The mercury column resistance standard of Werner von Siemens, ca. 1860. This resistance is known as the <a href="https://en.wikipedia.org/wiki/Siemens_mercury_unit" class="extiw" title="en:Siemens mercury unit">Siemens mercury unit</a> and is a little less than the modern ohm. (Munich 2015)
<a href="//commons.wikimedia.org/wiki/User:Reinraum" title="User:Reinraum">Reinraum</a>
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