382 H. A. Rowland—Maynetic Effect of Electric Convection. 
the latter, in many experiments, were divided into small por- 
tions by radial scratches, so that no tangential currents could 
take place without sufficient difference of potential to produce 
sparks. But to be perfectly certain, the gilded disc was re- 
placed by a plane thin glass plate which could be electrified by 
Neb on one side, a gilder induction plate at zero potential 
eing on the other. With this arrangement, effects in the same © 
direction as before were obtained, but smaller in quantity, see- 
ing that only one side of the plate could be electritied. 
The inductor plates were now removed, leaving the disc per- 
fectly free, and the iatter was once more gilded with a continu- 
ous gold surface, having only an opening around the axis of 
35cm. The gilding of the dise was connected with the axis 
and so was ata 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 induc- 
tors 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 elec- 
8. 
electricity is being constantly conducted in the plate so as to 
retain its position. Now the function which expresses the po- 
tential producing these currents and its differential coefficients 
must be continuous throughout the disc, and so these currents 
must pervade the whole disc. 
To calculate these currents we have two ways. Hither we 
ean consider the electricity at rest and the motion of the dise 
through it to produce an electromotive force in the direction of 
motion and proportional to the velocity of motion, to the elec- 
trification, and to the surface resistance; or, as Professor Helm- 
oltz has suggested, we can consider the electricity to move 
with the dise and as it comes to the edge of the inductor to be 
set free to return by conduction currents to the other edge of 
the inductor so as to supply the loss there. The problem is 
eapable of solution in the case of a dise without a hole in the 
center but the results are too complicated to be of much use. 
Hence scratches were made on the disc in concentric circles 
about °6 em. apart by which the radial component of the cur- 
rents was destroyed and the problem became easily calculable. 
For, let the inductor cover Me the part of the circumference 
_of any one of the conducting circles; then, if C is a constant, 
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