A Hertzian <a href="https://en.wikipedia.org/wiki/spark-gap_transmitter" class="extiw" title="w:spark-gap transmitter">spark-gap transmitter</a>, the earliest type of radio <a href="https://en.wikipedia.org/wiki/transmitter" class="extiw" title="w:transmitter">transmitter</a>.   Used by <a href="https://en.wikipedia.org/wiki/Heinrich_Hertz" class="extiw" title="w:Heinrich Hertz">Heinrich Hertz</a> in his historic 1889 experiments demonstrating the existence of radio waves.  It consists of an <a href="https://en.wikipedia.org/wiki/induction_coil" class="extiw" title="w:induction coil">induction coil</a> <i>(R)</i> that produces high voltage pulses, and a spark gap <i>(B1, B2)</i> attached to a <a href="https://en.wikipedia.org/wiki/dipole_antenna" class="extiw" title="w:dipole antenna">dipole antenna</a> consisting of two metal plates <i>(A1, A2)</i> attached to brass rods <i>L1, L2</i>.  When a spark jumps across the spark gap, it discharges the electric charge stored on the antenna, creating damped sinusoidal oscillations in the antenna that were radiated as radio waves in the <a href="https://en.wikipedia.org/wiki/Ultrahigh_frequency" class="extiw" title="w:Ultrahigh frequency">UHF</a> range.   The plates, called "capacity areas", functioned to add capacitance to the antenna to increase the charge stored on it by the induction coil before the spark gap fires, increasing the output power.  They also decreased the <a href="https://en.wikipedia.org/wiki/resonant_frequency" class="extiw" title="en:resonant frequency">resonant frequency</a>  of the dipole, making the waves longer.  The plates could be slid in or out on the rods to "tune" the transmitter.    From article "Electric Oscillations" in Brockhaus &amp; Efron encyclopedia.
Брокгауз и Ефрон
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