A mechanical Rectifier

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Look at this link, and at least halfway down the page.

They use a ( red) magnet along with a coil, the magnet only allows the positive portion of the ac sine wave to attract the reed and complete the circuit. So what is produced is a pulsating positive going wave. It is of course the same principle used in in synchronized vibrators to obtain a pulsating DC square wave. Another use was to make noise as a signal source for wire tracing, because of the rich harmonics up into the RF field.