When electricity was introduced in the budget, is mainly used for lighting. At that time, many power companies controlling a charging system, including the cost of electricity for Power Cords was lower than for other purposes. This took place bulb attached to Power Cords portable devices made.
But as electricity became a common method of lighting houses and operating requirements of labor-saving appliances, a safe way to connect to the network with a different version was a lamp. The electrical properties of two plug pins and socket were invented by Harvey Hubbell and patented in 1904.
Consumers ground several claimants to its invention. The first patent of land appears to be an application on January 11, 1915 by George P. Knapp, on behalf of the Company and Harvey Hubbell granted April 18, 1916. This patent covers the use of a tongue of land that is wider than the two other contacts to ensure that is used for the first time. However, the proposed configuration pins used in type I today, especially in Australia and China, which found no interoperable with the two sheets of contact with the ground. Other records on earth, which is very common today were developed later by others to ensure interoperability with non-grounded plugs.
The security system has been taken by Albert Büttner, who invented it in 1926, patented. The latest American version of the land with two vertical fins and a round grounding pin was invented by Philip F. Labre, while he was here the Milwaukee School of Engineering. It is said that the owner of a cat fan fall when he came through the window was. When connecting the new fan would be a shock. Labre has found that if the plug was based, not the current of the earth through the plug and go to their patron saint. It applied for a patent May 12, 1927 and obtained a U.S. patent for land and taken in June 1928.As the need for more secure facilities clearly grounded systems to three contacts have been made mandatory in most industrialized countries.