FLASH LAMP (Fr., Lampe eclair; Ger., Blitz lampe, Magnesiumlampe) In its original sense, a lamp for burning mag nesium powder, which is blown through a spirit or gas flame. A simple type of flash lamp may be made as shown at A, with a short metal pipe bent to a right angle and surrounded with cotton wool soaked in methylated spirit. A charge of powdered magnesium is placed in the pipe, and the free end is attached to a rubber tube ter minating in a ball. The spirit is then lit, and pressure on the ball at the desired moment drives the magnesium into the flame and ignites it. A number of such lamps may be arranged case, much of the powder may simply be driven through the flame without being consumed, and so be wasted. Flashlight mixtures should on no account be used in enclosed lamps ; these are only suitable for plain magnesium powder. A popular commercial flash lamp of the enclosed type, which gives either an instantaneous or a continuous flash, is shown at B. To use this, the metal chamber is half filled with magnesium powder, and about an ounce of methylated spirit is poured into the vessel holding the wick, taking care that none gets into the discharge orifice or over the sides of the reservoir. The rubber tube is then closed by means of the spring clip, and the bladder is inflated until it fills the net. The wick having been lit, a flash of the required duration can now be obtained by pressing the clip. After exposure, the flame is extinguished
by replacing a metal cap.
The term flash lamp is now also used for open arrangements in which a flashlight mixture, con sisting of magnesium powder mixed with potas sium chlorate, or other detonating ingredients, is burnt in a metal tray or pan, by means of a fuse which is operated from a small accumulator, the mechanical striking of a match or percussion cap, or other means. A typical flash lamp of the open type is illustrated at C. The ignition of the powder is obtained in a somewhat novel fashion. A milled disc of a special pyrophorous or spark giving metal is caused to rotate against a similar metal surface by means of a coiled spring, thus giving off a small stream of sparks as it revolves. The spring is wound up by a key and the powder is spread on the tray around the milled disc, when pressure on the pneumatic bulb at once starts the mechanism and ignites the flash mixture. For professional flashlight purposes, longer trays are commonly used, the powder being laid in a thin, heaped-up line. The lamp is raised to a suitable height by a jointed metal rod attached to a stand ; and a screen or cover of muslin, or other light, translucent material, is supported on a frame over the tray to diffuse the illumination.