The electric chair was a device commonly used for the execution of human beings during the 20th century in the United States of America.
The first practical electric chair was invented by Harold P. Brown. Brown was an employee of Thomas Edison's hired for the purpose of researching electrocution and for the development of the electric chair. Since Brown worked for Edison, and Edison promoted Brown's work, the development of the electric chair is often erroneously credited to Edison himself. Brown's design was based on Alternating Current (AC), which was then just emerging as the rival to Edison's less transport-efficient Direct Current (DC), which was further along in commercial development.
Apparently, Edison's primary motivation for the development of the electric chair was an attempt to make people associate AC electricity with death, and thus increase the market for his own DC technology. The design was adopted in 1888 for use in New York's State Prison system. 
The first execution via the electric chair was carried out on William Kemmler in New York's Auburn Prison on August 6, 1890. The first woman to be executed in the electric chair was Martha M. Place, executed at Sing Sing Prison on March 20, 1899. Before long, it had become the prevalent method of execution in the USA, and remained so until the mid-1980s despite the increased popularity of the gas chamber beginning in the 1950s.
The condemned prisoner was typically strapped into the chair, with one electrode attached to the head and a second attached to the leg. At least two applications of an electrical current would be applied for several minutes depending on the person. An initial volatge of around 2,000 volts is used to break the intital resistance of the skin and cause unconsciousness (in theory). The voltage is then lowered to reduce current flow so as to prevent burning. A current flow of around 12 ampss is usual. The body of the condemned would heat up to 138°F (59°C) and the electric current would cause severe damage to internal organs.
In theory, unconsciousness occurs in a fraction of a second. However, there have been reports of victims' heads on fire, of burning transformers, and of letting the crying victim wait in pain on the floor of the execution room while the chair was fixed. Further, regardless of how well the execution was performed, some skin is always burned and it is unpleasant for the guard charged with separating the burned, oozing skin from the seat belts. The victim loses control of his muscles after the initial jolt of electricity, and may start to defecate and urinate on the floor beneath the chair. This led to a refinement in modern electric chairs: they were padded, and came with automatic car-style seatbelts.
After Texas adopted lethal injection as a method of execution in 1982, the use of the electric chair reduced rapidly. As of 2003, the only places in the world still using the electric chair are the U.S. states of Alabama, Nebraska, and Virginia, and it is in the process of being phased out of use in Alabama. The method is only used if the prisoner chooses to be executed in this manner. The electric chair has come under criticism because of many instances in which victims were not instantly killed, but had to be repeatedly electric shocked, leading to a call for ending of the practice because many see it as cruel and unusual punishment.
See also: Electric shock