||ME mercury lamps have air-cooled quartz arc tubes loaded above 100 watts per centimetre, and this 250W lamp operates at a mercury vapour pressure of around 30 atmospheres. The /H suffix indicates that the lamp is for burning in the horizontal position only.
This particular lamp is more commonly known under its Air Ministry reference '5L/457 Bombing Teacher'. It was a war time development for the Royal Air Force, who required a lamp of extreme high brightness for use with a projection system from an aircraft. A short arc mercury lamp was chosen over Xenon because of its higher efficiency. To aid in rapidly replacing these lamps, which had a short lifetime, they are equipped with two pre-focussed caps. The main cap which supplies current is a standard P40s type, and is pre-focussed as normal. A second brass shell is cemented over the crown of the bulb, and a small indent is drilled in its end face. An imaginary line drawn between this and the centre of the main cap intercepts perfectly the axis of the electrodes, providing a reliable vibration-resistant design.
The arc burns between electrodes of solid tungsten to provide a compact source of extremely high brightness. Electrodes have flat ends on account of the high current, and a small overwound tungsten coil acts as a heat radiator. Seals are of the hand-made vacuum-shrunk type to double molybdenum foils capable of handling the high current. The arc tube is filled with mercury and argon and was ignited with a Tesla coil. A very rugged mounting was required to prevent arc tube movement during vibration. Metal cups supporting either end are bolted to a steel channel, clamped to the stem and at the other end to a dimple in the bulb.