Mirror galvanometer, used in physics education probably around 1900, on display in the <i>Schulhistorische Sammlung</i> (School Historical Museum), Bremerhaven, Germany.  It was used to measure small currents.  Wires from the current source were attached to the binding posts at either side of the base.  The current passed through the wire coil and caused a magnetized needle suspended from a fiber in the vertical column to turn.  This also turned a mirror attached to the fiber.  The current was measured by the deflection of a beam of light reflected from the mirror and striking a graduated scale <i>(not shown)</i>.
<a href="//commons.wikimedia.org/wiki/User:Hgrobe" title="User:Hgrobe">Hannes Grobe</a> (<a href="//commons.wikimedia.org/wiki/User_talk:Hgrobe" title="User talk:Hgrobe"><span class="signature-talk">talk</span></a>)
CC BY 3.0
own work, Schulhistorische Sammlung Bremerhaven