<P> Lamp bulbs with dual (carbon) filaments were built as early as 1902 to allow adjustable lighting levels . Large lamps of 500 - watt rating were used for street lighting in France, which had a second filament at 200 watts to reduce lighting costs . In 1933 in the United States, Westinghouse announced a dual - filament lamp for commercial lighting and a smaller lamp for residential use . </P> <P> A 3 - way incandescent bulb has two filaments designed to produce different amounts of light . </P> <P> The two filaments can be activated separately or together, giving three different amounts of light . A typical 3 - way incandescent bulb is a 50 W / 100 W / 150 W bulb . It has a low - power 50 W filament and a medium - power 100 W filament . When they are both energized at the same time, 150 W of power is delivered, and a high level of light is produced . Usually screw - base 3 - way bulbs fit into regular Type A sockets . Larger 3 - way bulbs (up to 300 W) have a larger "mogul" base . These 3 way bulbs can also come in spiral designs . </P> <P> Some compact fluorescent lamps have similar circuitry to produce the different amounts of light . </P>

How does a three way light bulb work
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