These are three-way dimmable light fixtures. Three way bulbs work by having multiple points of contact, each lighting up a separate filament inside the bulb. This allows you to get three levels of brightness out of one bulb.
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What’s happening is that people are installing regular bulbs into fixtures designed for three-way bulbs. This means when you flip the switch once, it tries to turn on one of the two filaments in the bulb, which the bulb doesn’t have, so nothing happens. On the second switch, what should be the second filament gets powered, but on this bulb it’s just the only filament, so the light turns on. A third switch should power both filaments at the same time, for full brightness, but on single filament bulbs there won’t be a difference.
The practical effect is that the bulb is dark, dark, light, light. If you put in a three-way bulb as the fixture was designed, you would get dark, dim, light, bright.
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