A galvanometer is a sensitive device which deflects when a current passes through it. The principle of the galvanometer is used in the construction of voltmeters and ammeters.
The galvanometer consists of a coil which is placed in a magnetic field. The coil deflects when a current is passed through it. The coil drives a pointer across a dial which indicates the value of the current 
or the voltage. However, when very small currents are to be measured, the current passing through the galvanometer will not be able to drive the pointer against the spring tension. The pointer may not even deflect for small currents.
Hence, to measure small currents a mirror galvanometer is used.
The mirror galvanometer consists of a coil which is suspended by means of silken threads. A small mirror is attached to the coil. The light from a source is made to fall on the mirror. The reflected light will fall on a screen. When the coil deflects, the mirror deflects with it. The light which is reflected also moves in the screen. The distance of the screen determines the sensitivity of the galvanometer. As the distance increases, the deflection of the reflected light even for a small current will be large. Thus, even low values can be measured with high precision. Even microamperes can be measured by the deflection galvanometer.
The mirror galvanometer is used in engineering, seismology, communication and in any application where high sensitivity is required in measurements.