30 H. A. Rowland—Magnetic Effect of Electric Convection. 
Art. IV.—On the Magnetic Effect of Electric Convection 7 by 
Henry A. Rowuanp of the Johns Hopkins University, 
Baltimore. 
current. Hence an experiment is of v 
well, in his “ Treatise on Electricity,” Art. 770, has computed 
The apparatus employed consisted of a vulcanite disc 21-1 
and ‘} centimeter thick which could 
be made to revolve around a vertical axis with a velocity of 
61: turns per second. On either side of the disc ata distance 
of 6 cm. were fixed glass plates having a diameter of 38-9 
em. and a hole in the center of 7-8 em. The vulecanite disc 
readily and there should be no uncertainty as to the electrifica- 
ion. The outside plates were usually connected with the earth ; 
and the inside dise with an electric battery, by means of a point 
not discharge unless there was a difference of potential between 
it and the edge. Between the electric battery and the disc, a 
were of non-magnetic material. 
Over the surface of the dise was suspended, from a bracket 
in the wall, an extremely delicate astatic needle, protected from 
electric action and currents of air by a brass tube. The two 
es were 15 cm. long and their centers 17-98 em. distant 
od 
other. readings 
The opening in the tube for observing t 
* The experiments described were made in the laboratory of the Berlin Uni- 
versity through the kindness of Professor Helmholtz, to whose advice they are 
; ir com : idea of the experiment first occurred 
to me in 1868 and was in a note book of that date. 
