This Leeds and Northrup galvanometer was used to detect and measure very small currents before the advent of digital ammeters. The current, passing through a coil on which a mirror is attached, induces a magnetic field which causes the mirror to deflect. The direction of deflection, as well as the extent of the deflection, indicates the polarity and size of the voltage applied. Low resistance shunts, one of which can be seen in the left-hand photo, could be used to vary the sensitivity of the instrument. These would be attached across the terminals seen in the right-hand photo. The unit shown was manufactured by Leeds and Northrup in Philadelphia, USA.
