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oblong wooden case, with a glass front. It is intended for workshop or similar use, where approximate results only are needed, and to that end is provided with a pointer or index in addition to a mirror, the said index travelling over a vertical semicircular scale. An appreciable deflection of the index is obtained with a current of 1.6 x 10 6 ampère, passing through the suspended coil.
Their next simplest form is illustrated below, and is very similar to that of Messrs. Nalder Bros., which has been already described and illustrated. It consists of the usual vertical horseshoe magnet, between the upturned poles of which is suspended the coil, which may be of any resistance from 10 to 1,000 ohms, according to the requirements of the customer. The suspensions consist
D'Arsonval Galvanometer, by Elliott Bros. of fine phosphor bronze strips above and below the coil; a plane or focus mirror is provided, and the instrument is mounted on a mahogany base fitted with the usual levelling screws and provided with the cylindrical brass cover shown in the illustration.
A somewhat more elaborate type of instrument is provided in Professor Rowland's form of D'Arsonval galvanometer, which is shown in the accompanying illustration. This instrument is more sensitive than the foregoing types in that it possesses a somewhat longer suspension. It is provided with a lifting attachment to receive the weight of the coil, and so protect the suspension from injury when the instrument is packed for transit. At a scale
D'Arsonval Galvanometer (Rowland's), by Elliott Bros.
range of one metre this instrument has a constant of one millimetre scale division, with a deflective current of 1x 10.8 ampère through its coil.
Another type of D'Arsonval galvanometer manufactured
by Messrs. Elliott Bros. for cable testing work is depicted in the accompanying illustration. It consists of a substantial metal frame, mounted on a broad base, with or without levelling screws (they are shown in the figure). The moving coil is mounted in a separate frame supported by the poles of the magnet and removable therefrom. The suspensions consist of a fine strip above and a spiral strip below, giving greater sensibility; the connecting wires to the top and bottom suspensions are secured with good electrical contact by screws, and are easily detachable when it is desired to remove the fraine carrying the suspension. Spare suspended coils of varying resistances may be obtained for this instrument, in frames complete, and are easily placed in position, being interchangeable ; the ordinary stock pattern has a resistance of 1,000 ohms.
A feature of this instrument lies in the small glass window provided in the front of the case for the passage of the beam of light to and from the mirror. This window, as will be seen in the figure, is given a slight upward tilt, such that the light reflected from the surface of the glass itself shall not, as it frequently does in the ordinary form of vertical window, interfere with the actual reflected beam, and so destroy the definition of the image due to the latter.
Still another type of D'Arsonval instrument, complete with scale, is manufactured by this firm, and is also illustrated. It is designed with a view to portability, and to that end, the usual form of scale and lamp is dispensed with, being replaced by the form of scale shown, in which the deflections are read through the sighting hole just above the scale. The device is sensitive and self-contained, and can be fixed either against a vertical support or on a horizontal table or basis as required.
Besides the dead-beat properties already alluded to, D'Arsonval galvanometers also possess the additional advantage of being uninfluenced by the proximity of masses of magnetic material, such as dynamos, etc., which renders them eminently suitable for engine-room work and similar purposes involving the neighbourhood of otherwise disturbing influences.
Whilst on the subject of magnetic disturbances, the writer will proceed to deal briefly with the systems of magnetic control and shielding adopted when using the
Referring to previous illustrations, it will be noticed that the instruments in question are provided with an ordinary bar magnet fitted to, and capable of adjustment on, a vertical stem rising from the centre of the upper portion of the galvanometer case. This bar magnet performs a useful office, in that it serves, when properly adjusted, to counteract any effects upon the needle, due to the earth's magnetic field; the sensibility of the instrument is, in act, to a considerable extent, controlled by it. If the magnet be so placed as to approximate as nearly as possible to the suspended magnets inside the case, it will exercise a considerable influence upon them, and their sensibility to motion under the influence of a current passing round the coils will in consequence be diminished. if, on the other hand, the magnet be adjusted at the opposite extremity of the supporting stem, the earth’s magnetism (in the case of single-needle instruments) comes into play, and the galvanometer must, in consequence, be placed with its needles in the magnetic meridian, a condition of things not altogether convenient. In order to procure the best possible conditions for working, the controlling magnet should, by dint of experiment, be so adjusted that its magnetic field just overcomes that due to the earth ; when this is the case, the suspended system can be brought to zero by gently turning the magnet in the required direction, whilst still in the same horizontal plane, i.e., without altering its vertical position on the supporting stem, an operation facilitated in modern types by the addition of a milled screw and pinion.
As regards magnetic shielding; in cases where it is necessary to employ a reflecting galvanometer of the first type in the neighbourhood of masses of magnetic material, it must be shielded from disturbing influences by the addition of a surrounding iron case, which need not be of excessive thickness, from in to fin. being ample under ordinary circumstances. An oblong opening closed by a thin sheet of glass, or, better still, clear mica, is provided in the surrounding shield for the passage of the beam of light, and the ordinary simple controlling magnet is replaced by two equal magnets mounted on separate telescopic and insulating axes in the interior of the case, either above or below the needles, controlled by convenient milled heads or turning-screws on the exterior. By approaching