Named after its inventor Thomas Edison, the Edison Screw Base was patented in 1881 and became the most popular light bulb base in America.
The base is a right-hand threaded metal base which screws into matching threaded sockets.
In the early days of lighting, the 1880’s, pioneering incandescent lamp inventors used several different kinds of bases and sockets, but the Edison base helped to standardize them.
Edison found inspiration for his base in the lid of a kerosene can in his workshop. He even was said to have sawed off the lid of the can to use as his first prototype in 1880.
After some design tweaks, Edison settled upon a screw 1 inch in diameter with 7 threads per inch of length – which much later became the E26 base. Screw shells produced as early as 1888 had a lighter taper than our modern ones.
In 1892, Edison General Electric Company merged with Thomson-Houston to found General Electric, which gradually adopted the Edison screw and made it prevalent.
By 1908, the Edison base was most common in the U.S., with the others falling out of use.
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. 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, and another for Americas. These standards were endorsed by the IEC in 1930 and 1931 respectively.
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.
Most Edison screw bases have right-hand threads (lamp is turned clockwise to tighten), but left-hand threaded screws are sometimes used, usually for a non-standard voltage or wattage bulb. This prevents the use of an incorrect bulb, which could cause damage. Left-hand threaded Edison screw bases have also been used to prevent theft in public spaces like subways and hotels.
Featured image courtesy of Shield Home Watch.
