Electric light be made with a screw-thread base, two metal pins, two metal caps or a bayonet cap. The three main categories of electric lights are incandescent lamps, which produce light by a filament heated white-hot by electric current, gas-discharge lamps, which produce light by means of an electric arc through a gas, and LED lamps, which produce light by a flow of electrons across a band gap in a semiconductor. Before electric lighting became common in the early 20th century, people used candles, gas lights, oil lamps, and fires. Cornish chemist Humphry Davy developed the first incandescent light in 1802, followed
Royal Society and acting as trustees for the society's possessions and estates. Members are elected annually via a postal ballot, and current standing orders mean that at least ten seats must change hands each year. The council may establish (and is assisted by) a variety of committees, which can include not only fellows but also outside scientists. Under the charter, the president, two secretaries and the treasurer are collectively the officers of the society. The current officers are: The President of the Royal Society is head of both the society and the council. The details for the presidency were set out in