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The Edison screw fitting is a system of light bulb connectors, developed by Thomas Edison and licensed starting in 1909 under the Mazda trademark. Most have a right-hand threading, so that it goes in when turned clockwise and comes out when turned counterclockwise, like a hardware screw. There are rare instances where these are reversed to deter theft, so that they cannot be used in other light fixtures.

Screw bases suffer from two disadvantages. Firstly, the metal screw itself forms one of the contacts for the circuit. If the lighting system is not correctly wired, the metal screw can become energized, presenting an electric shock hazard to anyone attempting to change the bulb. (This is also a problem for the bayonet base, and is also the reason why the electric plug is polarized, since the switch on the socket only controls the center contact.) Secondly, if the bulb unscrews in the socket for any reason, it can lose contact with the center contact and stop working until it is screwed-in tighter. This is not always a disadvantage, however: it provides a method for turning a single bulb off without using a wall or fixture switch, with appropriate precautions against the high heat of the bulb.

History

Early U.S. lamp manufacturers used different and incompatible bases. The Thomson-Houston company used a threaded stud at the bottom of the socket, and a flat contact ring. The Sawyer-Mann or Westinghouse base used a spring clip acting on grooves in the bulb base, and a contact stud at the bottom of the lamp. By about 1908 the Edison base was most common, with the others falling out of use.

In response to Edison's patent, Reginald Fessenden invented the bi-pin for the 1893 World's Fair. Other lamp bases include the bayonet mount and wedge base.

Sizes and applications

The designation Exx refers to the diameter in millimeters, even in the U.S., where the bulb glass is listed in eighths of an inch. (For example, E12 has a diameter of 12 mm.) There are four common sizes of screw-in sockets used for line-voltage lamps:

• candelabra: E12 North America, E11 in Europe
• intermediate: E17 North America, E14 (SmallES) in Europe
• medium or standard: E26 (MES) in North America, E27 (ES) in Europe
• mogul: E39 North America, E40 (GoliathES) in Europe.

Other screw thread sizes exist for other uses.

The large E39 is used on street lights, and high-wattage lamps (such as a 100/200/300- watt three-way) and many non-incandescent high-intensity discharge bulbs. Medium Edison screw (MES) bulbs for 12 V are also produced for recreational vehicles. Large outdoor Christmas lights use an intermediate base, as do some desk lamps and many microwave ovens. Previously, emergency exit signs also tended to use the intermediate base, but U.S. and Canadian rules now require long-life and energy-efficient LED lamps, which can be purchased inside a bulb as a retrofit. A medium screw base should not carry more than 25 amperes current; this may limit the practical rating of low-voltage lamps.

In countries which use 220–240 volts AC domestic power, E27 and E14 are the most common sizes, although the bayonet mount is also used. In 120-volt North America and 100-volt Japan, the standard size for general-purpose lamps is E26. E12 is typically used for candelabra fixtures. E17 is also sometimes used, especially in small table lamps and novelty lighting, and occasionally the lights on newer ceiling fans. Christmas lights use various base sizes: E17 for C9 bulbs, E12 for C7 bulbs, possibly E10 for some decades-old series-wired sets in the U.S., and an entirely different wedge base for mini lights.

Larger sizes (E39, E40) are used for higher-powered lighting, generally for lamps of over 250 watts. A tiny E5 or E5.5 size is used only for extra-low voltages, such as in interior illumination for model buildings, and model vehicles such as model trains. These are often called "pea bulbs" if they are globe-shaped, but they commonly look like mini Christmas bulbs, or large "grain-of-wheat" bulbs. E10 bulbs are common on battery-powered flashlights, as are bayonet mounts (although those are usually held in with a circular flange located where the base meets the bulb). The E11 base is sometimes used for expensive 50/75/100-watt halogen lights in North America, where it is called the "mini-can", and tighter threads are apparently used to keep them out of E12-base nightlights and other places where they could start a fire.

There are also adapters between screw sizes, and for adapting to or from bayonet caps. A socket extender makes the bulb stick out further, such as to accommodate a compact fluorescent with a self-ballast that will not fit in a recessed lighting fixture.

Other uses

Some adapters for wall outlets use an Edison screw, allowing a light socket to become an ungrounded electrical outlet (such as to install Christmas lights temporarily via a porch light), or to make a pullchain switch with two outlets, or to split it for two bulbs. Another adapter will make a wall outlet into a light socket.

The Edison screw was in fact used as an outlet itself (such as for toasters) when mains electricity was still mainly used for lighting, and before wall outlets became common. Various other accessories have been made, including a smoke detector that recharges over a few hours and lasts for a few days or weeks thereafter, and still allows the attached bulb to operate as normal. There have also been electronics that stick onto the end of the screw base and will allow the attached bulb to flash (such as to attract the attention of arriving guests or emergency vehicles), or function as a dimmer or timer, or dim gradually in a child's bedroom in the evening.

Edison screw-cap fittings

Three-way bulbs have a d suffix to indicate double contacts, usually E26d or E27d, or rarely E39d. The second contact is used for the lower-wattage filament of the two inside the bulb. This extra contact is a ring located around the main contact, which is used for the higher-wattage filament. Unlike bayonet sockets, three-way and regular bulbs are interchangeable, although the low filament or low setting will not work if they are mismatched.

The medium Edison screw has seven threads per inch, or about 3.6mm per thread. In the U.S., the Energy Independence and Security Act of 2007 requirement for greater energy efficiency only applies to the medium Edison screw, all other being considered "specialty" bulbs.

See also

• Multifaceted reflector
• GU10
• Kovar material for joining bulb glass to metal

References

1. I.C.S. Reference Library volume 4B, International Textbook Company, Scranton PA 1908, page 43-41
2. General Electric TP 110 page 12
3. http://homerepair.about.com/od/electricalrepair/ss/2007_energybill_4.htm

• ANSI C81.61-2007 American National Standard for electrical lamp bases - Specifications for Bases (Caps) for Electric Lamps, available at www.nema.org, retrieved 2009-01-20

• Lighting
• Electrical connectors
• Mechanical standards