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Fig. 2 represents diagrammatically its internal construction.

A and B are two equal coils of wire wound in the same direction, to a resistance of about 50 ohms apiece, or 100 obms in all, on hollow bobbins, or “formers,” mounted on a suitable base plate, and enclosing between them a magnetic needle, N s, mounted on a delicately-pivoted horizontal axis. The needle, N S, is made slightly heavier at its (S) extremity, in order that it may, by force of gravity, always remain in the vertical position when when no current passes through the coils A B. The two coils are connected together, and their free extremities led out to suitable terminals, 1 and 2, on the exterior of the case, which is usually of wood. Attached to the horizontal axis in a plane with the needle N S is a pointer or index, not shown in the figure, which indicates the movements of NS consequent upon the passage of a current, upon an engraved dial plate mounted behind a protecting glass front, in one of the sides of the case.

Such an instrument as that described above, although somewhat crude, is nevertheless of great use in ordinary workshop tests, and is principally employed, as its name implies, by telegraph and telephone linesmen for testing the continuity of circuits and similar purposes. At best it is only on indicator.

A similar instrument of a slightly more sensitive type, in which the needle is pivoted horizontally instead of vertically, and which is mounted in a circular metal case with a glass top, is shown in the accompanying illustration. It is wound to a higher resistance than the foregoing, and

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Portable Galvanometer, about 1,000w, by Nalder Bros. & Co.

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is fitted in a leather case with accompanying shoulderstrap for the sake of portability.

The Tangent Galvanometer.—This galvanometer, though a very familiar one in text-book practice, is seldom used in general electric testing. It is, however,

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largely adopted in the Telegraph Department of the G.P.O. for testing purposes, and the standard Post Office pattern, as manufactured by Messrs. Elliott Bros., is depicted in the accompanying illustration.

In its simplest form it consists of a circular vertical coil A, Fig. 3, surrounding a delicately pivoted magnetic needle N S placed at the centre, and capable of motion in a horizontal plane. The needle N S must be very short (some } in. to 1 in. for a 6-in. coil), as,

theoretically, it stands in the place of a point pole, and, in consequence, if too long, erroneous results will be obtained. It depends in principle on the fact that the "strength of currents circulating in the coil are directly proportional to the tangents of the angles of deflection of the needle."

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An index finger is attached to the needle or pivot, and the scale is divided up into tangents; 45 degs. is the angle of maximum sensitiveness of this instrument, and it is therefore advisable, wherever possible, to obtain a deflection as near this value as possible.

The Post Office form of tangent galvanometer illustrated above is wound differentially, its respective windings having a resistance of 160 ohms apiece. The extremities of these windings are brought to the four terminals shown on the base, so that they can be connected in any desired manner. In addition to the above, seven shunts, having the respective values of 1-5th, 1-10th, 1-20th, 1-40th, 1-80th, 1-160th, and 1-320th, are fitted in the base of the instrument and connected to the multiple plug switch. Their terminals are connected to the outer terminals of the galvanometer winding, so that they can be connected across the two 160-ohm coils in series by depressing the key shown in the figure. When inserted in the order mentioned above, they reduce the sensitive ness of the instrument in direct proportion, and the ohmio resistances between the outside terminals from an original total of 320 ohms to 32, 16, 8, 4, 2, 1, and

ohm respectively.

We pass on now to a much more sensitive type of instrument, to wit, the reflecting galvanometer, the general construction of which, in its simplest form, is roughly indicated in Fig. 4, where a b represents the coil as before.

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The needle, N s, in this case takes the form of a short strip, or series of strips, of magnetised watch-spring, attached, with their like poles together, to the back of a small circular mirror, c, about 1 in. in diameter, by means of a suitable cement, such as shellac or beeswax (the latter is unsuitable if the instrument be intended for use in a warm room). The mirror is carried by a short vertical aluminium wire supported at its upper extremity from a single fibre of unspun cocoon silk which has been pre viously treated to remove all twists and kinks, and is in turn attached at its upper extremity to a stud or screw, d, fitted in the body of the instrument, and by means of which it can be raised or lowered in order to bring it to the centre of the coil. The complete instrument, in its practical form, is shown in the block below. In this case no actual index is attached to the moving system, its place being filled by a beam of light from a suitable source such as an oil lamp, or, better still, one of the electric incandescent variety, which is thrown on to the mirror c and reflected back on to a transparent scale, such as that illustrated below, the requisite definition being rendered by a black line in the shape of a fine blackened wire or hair stretched across the aperture of emission, and the indications being taken from the coincidence of this line with those of the divisions on the scale.

It will be seen from the above description that a much more sensitive indication is recorded by this type of instrument, in that the support for the moving system is frictionless, and the range of the index only limited by the confines of space and the intensity of the reflected beam of light.

Thomson Tripod Galvanometer, by Elliott Bros.

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In order to render this instrument independent of the earth's magnetism, and consequently more sensitive still, it has been constructed in astatic ” form, as indicated in Fig. 5, where ab and e f represent two coils, sc connected as to exercise a similar comboned influence upon the system of magnets N S, which is doubled in this type of instrument, one set being attached as before to the back of the mirror c, and the other, an equal and similar set, with their N and S poles in an exactly opposite direction to those of the foregoing are attached at a negligible distance

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