149.  Guided by the law already expressed (114.), I expected that all the electric phenomena of the revolving metal plate could now be produced without any other magnet 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 the plate was a maximum.