Galena <a href="https://en.wikipedia.org/wiki/cat%27s_whisker_detector" class="extiw" title="wikipedia:cat's whisker detector">cat's whisker detector</a> from an antique <a href="https://en.wikipedia.org/wiki/crystal_radio" class="extiw" title="wikipedia:crystal radio">crystal radio</a>.  This antique radio component consists of a wire on an adjustable arm that touches the surface of a crystalline pebble of galena (lead sulfide).  It formed a crude semiconductor diode, which allows current to flow in one direction but blocks current flowing in the other direction.  It's function was to rectify the radio signal, changing it from alternating current to a pulsing direct current, to extract the audio (sound) signal from the radio frequency carrier wave.
<a href="//commons.wikimedia.org/wiki/User:F1jmm" title="User:F1jmm">F1jmm</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span> • <span class="int-own-work">Self-photographed</span>