The object of this paper is, first, to describe a galvanometer in which a principle which is, we believe, novel in its application to this particular purpose, has been employed to give an instrument of very great sensibility; and, second, to describe some forms of astatic galvanometers which are, we think, improvements on astatic combinations as ordinarily made and arranged for use. Fig. 1 shows in elevation and horizontal section the coils and arrangement of needles adopted in an instrument of very great sensibility which we designed in the summer of 1882, and which with assistance from the Government Research Fund, we have had,constructed for use in our experiments on the Electric Resistance of Glass and Allied Substances. It consists of two pairs of. coils,
c c c c
, with hollow cores, arranged so that the axes of each pair are parallel and in a vertical plane. Each pair is carried by a vertical brass plate, and the two plates are inclined to one another at an angle of about 106°; and thus the vertical planes containing the axes of the coils are inclined to one another at an angle of about 74°. Two horseshoe magnets,
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, made of steel wire of about 1 millim. in diameter, are connected together by a very light frame of aluminium, and are at such a distance apart as to hang, when the needle system is in equilibrium, with no current in the coils, freely within and nearly along the axes of the cores of the coils. The horseshoes are not plane, but are bent round so that they form approximately portions of one vertical cylinder of which the suspension-thread is the axis, and to which the axes of the coils form horizontal tangents near their middle points at the approximate positions of the poles of the needles.