In 120-Volt North America, 100-Volt Japan and 110-Volt Taiwan, the standard size for general-purpose lamps is E26. In countries that use 220–240 Volt AC domestic power, standard-size E27 and small E14 are the most common screw-mount sizes and are prevalent throughout continental Europe and China.
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Į29 "Admedium" bases are used for special applications, for example UV spotlight lamps in magnetic crack detection machines. A medium screw base should not carry more than 25 amperes current this may limit the practical rating of low voltage lamps. and Canadian rules now require long-life and energy-efficient LED lamps, which can be purchased inside a conventional Edison base bulb as a retrofit. Previously, emergency exit signs also tended to use the intermediate base, but U.S. Large outdoor Christmas lights use Intermediate base, as do some desk lamps and many microwave ovens. Medium Edison screw (MES) bulbs for 12 V are also produced for recreational vehicles. National Electrical Code, general-use lamps over 300 W cannot use an E26 base and must instead use the E39 base. The large E39 "Mogul" and E40 "Goliath" base are used on street lights, and high-wattage lamps (such as a 100 W / 200 W / 300 W 3-way) and many high-intensity discharge lamps. Other semi-standard screw thread sizes are available for certain specific applications. The E26 and E27 are usually interchangeable, as are the E39 and E40, although less so although there is only a 1 mm difference in thread outside diameter, there is a small difference in pitch an E40 cap will often fit in an E39 holder but not the other way around. Mogul: E39 North America, E40 (Goliath ES) in Europe.Medium or standard: E26 (MES) in North America, E27 (ES) in Europe.Intermediate: E17 North America, E14 (Small ES, SES) in Europe.Candelabra: E12 North America, E11 in Europe.There are four commonly used thread size groups for mains supply lamps: is given in eighths of an inch, e.g., A19, MR16, T12. This is distinct from the glass envelope (bulb) diameter, which in the U.S. In the designation "Exx", "E" stands for "Edison" and "xx" indicates the diameter in millimeters as measured across the peaks of the thread on the base (male), e.g., E12 has a diameter of 12 mm. Generally, the two standards are harmonized, although several types of screw mount are still defined in only one standard. Guidelines for Electrical Lamp Bases, Lampholders and Gauges-ANSI C81.64 and IEC 60061-4.
Proposals to introduce one or several international standards for Edison screws began in 1918, when France suggested to the International Electrotechnical Commission (IEC) to take up the issue of sockets and holders. By about 1908, the Edison base was most common in the U.S. In 1892, Edison General Electric Company merged with Thomson-Houston to found General Electric, which gradually adopted the Edison screw and made it prevalent. Screw shells produced as early as 1888 had a light taper than the modern ones. In response to Edison's patent, Reginald Fessenden invented the bi-pin connector for the 1893 World's Fair.Īfter some design tweaks Edison settled upon a screw 1 inch in diameter with 7 threads per inch of length, which much later became E26. Other lamp bases include the bayonet mount and wedge base.Īll three major designs were patented, Edison himself filed his applications in 18. : fig.7 Most smaller competitors had to produce lamps for all three types, and some used their own designs as well. : fig.8 The Sawyer-Man or Westinghouse base used a spring clip acting on grooves in the bulb base and a contact stud at the bottom of the lamp. Another company, the Thomson-Houston Electric Company, used a threaded stud at the bottom of the socket and a flat contact ring. In inventing his screw, Edison was inspired by a lid of a kerosene can in his workshop, he even sawed it off to make a prototype in 1880.
In the United States, early manufacturers of incandescent lamps used several different and incompatible bases in 1880s-1890s.