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who have been indefatigable in their efforts to assist in the compilation.
In concluding the series, the writer ventures to hope that its contents will prove of practical utility to the practical man, for whom they are mainly written, and that their present compilation in book form will serve to fill a vacant space in the library of such members of the electrical profession as require a brief but practical treatise on the various methods and applications of electrical testing, without the concomitant theoretical and mathematical proofs which usually accompany kindred works on the subject.
Great care has been exercised in the compilation of this series to render all formulæ, etc., correctly, and the author will esteem it a favour if those of his readers who may note errors in the subsequent context will, by drawing his attention thereto as promptly as possible, assist in the eradication of such errors from future issues.
THE PRACTICAL ELEMENTS OF
INSTRUMENTS AND APPARATUS. Galvanometers. The most essential unit in an electrical testing outfit of almost any description is undoubtedly the galvanometer, an instrument which is constructed in many varied forms, and whose indications, modified by the local conditions under which it is working, are taken as visible records of the results of the experiment or test in which it performs its office.
The principle underlying the action of all galvanometers is as follows:-A magnetic needle, N S, Fig. 1, is suitably supported at the centre of a coil, a b, which may consist
of one or many turns of wire, in such a manner that, when in its normal condition, i.e., when in the earth's magnetic meridian, it assumes the condition shown in the figure, it lies in a plane with the coil. If, however, an electric current be passed round the coil a b in either direction, the needle, N S, following a well-known law, tends to set itself at right angles to the plane of the coil, and the measure of this tendency, as indicated by suitable means,
is an indication of the current passing through the coil ab, either directly or indirectly, according to the secondary principles of the particular form of galvanometer under consideration. In some types the action is reversed, the needle in the form of a permanent magnet or otherwise) being fixed and the coil movable, but, in either case, the fundamental principle remains the same.
One of the simplest types of galvanometer used in practical testing, and that only for very rough indications of the existence of a current, as in simple continuity tests, for example, is the ordinary linesman's “detector, which is made up very simply with a view to portability and rough handling.
Linesman's Detector Galvanometer, by the India Rubber, Gutta.
Percha, and Telegraph Works Co., Ltd., Silvestown,