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How can you distinguish between a standard lamp socket and a three-way? It’s simple! Standard sockets have only two electrical contacts. The hot contact, a small metal tab, sits at the bottom of the socket, while the negative contact is the threaded metal shell (the one the bulb screws into).
Listen to BOB VILA ON STANDARD SOCKETS or read the text below:
Three-way sockets look very similar. The difference is that next to the metal tab at the bottom, there’s a third contact—a bit smaller than the metal tab and set slightly off center. That extra contact matches a small ring-shaped contact on the bottom of a three-way fluorescent bulb. Such bulbs are characterized by having two filaments, not one. When you twist the switch on a three-way lamp, the first click illuminates a single filament. The next click deactivates the first and illuminates the second, this one with a somewhat higher wattage. The final, third click illuminates both filaments at once, giving you the brightest light.
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