The Edison screw is a popular light bulb fitting with a male screw pattern on one end and a female screw socket in the socket. It can be manufactured in various sizes, denoted by the letter E followed by the diameter of the base in millimeters. The most common household light bulb in the US is the E26, while the E27 is popular in Europe. Other sizes of Edison screws fall into four categories, with specialized uses for smaller or larger sizes. The Edison screw can also be used for DC light bulbs and three-way bulbs with two center contacts.
Edison screw is a type of fitting commonly used with light bulbs. Sockets designed for use with this type of fitting contain a center contact and a female screw socket, while bulbs have a male screw pattern on one end. The screw base forms one side of the electrical connection and the center contact serves as the other. Edison screws can be manufactured in a range of sizes and are usually denoted by the letter E followed by the diameter of the base measured in millimeters. There are a number of competing bulb lamp designs, although the Edison screw is the most popular in the United States, much of Europe, and other countries around the world.
When electric lighting was first invented, each lamp manufacturer tended to create their own proprietary fixtures. Some used spring clips to hold the bulbs in place, while others used threaded studs or other designs. The Edison screw was originally introduced in the first decade of the 20th century and eventually became the most common light bulb base in the United States and other countries around the world. Other bulb fittings, such as the bayonet cap, are still popular in Europe and are also used in many 20-volt applications in the United States.
Any light bulb that has an Edison screw base can easily be defined by the size of that base. The common designation for Edison screw light bulbs is the letter E and a number representing the size of the base. In the United States, the most common household light bulb is the E26, which has a base with a diameter of 26 mm (about 1.02 inches). A similar bulb, the E27, is typically found in European countries and other areas with 240V electricity.
There are many other sizes of Edison screws, although those that are typically used with 120V or 240V electricity fall into four general categories. Candelabra bulbs are typically the smallest and have a designation of E11 in the US and E12 in Europe. The intermediate bulbs are E17 and E14, the sizes E26 and E27 are indicated as medium and the goliath or mogul bulbs are E39 and E40. Light bulbs that have an Edison screw smaller than E11 or larger than E40 typically have less common and specialized uses.
The Edison screw can also be used for direct current (DC) light bulbs in devices such as flashlights and toy trains, although bayonet cap and bi-pin bases are also common in these settings. Bulbs with screw diameters of 10 mm (about 0.39 inches) and smaller are usually rated at less than 30V, unlike larger bulbs which can be used on 120V or 240V. There is also a variant on the Edison screw referred to as a three-way bulb which contains two center contacts instead of one. These bulbs contain an extra filament with a lower wattage which can be used when installed in a compatible fixture.
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