than the earth. The plate so often referred to
(85.) was therefore fixed so as to rotate in a horizontal plane. The
magnetic curves of the earth (114. _note_), i.e. the dip, passes through
this plane at angles of about 70 deg., which it was expected would be an
approximation to perpendicularity, quite enough to allow of
magneto-electric induction sufficiently powerful to produce a current of
electricity.

150. Upon rotation of the plate, the currents ought, according to the law
(114. 121.), to tend to pass in the direction of the radii, through _all_
parts of the plate, either from the centre to the circumference, or from
the circumference to the centre, as the direction of the rotation of the
plate was one way or the other. One of the wires of the galvanometer was
therefore brought in contact with the axis of the plate, and the other
attached to a leaden collector or conductor (86.), which itself was placed
against the amalgamated edge of the disc. On rotating the plate there was a
distinct effect at the galvanometer needle; on reversing the rotation, the
needle went in the opposite direction; and by making the action of the
plate coincide with the vibrations of the needle, the arc through which the
latter passed soon extended to half a circle.

151. Whatever part of the edge of the plate was touched by the conductor,
the electricity was the same, provided the direction of rotation continued
unaltered.

152. When the plate revolved _screw-fashion_, or as the hands of a watch,
the current of electricity (150.) was from the centre to the circumference;
when the direction of rotation was _unscrew_, the current was from the
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 deg. 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 deg., when the electricity for a given velocity of t