||This is the first type of medium pressure mercury lamp to have been made in the USA. Its design is essentially a direct copy of the MA 400W lamp, which had been invented by Osram-GEC in England two years earlier. Both its physical and electrical characteristics are virtually identical.
In America it quickly became known as the H1. Several different variants emerged over time, This A-H1 is intended for burning in the vertical cap-up position. Type B-H1 has an inverted arc tube to permit cap-down use. It is important that the tip-off area is always uppermost, otherwise its slightly lower temperature may lead to the condensation of mercury inside the tip, thus preventing the lamp from fully running up. The D-H1 and E-H1 lamps were GE and Westinghouse's respective attempts at quartz lamps, whose more refractory arc tubes could be burned horizontally. The higher mercury vapour pressure also delivered increased luminous efficacy. A later F-H1 lamp was identical to the A-H1 but featured a more rugged mechanically fixed cap.
The discharge operates at a mercury pressure of 1.2 atmospheres, the same as the standard MA 400W. The arc tube is fabricated from a blown aluminosilicate bulb. The main electrodes take the form of open tungsten coils impregnated with barium, strontium and calcium oxides as the emissive material. They are supported on a moly t-piece, and the auxiliary ignition electrode is a simply moly rod alongside one of the main electrodes. As was standard practice with the American lamps, there is no platinum heat-reflective coating in the area behind the electrodes - hence the sensitivity to burning position, and the need to offer both cap up and cap down variants.