Light bulbs come in a much broader variety than the simple, standard screw-in bulb. The bases on light bulbs are standardised to fit a variety of sockets, but the standards vary by the use and type of bulb, among other factors. Understanding how light bulb bases are designated can help you find the right bulb for your light fixture.
The standard types of light bulb bases are screw, pin and bayonet. Each of these base types comes in a variety of sizes to fit into different fixtures. Screw bases have threaded metal bases that screw into threaded receptacles, such as household light fixtures. Pin bases have two prongs that fit into matching holes in the socket. Bayonet bases have a metal base with one or two small, round pins that stick out slightly to the sides. The lamp socket has channels that the pins slide into when the lamp is pressed into it. Then, the lamp is turned slightly to one side and the pins lock the lamp in place so it cannot be pulled straight out.
Each of the base types comes in a variety of sizes. For screw bases, medium size is the most common, and it is the one generally used for household fixtures. Intermediate bases are slightly smaller than medium bases, and may be used for appliance lights. Candelabra bases are the smallest screw-in bases, and are often used in decorative sconces. A large size of screw-in base, called a mogul base, is often used for industrial fixtures. Bayonet bases may also be noted as candelabra, medium or mogul size, along with smaller sizes that are denoted by their physical measurements. Pin-based light bulbs vary not only in base size, but in distance between the pins. When seeking a replacement lamp, check the old lamp or base for size notation, as there is a wide variety.
Screw bases are commonly used for incandescent light bulbs. Newer compact fluorescent light bulbs also use screw bases, making them compatible with older light fixtures. Older compact fluorescent light bulbs and many types of halogen lamps use pin bases, as do many theatrical lighting instruments. Bayonet base lamps are common in electronic equipment and other low-wattage or low-voltage applications.
Pin base lamps must be fully seated in their sockets to operate properly; otherwise, electricity will arc across the gap between the socket and the lamp pin, causing build-up on the pin that may fuse it to the socket over time. Screw base lamps generally do not have this problem, but will not work as well if not fully screwed in, and may be sensitive to vibrations. Bayonet base lamps are easy to seat fully, as that is the only way they will turn and lock into place, so arcing due to incorrect contact is rarely a problem.
Many other types of light bulb bases exist for speciality applications. Some theatrical lighting has a variation on the mogul bayonet base that uses broad, flat pieces of metal rather than smaller pins to lock the lamp in place. Christmas tree-type string lights have a simple press-in base that often relies on a strip of metal rather than pins, and tube-style lamps, such as those used in workshop halogen lights, have a contact on either end of the lamp rather than a true base.
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