Micrometer spark gap detector used by German physicist <a href="https://en.wikipedia.org/wiki/Heinrich_Hertz" class="extiw" title="w:Heinrich Hertz">Heinrich Hertz</a> in his historic 1889 experiments in which he demonstrated the existence of radio waves.  It consisted of a loop antenna consisting of a loop of wire with a spark gap that could be finely adjusted with a thumbscrew.  With this he detected radio waves in the <a href="https://en.wikipedia.org/wiki/Ultrahigh_frequency" class="extiw" title="w:Ultrahigh frequency">UHF</a> range from a <a href="https://en.wikipedia.org/wiki/spark_gap_transmitter" class="extiw" title="w:spark gap transmitter">spark gap transmitter</a>.  From article "Electric Oscillations" in Brockhaus and Efron encyclopedia.
Брокгауз и Ефрон
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