If a reflecting method be used with a straight scale, the observed deflection depends upon the tangent of twice the angle of movement of the needle. For small deflections, however, the change of flux can be taken as directly proportional to the observed deflection.
Fig. 527.--Thompson galvanometer with mirror reflecting system for reading the deflections of a galvanometer needle by the movements of a spot of light reflected from a mirror attached to the needle or movable magnetic system.
Use of Mirrors in Galvanometers.--In order that small currents may be measured accurately, some means must be provided to easily read a small deflection of the needle. Accordingly, it is desirable that the pointer be very long so that a large number of scale divisions may correspond to small deflections. In construction, since sensitive galvanometers must be made with the moving parts of little weight, it would not do to use a long needle, hence a ray of light is used instead, which is reflected on a distant scale by a small mirror attached to the moving part.
In the Thompson mirror reflecting galvanometer, as shown in [fig. 528], a small vertical slit is cut in the lamp screen below the scale, and the ray of light from the lamp, passing through the slit, strikes the mirror which is about three feet distant, and which reflects the beam back to the scale. It should be noted that the angle between the original ray of light and the reflected ray is twice the angle of the deflection of the mirror; the deflections of the ray of light on the scale, however, are practically proportional to the strength of currents through the instrument. The mirror arrangement as shown in [fig. 528], requires a darkened room for its operation, but such is not necessary when a telescope is used as in [fig. 529]. Here the scale readings are reflected in the mirror and their value observed by the telescope without artificial light.
Fig. 528.--Telescope method of reading galvanometer deflections by reflection of scale reading in mirror. Here two mirrors are used, but in most cases the telescope is pointed directly toward the mirror on galvanometer shown in [fig. 527], because the two mirror system, as illustrated in the figure, is used on portable galvanometers since it is the more compact.
Damping.--This relates to the checking or reduction of oscillations. Thus, a galvanometer is said to be damped when so constructed that any oscillations of the pointer which may be started, rapidly die away. Galvanometers are frequently provided with damping devices for the purpose of annulling these oscillations, thus causing the moving part to assume its final position as quickly as possible.
Sometimes the instrument is fitted with a damping coil, or closed coil so arranged with respect to the moving system that the oscillations of the latter give rise to electric currents in the closed coil, whereby energy is dissipated. Again, air vanes are employed, but anything in the nature of solid friction cannot be used.
