||The first mass produced American mercury lamp was the H1 style from General Electric, a 400W hard glass arc tube operating inside a T-16 (50mm diameter) tubular outer envelope of soft glass material. Offered in two formats, the A-H1 for cap up operation and the B-H1 for cap down burning, this remained the most popular choice in mercury lighting for several decades. Horizontal operation of either was feasible, but only with an electromagnet situated above the lamp and in series with the arc, its field serving to deflect the natural upward-bow of the discharge column and prevent overheating of the arc tube wall which would shorten life.
In 1948 an improved lamp having a quartz arc tube and designated the E-H1 came on the scene and this was suitable for universal burning. However the quartz lamp technology came with a hefty price penalty in its infancy, and the old style lamps continued to be mass produced for a further ten years alongside their more modern alternatives.
Consequently, many improvements were still being made to the glass arc tube lamps, most notably around 1956 when the H25-HC lamp was introduced. Appearing physically identical, it made use of an improved aluminosilicate arc tube glass permitting hotter operation for greater light output, and enhanced longer-life electrodes. The hotter arc tube however did lead to occasional problems of the T-16 tubular outer jacket reaching such a temperature that cracking by thermal shock would occur. This was quickly overcome by adopting the BT-24 bulged outer jacket shown here, fabricated in lead-borosilicate hard glass. It was recommended that this new lamp, termed H25-DE, be used in all luminaires which could accept the larger bulb size.