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
President of the Royal Astronomical Society William Herschel in 1821, though he never chaired a meeting. Since then the post has been held by many distinguished astronomers. The post has generally had a term of office of two years, but some holders resigned after one year e.g. due to poor health. Francis Baily and George Airy were elected a record of four times each. Baily's eight years in the role are a record (Airy served for seven). Since 1876 no-one has served for more than two years in total. President of the Royal Astronomical Society The President of the Royal Astronomical Society (prior to 1831 known