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In the series-string sets with a half wave power supply, a reverse-connected diode across the power switch will perform both the functions of reducing the apparent voltage to the filaments by cutting off half of the AC cycle, and since the pulses are on the negative half cycles they are blocked by the rectifier diode and the DC supply to the set is thus shut off.
In more elaborate sets like this which may use an autotransformer or voltage doubler there is usually a double pole power switch to disable the DC supply while engaging the instant on reduced voltage system.
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