he plate was
a maximum.

154. It is a striking thing to observe the revolving copper plate become
thus a _new electrical machine_; and curious results arise on comparing it
with the common machine. In the one, the plate is of the best
non-conducting substance that can be applied; in the other, it is the most
perfect conductor: in the one, insulation is essential; in the other, it is
fatal. In comparison of the quantities of electricity produced, the metal
machine does not at all fall below the glass one; for it can produce a
constant current capable of deflecting the galvanometer needle, whereas the
latter cannot. It is quite true that the force of the current thus evolved
has not as yet been increased so as to render it available in any of our
ordinary applications of this power; but there appears every reasonable
expectation that this may hereafter be effected; and probably by several
arrangements. Weak as the current may seem to be, it is as strong as, if
not stronger than, any thermo-electric current; for it can pass fluids
(23.), agitate the animal system, and in the case of an electro-magnet has
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
severa