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and negative poles of the battery enclosed in the drawers u and u'. The finer and longer wire is rolled round the thicker and shorter coil and forms the secondary coil. Its ends terminate at the two springs of the commutator of the coils (E, fig. 14). This commutator is for the purpose of transmitting rapidly and alternately the current either of the primary or secondary coil to the conductors attached to the knobs P and Q, (fig. 11), according as the needle (F, fig. 14) is turned to the right or left, as is shown on a plate situated above the needle. The battery which works the instrument is composed of three pairs of elements contained in the drawers u and u', two pairs in the upper and one in the lower. Each pair consists of a carbon plate (c d', fig. 12) fixed to a cell of hard caoutchouc, and of a
zinc plate, z z, of the same surface as the carbon and separated from it by a cloth diaphragm. The platinum wires which form the carbon contacts are arranged as in the smaller instrument (fig. 17, page 46), while the zinc contacts differ in each of the pairs :-1. In the lower drawer (u, fig. 11) a strip of iron, (b, fig. 13), riveted and soldered to the zinc, 2, is bent upwards at a right angle at its anterior extremity, in such a way that, when the drawer is closed, the iron comes in contact with a small plate of platinum fixed in the front of the apparatus on a level with the knob, L. 2. In the further compartment of the upper drawer, the zinc (z, fig. 12), is also prolonged by a strip of iron, b. This strip is also bent upwards at a right angle at its anterior extremity, and when the drawer is closed can be brought in contact with another small plate of platinum fixed in front of the apparatus on a level with the knob, G. 3. Lastly the zinc (z', fig 12), of the pair in the front compartment of the upper drawer rests on a platinum wire which winds over the posterior wall of the cell, and comes in contact with a double spring, a, fixed to the partition of the drawer, which spring in the further compartment rests by a platinized surface upon the carbon, c'. The left-side drawer is absent in the figure, in order to display the arrangement of the parts forming the two pairs. The graduator (B, fig. 11) is a cylinder of copper, which surrounds the coils, and which has a scale marked upon its upper part. The knob, B', fixed to its extremity, is a handle by which it can be drawn out or pushed back.
The moderator is a glass tube (F, fig. 11), terminating below in a metallic base, to which is attached a knob, I, and above is a collar, K, from which proceeds a hook, which serves to connect the moderator with one of the knobs, P, which receive the conducting wires to the electrodes, and in which the poles of the coils terminate. In the collar is a small opening traversed by the stem of the moderator, J. The tube is filled with water.
The trembler is composed of a piece of soft iron (A, fig. 11), and of a platinized screw, s, against which the soft iron is pressed by a small spring. The pedal (1, fig. 11) is to allow slow intermissions to be produced with the foot, an arrangement which leaves the hands of the operator free, either to hold the conductors or to gradulate the currents.
Duchenne's large Uncovered Apparatus (fig. 14) differs from that just described in the following respects :-1. It has no external covering, so that the arrangement of the coil A, and the movement of the cylinder B over it, may be seen. 2. It possesses two graduator tubes ; one, B, which acts upon the secondary coil; and another, C, upon the primary. 3. Its core of soft iron, D, is movable, and may be withdrawn so that we may study the influence of the tubes B and c, independently of temporary magnetization. 4. Besides the commutator of the coils, E, it possesses a commutator of the poles, H, forming part of its structure, and by which the directions of the currents may be rapidly changed without displacing the conductors. 5. The trembler is so constructed that the rapidity of the intermissions can be progressively increased from four or eight in the second, to an almost incalculable number in the same period of time.
Fig. 15. Fig. 14.-Large uncovered Volta-Faradic Instrument. Fig. 15.-Bundle
of soft iron wire.
To charge the battery water must be poured over the carbon until it is well saturated; then spread over its surface bisulphate of mercury in quantity sufficient to form a thin layer. Place upon this layer the cloth diaphragm well wetted, and over this the plate of amalgamated zinc, which then comes in contact with the platinum wires. Thus prepared, the elements are placed in their compartments, which are distinguished by numbers marked on the sides of the caoutchouc cells. Lastly, the drawers (U U', fig. 11) must be shut, and the button, A', turned in such a manner that its bar assumes a vertical direction and prevents them from opening. The communications of the elements of the battery with each other, and with the circuit of the primary coil, are then established. The knob, L', must be turned from left to right, and the knobs, E, G, L, N, screwed into contact with their platinum connections.
If it be desired to obtain rapid intermissions of the trembler (A, fig. 11), turn from right to left the knob c, which fixes the movable plate, and then turn from left to right the buttons E, G, L, N, so as to bring them into contact with the pieces of platinum to which they correspond. When this is done the plate oscillates rapidly between the screw, s, and the temporary magnet of the central coil. If intermissions, more or less separated from one another are desired, the arrangement is made as before, except that the button, c, is turned from left to right so as to render the plate, A, of the trembler immovable, and the button, L, from left to right so as to separate it from its platinum connection. Then connect with this button one of the conductors fixed to one of the knobs, I, of the pedal rheotome, y, and the other conductor from the knob, l' of the rheotome, is attached to the button, I'. It is then only necessary to govern with the foot the spring contact, 2, of the pedal rheotome, in order to make and break contact at pleasure. It is therefore per