166 MR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
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 con- 
ductor (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 per- 
pendicular to the magnetic meridian, and when consequently its own plane 
was inclined only 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 elec- 
tricity 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 sub- 
stance 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 lias 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 
