A <a href="https://en.wikipedia.org/wiki/selenium_rectifier" class="extiw" title="w:selenium rectifier">selenium rectifier</a>. These were used from the 30s to 50s until they were replaced by silicon diodes.
<a href="//www.flickr.com/people/36473423@N02" class="extiw" title="flickruser:36473423@N02">Nick Ames</a>
CC BY-SA 2.0
<a href="//commons.wikimedia.org/wiki/Flickr" class="mw-redirect" title="Flickr">Flickr</a>: <a rel="nofollow" class="external text" href="https://www.flickr.com/photos/36473423@N02/3756304358">Mallory Selenium Rectifier</a>