EXPLICATION OF ARAGO’S MAGNETIC PHENOMENA. 
151 
magnet was brought near to the plate ; they were of the same kind as to 
direction, &c., but by no means so powerful. 
98. All care was taken to render these results independent of the earth’s 
magnetism, or of the mutual magnetism of the magnet and galvanometer 
needles. The contacts were made in the magnetic equator of the plate, and 
at other parts ; the plate was placed horizontally, and the poles vertically; and 
other precautions were taken. But the absence of any interference of the kind 
referred to, was readily shown by the want of all effect when the disc was re- 
moved from the poles, or the poles from the disc ; every other circumstance 
remaining the same. 
99. The relation of the current of electricity produced, to the magnetic pole, 
to the direction of rotation of the plate, &c. &c., may be expressed by saying, 
that when the unmarked pole (44. 84.) is beneath the edge of the plate, and 
the latter revolves horizontally, screw-fashion, the electricity which can be 
collected at the edge of the plate nearest to the pole is positive. As the pole 
of the earth may mentally be considered the unmarked pole, this relation of 
the rotation, the pole, and the electricity evolved, is not difficult to remember. 
Or if, in fig. 15, the circle represent the copper disc revolving in the direction 
of the arrows, and a the outline of the unmarked pole placed beneath the 
plate, then the electricity collected at b and the neighbouring parts is positive, 
whilst that collected at the centre c and other parts is negative (88.). The cur- 
rents in the plate are therefore from the centre by the magnetic poles towards 
the circumference. 
100. If the marked pole be placed above, all other things remaining the 
same, the electricity at b, fig. 15, is still positive. If the marked pole be placed 
below, or the unmarked pole above, the electricity is reversed. If the direction 
of revolution in any case is reversed, the electricity is also reversed. 
101. It is now evident that the rotating plate is merely another form of the 
simpler experiment of passing a piece of metal between the magnetic poles 
in a rectilinear direction, and that in such cases currents of electricity are 
produced at right angles to the direction of the motion, and crossing it at the 
place of the magnetic pole or poles. This was sufficiently shown by the fol- 
lowing simple experiment : A piece of copper plate one-fifth of an inch thick, 
one inch and a half wide, and twelve inches long, being amalgamated at the 
