Fig. 10.—Holden-d’Arsonval Mirror Galvanometer.
[Fig. 10] represents a mirror galvanometer working on the d’Arsonval principle, designed by Gen. Holden, F.R.S. The horse-shoe magnet is laminated, and an iron core, supported by a pillar, is placed between the poles. The coil, which moves in the space between the core and the poles of the magnet, is suspended by a thin, flat strip of phosphor-bronze, which carries a small circular mirror. A similar phosphor-bronze strip is fastened to the lower part of the coil, and is continued to an adjusting-screw in the base. The ends of the suspension strips communicate with the terminals of the galvanometer, and a current entering at one terminal passes through the metallic suspensions and the coil to the other. The effect of passing a current through the coil, which is located in a powerful magnetic field is to produce an axial movement tending to twist the suspension strips, which movement is greatly magnified by a spot of light reflected from the mirror on to a distant scale. When the current ceases, the untwisting of the strip restores the coil to its former position. Galvanometers of this type are remarkably “dead-beat” in action, that is, the movement and restoration of the coil are accomplished without vibration. A semi-transparent scale, placed at 1 metre distance, and 50 centimetres long, is suitable for use with this galvanometer. When used in workshops, it is necessary to protect a mirror galvanometer from the vibrations produced by machinery, which would cause the spot of light to become unsteady. The best method of effecting this is shown in [fig. 11], which represents the mode of suspension devised by W. J. Lambert for use in the Royal Gun Factory, Woolwich Arsenal. The usual supports of the galvanometer are abolished, and the instrument suspended from the ring of a brass tripod, so as to keep three springs partly in compression. When suspended in this manner, a mirror galvanometer is quite suited to commercial use; in the quiet of the laboratory the ordinary supports may be employed. The advantage gained by using the mirror type is that a much longer scale is possible than with instruments furnished with a pointer, and hence greater accuracy in determining temperature readings may be secured.
Fig. 11.—Lambert’s Anti-vibration Stand for Galvanometers.
