disc, if the inductors were very 

 small, the electricity would remain nearly at rest and the plate 

 would as it were revolve through it. Hence in this case we should 

 have conduction without motion of electricity, while in the first experi- 

 ment we had motion without conduction. I have used the term 

 " nearly at rest " in the above, for the following reasons. As the disc 

 revolves the electricity is being constantly conducted in the plate so as 

 to retain its position. Now the function which expresses the potential 

 producing these currents and its differential coefficients must be con- 

 tinuous throughout the disc, and so these currents must pervade the 

 whole disc. 



