produced sparks (32.).

155. A disc of copper, one fifth of an inch thick and only one inch and a
half in diameter, was amalgamated at the edge; a square piece of sheet lead
(copper would have been better) of equal thickness had a circular hole cut
in it, into which the disc loosely fitted; a little mercury completed the
metallic communication of the disc and its surrounding ring; the latter was
attached to one of the galvanometer wires, and the other wire dipped into a
little metallic cup containing mercury, fixed upon the top of the copper
axis of the small disc. Upon rotating the disc in a horizontal plane, the
galvanometer needle could be affected, although the earth was the only
magnet employed, and the radius of the disc but three quarters of an inch;
in which space only the current was excited.

156. On putting the pole of a magnet under the revolving disc, the
galvanometer needle could be permanently deflected.

157. On using copper wires one sixth of an inch in thickness instead of the
smaller wires (86.) hitherto constantly employed, far more powerful effects
were obtained. Perhaps if the galvanometer had consisted of fewer turns of
thick wire instead of many convolutions of thinner, more striking effects
would have been produced.

158. One form of apparatus which I purpose having arranged, is to have
several discs superposed; the discs are to be metallically connected,
alternately at the edges and at the centres, by means of mercury; and are
then to be revolved alternately in opposite directions, i.e. the first,
third, fifth, &c. to the right hand, and the second, fourth, sixth, &c. to
the left hand; the whole being placed so that the discs are perpendicular
to the dip, or intersect most directly the magnetic curves of powerful