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Spark-gap transmitter
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====AC transformer==== In higher power transmitters powered by AC, a [[transformer]] steps the input voltage up to the high voltage needed. The sinusoidal voltage from the transformer is applied directly to the capacitor, so the voltage on the capacitor varies from a high positive voltage, to zero, to a high negative voltage. The spark gap is adjusted so sparks only occur near the maximum voltage, at peaks of the AC [[sine wave]], when the capacitor was fully charged. Since the AC sine wave has two peaks per cycle, ideally two sparks occurred during each cycle, so the spark rate was equal to twice the frequency of the AC power<ref name="Hyder" /> (often multiple sparks occurred during the peak of each half cycle). The spark rate of transmitters powered by 50 or 60 Hz mains power was thus 100 or 120 Hz. However higher audio frequencies cut through interference better, so in many transmitters the transformer was powered by a [[motor–generator|motor–alternator]] set, an [[electric motor]] with its shaft turning an [[alternator]], that produced AC at a higher frequency, usually 500 Hz, resulting in a spark rate of 1000 Hz.<ref name="Hyder" />
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