Figure 43 exhibits a side view of the instrument: _a_, _a_, _a_, is a compound permanent magnet, consisting of three steel plates, put together, side by side. B and C are two wooden supports, upon the platform A A. To these supports the magnet is permanently secured, by a yoke, S, through which pass two screws into the wooden supports below. M is a cross bar, into which, and at right angles with it, are screwed two arms of round soft iron, R, about five-eighths of an inch in diameter, the whole forming the armature or keeper of the magnet. Upon these two projecting arms, are placed two coils, D′ and D, of copper wire, insulated with silk. The whole is very securely fastened upon the steel spindle, N, which has its journals in the supports, B and B′. On the end of the spindle, N, near the curve of the magnet, there is a small band pulley, F, which is driven by the band or cord of the large wheel, E, and the crank, J. The axis of the large wheel passes through a long socket, L, in the top of the column, H; on one end of the axis the band wheel is fastened and on the other the crank. By this arrangement a very rapid and quiet rotary motion is given to the armature. In the column, H, there is a socket, into which the stem of the upper part of the column, G, is fitted, which admits of the large wheel being raised or lowered, so as to prevent the band from slipping, and when properly adjusted it is secured by the screw, I. O is an ivory hub, sliding over that part of the spindle immediately projecting beyond the cross bar, M. Upon this ivory hub is a copper disc, C, with a socket, _n_: _b_, is a needle made of platinum, which, with its socket, _m_, is nicely fitted upon the end of the steel spindle, so as to be adjusted to any required angle with the armature, and when adjusted to retain its position. The two ends of the two coils, which leave the _centre_ of the helices, are made to form a contact with the soft iron arms, R R, passing through the coils, D′ and D, thus making the circuit complete with the needle _b_, upon the end of the spindle, N, by a continuous metallic connection of the arms, with the cross bar, M, and through the cross bar to the spindle, N, in contact with the needle, _b_. The two ends of the two coils, leaving the _outside_ of the helices, are joined in one, and as they pass through the cross bar, M, are insulated from it, by a piece of ivory, inserted in the cross bar. The united wire then passes into and through the ivory hub, _e_, forming a perfect contact with the copper disc, _c_, underneath its socket, _n_: _d_, is a cup of mercury, in which the copper disc, _c_, is always immersed, and the needle, _b_, twice in every revolution of the armature. The cup, _d_, is so constructed as to rise and fall, by means of a stem, _i_, sliding vertically into a socket, _e_, of its support, and is secured to its position by the screw, _h_. In this way, its proper height for breaking and closing the circuit may be easily