Two light bulbs are hooked in series. A third light bulb is connected directly to a variac (120v). The two series bulbs are half as bright and when the variac is adjusted to the brightness of the series bulbs it is at 50%. When a 75w bulb and a 25w bulb are connected in series only the 25w bulb lights up because it has a higher resistance and therefore most of the available voltage is dropped across it.
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