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below, to the face of a small aluminium vane, g, which oscillates with the mirror and aluminium wire support, and

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Lamp Stand, with Transparent Galvanometer Scale,

by Nalder Bros.

is closely surrounded by a small similarly-shaped chamber, closed, or nearly closed, on all sides, the interior surfaces

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of which only just clear the edges of the vane g. This latter device constitutes what is known as a damping” box, i.e., it “damps” or deadens the otherwise irresponsible oscillations of the suspended system, and reduces them to a series of steady and regular swings with a definite period, a result which renders the reading of the resultant deflections on the scale a much less arduous task than would otherwise be the case. This effect is the result of a small enclosed volume of air in the surrounding chamber which renders any rapid movement of the vane g an absolute impossibility under the circumstances. The device may be rendered adjustable by mounting the front wall of the damping chamber on a movable slide, by which means it can be adjusted with regard to its distance from the other walls of the chamber; by this means any requisite degree of exit for the otherwise enclosed air inay be obtained, and the consequent damping effect on the system regulated accordingly.

The usual procedure in constructing an instrument of the latter type is to arrange the two sets of coils a b and ef in pairs, making four in all, the eight extremities of which are brought to terminals on the exterior of the base, and can be connected up as desired, in order to vary the total resistance of the instrument windings for various purposes, or to render it available for use differentially."

The coils are mounted on metal frames, hinged to the main vertical supports of the galvanometer, so that they can be turned back out of the way in the event of any necessary repairs to the suspended system, such as that caused by the breaking of a silk fibre, etc.

The term “differentially,” which was employed in the preceding paragraph, requires some explanation. Referring back to Fig. 5, it will be recognised, from what has gone before, that if equal and similar currents be simultaneously passed round the two coils a b and e f in the same direction, i.e., from a to b and from e to f their respective effects upon the two

gnetic systems N S will neutralise one another, and no movement of the system will take place. If, however, the current through one of the coils preponderates slightly over that through the other, a slight movement, due to the excess of current, will result, and it will ultimately be shown how, by connecting one coil in a certain circuit and another in a different part of the same or similar circuits, this fact is utilised in certain tests, the system thus made use of being termed differential.

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Lord Kelvin's High-Resistance Four-Coil Galvanometer,

by Nalder Bros.

Reverting to the original theme, we now come to a consideration of that class of instrument known as deadbeat," i.e., those which record their indications directly, without any concomitant oscillation above or below the resultant division on the scale. Of this class, the d'Arsonval galvanometer, illustrated in the accompanying block, was the pioneer. It is constructed on the moving coil principle previously referred to, the magnet (permanent) being fixed, and the coil suspended; for this reason it is necessarily less sensitive than the foregoing types of reflecting galvanometer, in which the simple magnetic needle constitutes the moving unit, but this comparatively small disadvantage is amply compensated for by the attendant advantages offered by its dead-beat qualities, especially when, as is very often the case, time is an object while conducting certain tests.

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D'Arsonval Type Reflecting Galvanometer, by Nalder Bros.

Fig. 6 illustrates roughly the detailed construction of the d’Arsonval galvanometer depicted in toto in the above

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illustration. NS is a set of three strong permanent magnets, bolted with their like poles together. a and 6 are fine silver suspension wires, leading the current to, and, at the same time, acting in the capacity previously filled by the single silk fibre, to the coil e, which is extremely light and is wound on a closed metal frame of very thin silver or copper, the induced currents in which, set up by the originating current through the coil e, exercise a reactive effect upon the system, and cause it to rapidly assume a position of rest after having been set in motion by the original current previously referred to, or, in other words, the current which the instrument is required to indicate. c is the attached mirror as before, whilst d is a second permanent magnet cylindrical in shape, and constructed of horizontal laminæ. It is arranged with its North pole opposite the South pole of the original horse-shoe magnet Ñ S, and, by this means, a very intense magnetic field is set up in the small space between the two magnets, which space is occupied with just sufficient turning clearance by the suspended coil e. f is a small spring with adjustingscrew, to which the lower suspension wire b is attached, and the moving system thus maintained in sufficient tension to produce equilibrium, and at the same time freedom of motion for the working coil. The ordinary patterns of this type of instrument are wound to varying resistances from 150 to 750 ohms, according to the purpose for which they are ultimately required.

The following brief description of one or two of the leading types of this instrument which are at present on the market will perhaps assist the reader in his choice of an instrument suited to the special requirements of any tests which he may be called upon to perform by its aid.

The practical working principle of the D'Arsonval galvanometer pure and simple has already been dealt with (see Fig. 6 and accompanying illustration), so that I shall not touch further upon it at this point, but proceed to deal instead with some of the more important details which occur in connection with the various patterns of nstrument to be described.

The illustration accompanying Fig. 6 of this series represents the form of D'Arsonval galvanometer manufactured and sold by Messrs. Nalder Bros. and Co., of Westminster, and it is supplied either“ damped.” "undamped," as required-i.e., it may be either dead beat or ballistic.

For the benefit of the uninitiated, I may here state that the damping or dead beat properties in a D'Arsonval

or

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