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A 3-way lamp, also known as a tri-light, is a lamp that uses a 3-way light bulb to produce three levels of light in a ''low-medium-high'' configuration. A 3-way lamp requires a 3-way bulb and socket, and a 3-way switch. Unlike an incandescent lamp controlled by a dimmer, each of the filaments operates at full voltage, so the color of the light does not change over the three steps of light available. Certain compact fluorescent lamp bulbs are designed to replace 3-way incandescent bulbs, and have an extra contact and circuitry to bring about similar light level control. 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. 〔 http://home.frognet.net/~ejcov/3way.html Ed Covington, ''The 2-Filament 3-Way Incandescent Lamp'', retrieved 2013 March 12 〕 ==Three-way bulbs== A 3-way incandescent bulb has two filaments designed to produce different amounts of light. 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. 〔https://www.1lightbulbs.com/category/38/Spiral/1/base/E26D+Medium+Screw+3-Way/ 1 Light Bulbs, ''3-Way Incandescent Bulbs'', retrieved 2014 January 24 〕 Some compact fluorescent lamps have similar circuitry to produce the different amounts of light. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「3-way lamp」の詳細全文を読む スポンサード リンク
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