<P> Although most flashlights are designed for user replacement of the batteries and the bulb as needed, fully sealed disposable flashlights, such as inexpensive keyring lights, are made . When the batteries are depleted or the bulb fails, the entire product is discarded . </P> <P> Diving lamps must be watertight under pressure and are used for night diving and supplemental illumination where surface light cannot reach . The battery compartment of a dive lamp may have a catalyst to recombine any hydrogen gas emitted from the battery, since gas cannot be vented in use . </P> <P> People working in hazardous areas with significant concentrations of flammable gases or dusts, such as mines, engine rooms of ships, chemical plants or grain elevators, use "non-incendive", "intrinsically safe" or "explosion proof" flashlights constructed so that any spark in the flashlight is not likely to set off an explosion outside the light . The flashlight may require approval by an authority for the particular service and particular gases or dusts expected . The external temperature rise of the flashlight must not exceed the autoignition point of the gas, so substitution of more powerful lamps or batteries may void the approval . </P> <P> Inspection flashlights have permanently mounted light guides containing optical fibers or plastic rods . Another style has a lamp mounted at the end of a flexible cable, or a semi-rigid or articulated probe . Such lamps are used for inspection inside tanks, or inside structures such as aircraft . Where used for inspecting the interior of tanks containing flammable liquids, the inspection lights may also be rated as flame - proof (explosion - proof) so that they cannot ignite liquids or vapors . </P>

When was the first battery powered torch invented