allow of revolution either vertically or horizontally, its edge being at
the same time introduced more or less between the magnetic poles (fig. 7.).
The edge of the plate was well amalgamated for the purpose of obtaining a
good but moveable contact, and a part round the axis was also prepared in a
similar manner.

86. Conductors or electric collectors of copper and lead were constructed
so as to come in contact with the edge of the copper disc (85.), or with
other forms of plates hereafter to be described (101.). These conductors
were about four inches long, one third of an inch wide, and one fifth of an
inch thick; one end of each was slightly grooved, to allow of more exact
adaptation to the somewhat convex edge of the plates, and then amalgamated.
Copper wires, one sixteenth of an inch in thickness, attached, in the
ordinary manner, by convolutions to the other ends of these conductors,
passed away to the galvanometer.

87. The galvanometer was roughly made, yet sufficiently delicate in its
indications. The wire was of copper covered with silk, and made sixteen or
eighteen convolutions. Two sewing-needles were magnetized and fixed on to a
stem of dried grass parallel to each other, but in opposite directions, and
about half an inch apart; this system was suspended by a fibre of unspun
silk, so that the lower needle should be between the convolutions of the
multiplier, and the upper above them. The latter was by much the most
powerful magnet, and gave terrestrial direction to the whole; fig. 8.
represents the direction of the wire and of the needles when the instrument
was placed in the magnetic meridian: the ends of the wires are marked A and
B for convenient reference hereafter. The letters S and N designate the
south and north ends of the needle when affected merely by terrestrial
magnetism; the end N is therefore the marked pole (44.). The whole
instrument was protected by a glass jar, and stood, as to position and