The Edison screw fitting is a system of connectors used for light bulbs, 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, or a lamp is plugged into a non-polarized outlet, the metal screw can become energized, presenting an electric shock hazard to anyone attempting to change the bulb. 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.
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Early U.S. lamp manufacturers used different and incompatible bases. The Thomson-Houston Electric 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. [1]
In response to Edison's patent, Reginald Fessenden invented the bi-pin connector for the 1893 World's Fair. Other lamp bases include the bayonet mount and wedge base.
Dimensions and tolerances for screw bases are standardized in ANSI standard C81.67 and IEC standard 60061-1. The two standards have mostly been harmonized, although several types exist in only one of the two standards with no equivalent in the other.
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:
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. In areas following the National Electrical Code general-use lamps over 300W cannot use an E26 base and instead use the E39 base, 300W lamps may use either base. 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.[2]
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, E10 for decades-old series-wired C6 bulb sets in the U.S., and an entirely different wedge base for T1¾ mini lights. For a short time early on, these mini lights were manufactured using E5 screw bases.
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 lamp with a self-ballast that will not fit in a recessed lighting fixture.
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 socket was used as an outlet (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 normally. There have also been electronics that stick onto the end of the screw base and will allow the attached bulb to flash, for example, to attract the attention of arriving guests or emergency vehicles; others function as a dimmer or timer, or dim gradually in a child's bedroom in the evening.
| Designation | Base Diameter (Crest of thread) |
Name | IEC 60061-1 standard sheet |
|---|---|---|---|
| E5 | 5 mm | Lilliput Edison Screw (LES) | 7004-25 |
| E10 | 10 mm | Miniature Edison Screw (MES) | 7004-22 |
| E11 | 11 mm | Mini-Candelabra Edison Screw (mini-can) | (7004-6-1) |
| E12 | 12 mm | Candelabra Edison Screw (CES) | 7004-28 |
| E14 | 14 mm | Small Edison Screw (SES) | 7004-23 |
| E17 | 17 mm | Intermediate Edison Screw (IES) | 7004-26 |
| E26 | 26 mm | [Medium] (one-inch) Edison Screw (ES or MES) | 7004-21A-2 |
| E27 | 27 mm | [Medium] Edison Screw (ES) | 7004-21 |
| E29 | 29 mm | [Admedium] Edison Screw (ES) | |
| E39 | 39 mm | Single-contact (Mogul) Giant Edison Screw (GES) | 7004-24-A1 |
| E40 | 40 mm | (Mogul) Giant Edison Screw (GES) | 7004-24 |
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. 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.[3]
Diazed fuses (IEC_60269#D-system_.28DIAZED.29) DII uses the same E27 thread as light bulbs but the diazed fuse holder is far deeper than an E27 light bulb socket so a light bulb screwed in a buse box wont make contact with the middle pole, and trying to screw a DII fuze in an E27 bulb socket will not work because the fuse will hit the bottom of the bulb socet before the threads on the fuse holder will be able to reach the threads of the bulb holder. However it's possible but not useable to screw a DII fuseholder without a fuse in an E27 bulb holder.