Mirror galvanometer

This astatical mirror galvanometer measures current strength. The operating principle behind a mirror galvanometer is that a small mirror is rotated in proportion to the strength of the interaction between permanent magnets and coils through which the current to be measured flows. The astatical version is not affected by outside influences, such as the Earth's magnetic field. This is because the device has a double arrangement: rather than just one, there are two sets of magnets placed in the field produced by two coils, and in such a way that any external influences affect them equally yet in opposite directions so they cancel each other out. The principle of the mirror galvanometer was patented by William Thomson in 1857.
This galvanometer has an upright brass tube on a brass pedestal with two black, drum-shaped boxes. Between them is an opening covered with glass, through which the spinning mirror can be seen.