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FIG. 149, II.

(3) A means of varying slightly the directive force of the control, and still ensuring that it remains constant afterwards.

(4) A means of indicating the motions of the needle accurately. The electrometer, which is largely used for insulation resistance and other testing in connection with cable work, etc., is shown in perspective in Figs. 150 and 149, I., and in sectional elevation in

FIG. 150

Fig. 149, II. The body of the instrument consists of an inverted glass shade, carried in an outer brass framework provided with a number of rectangular apertures, and supported on three ebonite levelling screws. This inverted glass shade, or bell jar, is coated externally with strips of tin-foil in such a way as to leave openings or windows at intervals through which the interior can be seen.

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only keeps the interior of the jar dry, owing to its avidity for moisture, but, from its being a conductor, it forms the other coating of the Leyden jar.

The top of the jar is closed by a flat metal lid, which we shall term the "main cover" (vide Fig. 149, II.), and which carries the whole mechanism of the instrument. Visible on the top of this main cover (vide Figs. 149 and 150) is an ebonite micrometer head, which when turned causes one of the quadrant boxes to close up to or recede from the others. This head, which is finely graduated on its periphery, rotates past a fixed index mark, and is seen slightly to the left of the front. On the right of Fig. 149, I. and II., can be seen the electrode or terminal of the induction plate, which will be referred to again later on.

The head of the replenisher is seen on the left (Figs. 150 and 149, I.). A circular spirit-level, seen most clearly in Fig. 150 to the front, is for the purpose of carefully levelling the instrument, so that the needle hangs freely, and always in the same position relatively to the quadrants.

The two similar-looking terminals (Fig. 150), seen above the segment-shaped glass top at the back of it, are the two electrodes of the two pairs of quadrants, and that by the side of them, to the right, is the charging electrode for charging the needle. The former can by

raising them be disconnected from the quadrants. The erection in the centre of the main cover is called the lantern, and has as seen a segment-shaped glass top and a flat glass side (at the back) not seen. Inside this is an attracting plate supported by a glass stem (Fig. 149, II.), which carries the mirror and needle by a bifilar suspension.

In the uppermost portion of the lantern is the gauge, by means of which the potential of the needle is regulated, and which is really an attracted disc electrometer.

The four quadrants and the induction plate above one of them are supported from the under side of the main cover by glass stems. The needle previously mentioned is of the shape there given, and is about 4 square cms. in area, and weighs about grm.

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P

h

The Gauge is shown more in detail in Fig. 151, I. Above the metal attracting plate just mentioned, in the upper part of the lantern, which plate is connected with the needle electrically, and hence with the inside of the Leyden jar, but insulated from all other parts of the instrument, is an arm, h, turning on a horizontal axis, f, consisting of a stretched platinum wire, f, supported by two pins or bridges, bb. The lighter end consists of a square plate, P, of thin sheet aluminium, which lies parallel to the attracting plate when the arm / is horizontal. This last-named plate charges P inductively, but with the opposite kind of electricity, and therefore attracts it. The heavier end of the arm ƒ terminates in a horizontal fork, K, across which a fine hair is stretched. A white enamel index fixed to the stand, and having two dots on it, projects up through the fork, and both this and the hair, which plays in front of them, are observed through a lens, L, just in front, and seen in the figures. The arm and dots at the onset are so adjusted that when the needle is properly charged, the hair appears exactly midway between the dots. If the needle is overcharged, P is attracted further down, and closer to the plate under it, and the hair rises; if, however, the needle is insufficiently charged, the hair falls.

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FIG. 151, I.

Fig. 151, II., shows the mirror and its bifilar suspension, together with the circular attracting disc at the top.

The Suspension, which is bifilar, consists of two silk fibres, the upper ends of which are fixed to and wound round two pins, c and d. These may be turned slightly in their sockets by a square pointed key seen hanging at the edge of the main cover (Fig. 149, I.); and in this way they equalize the tensions on the two fibres, and at the same time adjust the height of the needle so that it is midway between the upper and lower surfaces of the quadrants. a and b are two screw pins pivoted in blocks carried by the spring strips e and ƒ. When, therefore, a and b are turned by a key clockwise, they butt up against a fixed plate at the back of e and ƒ, and throw forward the suspending pin c and d, and the reverse is the case when they are screwed out.

In this way the needle can be given a slight turn until it takes the position of symmetry (Fig. 148, II.), the quadrants at the time being short-circuited and earthed. h is a conical pin which when pressed in forces the plates to which c and d are separately fixed apart, and so diminishes the sensibility of the needle to a deflecting force. Any adjustment of the distance between c and d can by this means be effected from inch to inch. The lower ends of the fibres are

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attached to a cross or T-piece formed on the upper end of a light but stiff platinum wire, to which the mirror and needle are rigidly fixed.

The "Replenisher," which was devised by Lord Kelvin, is for the purpose of maintaining the charge of the needle; in other words, to supply what portions of the charge are lost by leakage, and thus maintain the hair of the gauge midway between the dots.

It is an accumulating influence machine which builds up potential on what is usually termed "the compound interest law." Fig. 151, III., shows this Thomson replenisher in perspective, and Fig. 152 symbolically in plan. Two brass carriers, C and D, are fixed eccentrically to an ebonite arm, carried by the vertical ebonite spindle E (Fig. 151, III.), which can be rotated by the milled ebonite head M. A and B are two inductors, and S, S' two springs connected by a wire, M (Fig. 152), and which touch C and D as they rotate. In

Fig. 151, III., S, S' are connected by the brass strip M, which passes round the edge of the ebonite centre plate P, which carries the lower bearing of E. Fig. 151, III., shows, without need of further explanation, how the various parts are disposed and supported.

The action is as follows: When the carriers C, D simultaneously touch S, S', any charges they contained neutralize, and the next instant they are acted on inductively by the charges in the inductors A and B ; and on touching, s, s' part with their charges to A and B, and in this way the charges on A and B rapidly accumulate.

In practice there is always a sufficiently large P.D. between A and B to start the action on turning the spindle E. To avoid, however, A and B losing the greater part of their charge, when the replenisher

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is not in use, by touching C and D accidentally, the milled head M is prevented from turning by a pin fixed to the head H, catching a hole in M when H is turned to a certain position. When H is again turned slightly, the pin disengages from M, which is therefore free to revolve. His lightly locked in one or other of two positions by a spring, K, pressing on a flat formed on H. In one position H locks M in turn, and in the other it does not.

With the bell jar constructed of good glass, carefully washed with distilled water and dried thoroughly before the pure sulphuric acid is put in, the Leyden jar can be made to insulate so well as not to lose so much as I per cent. of its charge in the course of 24 hours, and quite a few turns of the replenisher will supply a much greater loss than this usually in a few moments.

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