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.
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.Gauges (to ensure interchangeability)-ANSI C81.63 and IEC 60061-3.
Specifications for all lamp mount types are defined in the following American National Standards Institute (ANSI) and International Electrotechnical Commission (IEC) publications: It was in this period when E-designations of screws first originated in Germany (where seven DIN VDE standards were enacted in 1924-1925 ) and then adopted by IEC. All IEC attempts to reach consensus by 1925 failed, but lamp makers continued the work in an independent committee and developed two standards-one for Europe, another for Americas-which were endorsed by the IEC in 19 respectively.
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.
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