148 MR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
84. The magnet has been already described (44.). To concentrate the poles, 
and bring them nearer to each other, two iron or steel bars, each about six or 
seven inches long, one inch wide, and half an inch thick, were put across the 
poles as in fig. 7, and being supported by twine from slipping, could be placed 
as near to or far from each other as was required. Occasionally two bars of 
soft iron were employed, so bent that when applied, one to each pole, the 
two smaller resulting poles were vertically over each other, either being upper¬ 
most at pleasure. 
85. A disc of copper, twelve inches in diameter, and about one fifth of an 
inch in thickness, fixed upon a brass axis, was mounted in frames so as to be 
revolved either vertically or horizontally, its edge being at the same time in¬ 
troduced 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 move- 
able contact ; a part round the axis was also prepared in a similar manner. 
86. Conductors or 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 galva¬ 
nometer. 
8/. The galvanometer was roughly made, yet sufficiently delicate in its in¬ 
dications. The wire was of copper covered with silk, and made sixteen or 
eighteen convolutions. Two sewing-needles were magnetized and passed 
through 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 mul¬ 
tiplier, and the upper above them. The latter was by much the most power¬ 
ful 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 conve¬ 
nient reference hereafter. The letters S and N designate the south and north 
