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A lamp socket, also known as the Edison screw fitting, is a threaded screw base used to hold and connect a light bulb to a power source. Invented by Thomas Edison and marketed in 1909, these sockets typically have right-hand threads, which allow the light bulb to be screwed in clockwise and removed counterclockwise. To deter theft in public places, the threads are sometimes reversed to prevent installation in common light fixtures.
Lamps, light fixtures, and adapters are common types of fixtures that include a lamp socket. Light fixtures as well as lamps incorporate Edison screw lamp sockets into their design not only for holding and energizing the bulb but also to take advantage of the many types of bulbs on the market that use this design. Alternate designs were implemented in the early 1900s that varied from Edison's design but in the end, they fell out of use due to the practicality of the Edison screw.
Over the years, many types of adapters have been invented to take advantage of the availability of the lamp socket. Common types include the adapters that screw into the lamp socket and convert it into a wall outlet. This is particularly convenient when installing outside Christmas lights when a standard outlet is not available. Other types convert the light socket into a switchable outlet while still allowing for the use of the light bulb. Smoke detectors have also been designed to connect to the lamp socket without interfering with the light bulb's operation.
A couple of disadvantages to the Edison screw system is that if it is not properly wired, the metal screw may become energized which creates a potential electric shock hazard. Another disadvantage is that if the bulb becomes loose in the socket, it will fail to function properly. Vibrations and other factors can lead to this problem.
Although the lamp socket has been around for over a hundred years, it has stood the test of time. Many variations have been tried, with a few succeeding, but none of them has been able to overcome the Edison screw's hold on the market. Advanced bulb manufacturing, such as the energy efficient light bulbs and even fluorescent light bulbs still use the Edison screw lamp socket system today.