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fectly easy to obtain intermissions separated by any desired interval..
When the graduator, B, is entirely contained within the instrument, the currents are at the minimum of intensity; and in order to increase them, it is only necessary to draw out this tube. The instrument being in action the conductors must be fixed to the knobs, P and Q, to which is brought the current of either the primary or secondary coil by turning to the right or the left the needle of the commutator. If it is proposed to measure infinitely weak doses the upper part, k, of the moderator, F, must be connected with one of the knobs, P or Q, and one of the conductors with the lower part of the moderator, I. Then the more the stem, J, of the moderator is drawn up, the greater will be the thickness of the water traversed by the current, and the more the latter will be weakened. The current may then be divided and measured by the tube graduator, B. In the intervals between faradization, the current is interrupted by turning back the screw, E, so as to preserve the force of the battery. At the same time plates of hard caoutchouc must be placed between the cloth and the zinc, care being taken to wipe the latter if the instrument will not be required again for some hours. A battery thus charged will last for two or three weeks, and may be used for several hours a day, without any other care than to renew the moisture of the cloth, and
to move a little every day, before use, the paste of mercury upon the carbon. It is also desirable to maintain the platinum connections in a state of cleanliness. When the mercurial paste is quite decomposed (its colour originally yellow will then be green), the carbon must be washed, and the battery charged with fresh bisulphate as before.
Duchenne also adapts to his uncovered instrument a special trembler, whose beats may be quickened or retarded at pleasure. It is shown in fig. 16, a front view of this instrument. One of the
extremities of the wire of Fig. 16.
the primary coil communicates with the knob, E, the other with the screw, c. The movable piece of copper, A, which is pressed back by a spring placed in front of it against the screw, c, communicates with the knob, F. As soon as
the knobs, E and F, are BB M placed
placed in communication with the poles of a battery,
the current passes through Front view of the instrument shown in fig. 14.
the wire of the primary
coil, and magnetizes its core of soft iron, which attracts the movable piece of soft iron, G. This draws with it the piece of
copper, A, which was resting against the screw, c. A solution of continuity is produced, demagnetizing the core, and causing the return of a to c. The same order of events continues, the current being re-established. When the screw, c, is tightened, the spring which presses the piece, A, against it, is sufficiently tense to cause the intermissions to occur without vibrations; but when the spring is relaxed by loosening the screw, each beat or intermission is followed by a vibration such that its slowest action (four intermissions per second) produces muscular contractions that are painful, although infrequent. These slow intermissions are sometimes highly useful. In the trembler the point of the screw is platinized and comes in contact with a plate also platinized, and soldered to the piece which beats against it, so that perfect contact is as little as possible interfered with by oxydation. In time, however, the platinum burnt by the electric spark does undergo oxydation; and the oxyde deposited on its surface checks the passage of the current and weakens the power of the instrument, the play of the trembler being either hindered or altogether stopped. This fault has been remedied by increasing the thickness of the piece of platinum, and by making it movable, so that when after working some hours a day for many months, it becomes oxydized, it may be shifted a little to the left or right, and a new place brought
into use. Clean also the point of the screw from time to time, by rubbing it with a piece of fine emery paper, to remove the oxyde.
Duchenne's small Volta-faradic Instrument, is a reduction of the instrument just described. It has the form of a flat oblong box (fig. 17). When it is open, as in fig. 19, it is seen to be divided into two parts. To the right is placed the induction instrument, properly so called. To the left is the
(closed). battery in the lower compartment, A, and the electrodes and their conductors are in the upper compartment B. The induction apparatus is composed of the two superposed coils, the wires of which are of different length and diameter, and of a core formed by a broad band of soft iron, rolled into a helix, as in figure 18, so that it can be