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contact with the spring, h, which rests against the platinized termination, F, of one of the extremities (poles) of the primary coil, while the zinc (n, fig. 22), is brought into communication with the other extremity (pole) of the primary coil, by the spring, H'. If it is wished that the intermissions should be made slowly, the knob L, (fig. 20), must be turned till the line traced upon it is directed transversely. If rapid intermissions are desired, the button, 1, must be turned from right to left until the bar, E, pushed forward by a platinized eccentric fixed to the button, is sufficiently near to the temporary magnet, J, which is in contact with the soft iron in the centre of the coil. The approximation is sufficient when the noise of the trembler can be heard. The movements of the trembler must be carefully regulated. Slow intermissions may also be produced with the pedal rheotome, y (already described with the large instrument). For this purpose its conducting wires must be attached to the knobs 3 and 4 (fig. 19), and then after having turned the button, L, from left to right, until the bar, E, is in contact with the magnet, J (in this position the battery current is interrupted), the intermissions are to be made with the foot in the manner already described. (By pressing on the knob, 6, fig. 21, the battery current is completed.) The electrode knobs, 1 and 2, receive the currents of each coil, and to them are fixed the conducting wires. To these knobs the

current either of the primary or secondary coil can be brought by pushing to the left or to the right, as far as it will go, the stem of the commutator of the coils, c. The figures engraved upon the small plate of copper traversed by this stem, point out the side towards which the stem should be pushed in order to bring one or other of the currents to the knobs 1 and 2. The graduation is effected by the tube, D, as in the other instruments. In the intervals between the applications the battery current is interrupted by pushing back the stem of the rheotome, L, until the bar, E, is in contact with the soft iron, J.

The power of this small instrument is very considerable, regard being had to the small size of its coil. This is not only due to the excellent proportions of length and diameter of its wires, and to good manufacture, but also, and chiefly, to the powerful magnetization of its band of soft iron rolled into a helix, which thus offers a considerable extent of surface.

Duchenne's Magneto-faradic Instrument.—This consists of a magnet; of an armature set in movement by a peculiar mechanism; of a regulator of the armature that is at once a moderator of the currents and a magnetic tensor ; of two coils of copper wire of unequal length and thickness; of a rheotome; of a regulator of the intermissions; of a


graduator of the currents; and of a commutator of the poles (fig. 23).

The magnet is formed of two parallel cylindrical branches, connected together at one of their extremities by a transverse bar of soft iron. The armature which, by its movement of rotation, produces the intermissions of the magnetic current, is traversed in its centre by a horizontal axis terminated at its extremities by a very hard steel point, received in steel sockets, which are screwed into two copper mounts. These mounts are firmly fixed to a square movable plate of copper (a, fig. 23), which rests on the base of the apparatus. Between these two mounts, and at their superior extremity, is fitted a large wheel, A, the axis of which traverses on one side the anterior mount in which it rotates, while the other extremity terminates in a point, and is received in a piece fixed to the posterior mount. The handle, m, which puts the large wheel in motion, can be removed at pleasure. The circumference of this large wheel is divided into sixty-four teeth, which set in action a small wheel of eight teeth, fixed upon the axis of the armature in such a manner that at every revolution of the great wheel the armature turns eight times upon its axis, and consequently produces thirty-two intermissions of the current of induction. As it is possible to make the larger wheel revolve twice in a second, we may obtain sixty-four intermissions in that time. The

movable plate, G, is brought near to, or removed from, the magnet by means of a strong screw, N, called the regulator of the armature. This screw turning in a nut fixed to the instrument, acts upon the base of one of the mounts which forms part of the plate, G, which it moves backwards and forwards. In order that the armature may exercise tension

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Duchenne's Magneto-faradic Instrument. upon the magnet, the screw, n, is so turned that the needle is brought to the centre of the arc of a circle, 0, which is called the indicator. The superposed coils are similar to those of the Volta-faradic instrument. The end of the one is soldered to the beginning of the other, while the other ends are brought—that of the thick wire to the springs, s

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and s', which produce the intermissions on the reel, B; and that of the fine wire to the right side, u, of the commutator of the coils, t, which conducts the current of the primary coil to the knobs, P and P, to which are attached the conductors of the rheophores. The spring, s', gives origin to a conducting wire which proceeds to the left side of the commutator, T. The rheotome is composed of a small wooden reel, B, and of two springs, s and s'. The reel is fixed upon an axis of soft iron. One of the springs, s, in relation with one end of the central wire, rests upon a metallic ring fixed upon the reel. This ring is divided into four teeth, two of which are very short. The second spring, s', which communicates with the other end of the central wire, is brought into contact with the four, or with the two teeth, by means of the regulating knob of the commutator (not shown in fig. 23), which is fixed to the right of the instrument, and which rotates from right to left, and vice versâ. The intermissions are thus regulated A piece of copper is fixed to the base of the instrument, to the left of the large wheel It is traversed by a screw, D, to which is soldered a brass spring, 1. By means of the screw, D, the spring, I, can be made to perform a to-and-fro movement, which brings it in contact sometimes with the pins a, upon the posterior face of the great wheel, A, and sometimes with the plate, G, which supports the latter. One of the extremities

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