H. A. Rowland—Magnetic Effect of Electric Convection. 31 
from electrical action by a metallic cone, the mirror being at its 
vert So perfectly was this accomplished that no effect of 
electrical action was apparent either on charging the battery or 
reversing the electrification of the disc. The needles were so 
far apart that any action of the disc would be many fold greater 
on the lower needle than the upper. The direction of the nee- 
dles was that of the motion of the disc directly below them, 
that is, perpendicular to the radius drawn from the axis to the 
needle. As the support of the needle was the wall of the lab- 
oratory and revolving disc was on a table beneath it, the needle 
was reasonably free from vibration. 
In the first experiments with this apparatus no effect was 
observed other than a constant deflection which was reversed 
with the direction of the motion. This was finally traced to 
many weeks, the needle always answered to a change of electri- 
fication of the disc. Also on raising the potential above zero 
the action was the reverse of that when it was lowe elow. 
The swing of the needle on reversing the electrification was 
about 10° or 15- millimeters and therefore the point of equili- 
brium was altered 5 or 74 millimeters. This quantity varied 
with the electrification, the velocity of motion, the sensitive- 
ness of the needle, ete. 
The direction of the action may be thus defined. Calling 
the motion of the dise +- when it moved like the hands of a 
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 — electrifica- 
tion, it moved toward the axis. The direction is therefore that 
in which we should expect it to : 
To prevent any suspicion of currents in the gilded surfaces, 
