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of the inner, and one of the outer, wires of the coil communicate with this plate, G. The brass spring, I, is in relation by a copper wire with the left side, u, of the commutator, T. The currents are graduated by two tubes of copper, H h, which glide over the reels, and can be pulled out or pushed in by the stem, R. When pushed home, the cylinders cover the reels, and the currents are at their minimum of intensity, and at their maximum when the stem is drawn completely out. The commutator of the coils has the same construction as in the Volta-faradic instrument.

In order to put the instrument in action the regulator of the armature, n, must be turned from left to right until the soft iron no longer comes in contact with the magnet during its rotatory movement. If the employment of a rapid current is desired, the regulator of intermissions, D, must be turned from right to left until it reaches its point of arrest. If, on the contrary, slow intermissions are desired, the same indicator, D, must be turned in the opposite direction, and stopped when its needle points to the number of intermissions which it is desired to obtain for each revolution of the large wheel. The handle, turned from left to right, should always be moved very quietly, making perhaps two revolutions in a second. In order to graduate the currents, it is sufficient to remember that when the stem, R, is pushed home, the current is at its mini

mum. If still more feeble doses are required, the armature must be moved further away from the magnet by turning the regulator, n, from left to right. The knob, T, of the commutator of the coils must be turned from right to left, when it is wished. to bring the currents of the primary coil to the knobs, P and p. On the contrary, the knob, t, is turned from left to right, to get the current of the secondary coil.

Stöhrer's Induction Instrument.—The battery of this instrument consists of carbon and zinc, without an earthenware cell. The carbon (fig. 24), hollow

within, filled with sand, and closed Fig. 24.

by a glass stopper, serves for the reception of a concentrated solution of chromic acid in water. Of this solution 10 or 12 drops should be added whenever the battery is recharged. The zinc surrounds the carbon, but is kept from contact by glass insulating buttons. These elements are placed in a circular glass cell, which serves for the reception of diluted sulphuric acid (one part

acid to seven parts water). This Stöhrer's Battery.

cell is so arranged in the completed apparatus, that it can be moved vertically up and down, and can be fixed at any point. By this arrangement the acid can be brought into contact with

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the whole, or with part of the zinc and carbon, or by shutting down the glass, can be excluded from them altogether. In the latter case, the acid will only fill the lower third of the glass. The advantages arising from being able to remove the

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elements at once, and without difficulty, from the exciting fluid, and from the facility with which they may be brought into action, are obvious.

Stöhrer constructs a smaller (fig. 25), and a larger (fig. 26) instrument. The battery of the former is constituted by a single cell of the latter, by two cells, which may, however, be arranged either as two pairs, or as a single pair of elements. Both possess a primary and a secondary coil, the

Fig. 26.

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Stöhrer's larger Induction Instrument. currents of each of which can be made use of separately; and in both the currents have a definite direction, positive electricity being set free at one terminal, and negative at the other, of each of the coils. The terminal from which the positive current proceeds may be ascertained easily by the decomposition of iodide of potassium. The larger instrument differs from the smaller in having a much greater range of power, more thorough means of graduating the currents, and a more elaborate arrangement of the interrupting hammer. With practice, however, the force and rate of interruption of the smaller instrument may be regulated with much nicety. To neither instrument is a water graduator attached, but if needed for any special nicety of application, one can readily be added, and it would be best carried loose in the drawer for accessories, to be attached only when required. Graduation of the strength of the currents is effected by the arrangement of the coils. The primary coil is fixed upon a pedestal; the secondary is movable, and is brought into and placed out of action by being lifted over or thrust away from the primary. The degree of action in the secondary coil is proportionate to the extent to which it is brought under the influence of the primary. The action of the primary coil is regulated in the smaller instrument by the extent to which it is masked by the secondary coil—the latter acting upon it as a metallic sheath would do. In the larger apparatus a special copper sheath is provided for the graduation of the current of the primary coil. It is to be regretted that a similar arrangement has not been adopted in the smaller instrument.

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