Resonator antenna that German physicist Heinrich Hertz used as a radio wave detector in his first experiments that demonstrated the existence of radio waves. It consists of a helix of brass wire in the shape of a ring, with a spark gap between the ends. It probably functioned as a quarter wave antenna. When the radio waves produced by his spark gap transmitter were at the resonant frequency of the loop of wire, it produced a high voltage between the ends of the wire, which jumped across a tiny spark gap. The spark gap "(bottom of loop)" was a micrometer with which he could measure the length of the spark, to get an estimate of the field strength of the waves.