Most light bulbs screw into a socket. Almost all household bulbs have a medium screw base, sometimes called an edison screw, 1 1⁄16 inches in diameter with 7 threads per inch. Bulbs with a base that looks like a medium base but isn't are made to defeat bulb thieves in public places. Some have a left-hand thread; others are the size called “admedium,” used in signs, just a bit bigger (1 5⁄32 inches) so they won't fit a residential socket. Other common screw bases, in order of decreasing diameter, are:
Householders usually describe bulb size by wattage, but the industry has a system for describing the actual dimensions of the bulb.
Bulb shapes are identified by letters: “A” is the shape of a typical household bulb; “B” is the shape of a candelabra base Christmas tree bulb; “C” is the shape of a miniature screw night light bulb, and so on.
The bulb size is then given by a letter for shape followed by the bulb's maximum diameter in eighths of an inch. For example, a T-8 bulb would be a tubular bulb 1 inch in diameter. Ordinary 40-watt and 75-watt bulbs are A-19, with a few A-21.
The size of the base or the shape and size of the bulb aren't sure signs of the bulb's wattage or voltage. For example, A-19, medium base lamps are made for 12 volts, 100 volts, 115 volts, 130 volts and 250 volts.
Collecting old incandescent bulbs is a thriving hobby. A valuable compendium of information on early, pre-standardization sizes can be found on Early Incandescent Lamps at www.frognet.net/~ejcov/index.html.
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Last revised: 9 November 2002.