circumference to the centre. These directions are the same with those
obtained when the unmarked pole of a magnet was placed beneath the
revolving plate (99.).

153. When the plate was in the magnetic meridian, or in any other plane
_coinciding_ with the magnetic dip, then its rotation produced no effect
upon the galvanometer. When inclined to the dip but a few degrees,
electricity began to appear upon rotation. Thus when standing upright in a
plane perpendicular to the magnetic meridian, and when consequently its own
plane was inclined only about 20° to the dip, revolution of the plate
evolved electricity. As the inclination was increased, the electricity
became more powerful until the angle formed by the plane of the plate with
the dip was 90°, when the electricity for a given velocity of the 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