safety lamp, oil lamp designed for safe use in mines and other places where flammable gases such as firedamp (see damp) may be present. Its invention (c.1816) is usually attributed to Sir Humphry Davy. The Davy lamp is based upon the principle that to be ignited a substance must first be heated to its kindling temperature and that if such heating is prevented combustion will not occur. The flame in the lamp is surrounded by a metal-gauze screen that distributes the heat over a large area so that the maximum temperature of the screen is below the ignition temperature of the flammable gas mixture (e.g., firedamp). Improvements devised by K. G. Bischof and others include special locks to prevent the accidental opening of the lamp and devices to permit the lamp to be held upside down without danger. If firedamp or related gas mixtures are present in a mine, the Davy lamp flame burns higher and with a blue halo; the height of the flame and color of the halo indicate the amount of combustible gas in the air. If the mine air is deficient in oxygen, the lamp flame is extinguished. Coal miners often placed the safety lamp close to the ground to detect gases, e.g., carbon dioxide, that are denser than air and thus collect in poorly ventilated depressions in the mine. Other gas-detecting devices are now more commonly used, e.g., the methanometer; as a light source the Davy lamp has been replaced by electric lighting.