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
Light crude oil on practical grounds than theoretical. The New York Mercantile Exchange (NYMEX) defines light crude oil for domestic U.S. oil as having an API gravity between 37° API (840 kg/m) and 42° API (816 kg/m), while it defines light crude oil for non-U.S. oil as being between 32° API (865 kg/m) and 42° API (816 kg/m). The National Energy Board of Canada defines light crude oil as having a density less than 875.7 kg/m (API gravity greater than 30.1° API). The government of Alberta, which produces most of the oil in Canada, disagrees and defines it as oil with a density