Plug-resistor box, used in demonstrations of Wheatstone bridge in science education around 1900, on display in the <i>Schulhistorische Sammlung</i> (School Historical Museum), Bremerhaven, Germany. Shorting plugs <i>(not shown)</i> could be inserted in the holes to make up a wide range of resistance values.
<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