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 currents
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 following 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 edges, was placed between the