RLOD#5 (2020.05.09) 1953 Westinghouse E-H1

Introduced in 1946, the E-H1 is the first modern quartz high-pressure mercury lamp of American origin to feature an elliptical (BT) outer jacket. This bulb was introduced by Westinghouse so as to increase the lamp's service life, whose limitation was identified as originating from the effusion of hydrogen from the glass vessel. This light gas diffuses into the quartz arc tube where its accumulation leads to the formation of mercury hydride, a volatile molecule with electronegative properties which impedes plasma ignition. A mitigating solution was found by increasing the size of the outer bulb so as to reduce its temperature, and by reducing the amount of hydroxyl ions in the glass material. The latter is of a soft soda lime silicate kind as the company did not have the capacity to produce BT bulbs with harder glasses suitable for an exposure to weather elements, as it would become the norm later on.

The E-H1 shown here is of the second-generation type released in 1950, featuring several design improvements devised by Westinghouse. First, the cement used to attach the metal cap to the lamp was upgraded to a silicone-based material retaining better adhesive properties at high temperature (up to 300 °C). Next, the burner mounting frame was improved with the use of stamped metal sheet supports which replaced the lighter and more fragile mica plates. Finally, the burner design was changed with a longer electrode gap distance of 7.7 cm (was 6.0 cm initially) and a wider bore diameter so as to reduce the power load to the quartz wall from 14 to 10 W/cm². These modifications improved the balance between life, efficacy and output flux maintenance. Interestingly, that burner also features the first molybdenum foil seals in a US-made general lighting lamp. The new feedthrough (at that time) is of the vacuum-shrunk type, developed by D. Gabor (Siemens Bros, UK) in the early 1930s. The introduction of this particular burner seal was motivated by the strong rise in market demand for medium-wattage quartz mercury lamps, a demand which could not be addressed in any economical way with the graded-glass seals used in first-generation quartz mercury burners. Nevertheless, the manufacture of vacuum-shrunk feedthrough was still very labor intensive. This construction was used exclusively in Westinghouse’s E-H1 from 1950 to 1955, until it was replaced by the much simpler pressed moly seal.