l part of the
copper plate. The galvanometer needle moved as in former cases, and the
_direction_ of motion was the _same_ as that which would have resulted, if
the copper only had revolved, and the magnet been fixed. Neither was there
any apparent difference in the quantity of deflection. Hence, rotating the
magnet causes no difference in the results; for a rotatory and a stationary
magnet produce the same effect upon the moving copper.

219. A copper cylinder, closed at one extremity, was then put over the
magnet, one half of which it inclosed like a cap; it was firmly fixed, and
prevented from touching the magnet anywhere by interposed paper. The
arrangement was then floated in a narrow jar of mercury, so that the lower
edge of the copper cylinder touched the fluid metal; one wire of the
galvanometer dipped into this mercury, and the other into a little cavity
in the centre of the end of the copper cap. Upon rotating the magnet and
its attached cylinder, abundance of electricity passed through the
galvanometer, and in the same direction as if the cylinder had rotated
only, the magnet being still. The results therefore were the same as those
with the disc (218.).

220. That the metal of the magnet itself might be substituted for the
moving cylinder, disc, or wire, seemed an inevitable consequence, and yet
one which would exhibit the effects of magneto-electric induction in a
striking form. A cylinder magnet had therefore a little hole made in the
centre of each end to receive a drop of mercury, and was then floated pole
upwards in the same metal contained in a narrow jar. One wire from the
galvanometer dipped into the mercury of the jar, and the other into the
drop contained in the hole at the upper extremity of the axis. The magnet
was then revolved by a piece of string passed round it, and the
galvanometer-needle immediately indicated a powerful current of
electricity. On reversing the order of rotation, the electrical current was
reversed. The direction of the electricity was the same as if the copper
cylinder (219.) or a copper wire had revolved round the fixed magnet in the
same direction as that which the magnet itself had followed. Thus a
_singular independence_ of the magnetism and the bar in which it resides is
rendered evident.

221. In the above experiment the mercury reached about halfway up the
magnet; but when its quantity was increased until within one eighth of an
inch of the top, or diminished until