||Soon after GE had placed the first medium pressure mercury lamp on the American market, the model A-H1 and B-H1 of 1932, Westinghouse was quick to finalise its lamp development and offered this very similar product.
The arc tube is made from a blown bulb, originally domed at the base end and having a constriction just before the open end into which the top electrode was sealed. The electrodes take the form of open coils, this arrangement acting as a hollow cathode to facilitate lamp ignition in a similar way to the shape of beehive cathodes in low pressure sodium lamps. The coil is impregnated with the oxides of barium and strontium to enhance electron emission, and the ends are welded to a molybdenum T-piece. One electrode is sealed into the centre of the domed end of the arc tube bulb, along with an auxiliary starting electrode and exhaust tube. See third thumbnail photo below. The other main electrode would then have been sealed into the constriction at the other end of the arc tube.
It is interesting to note that this lamp pre-dates the use of heat reflective platinum paint on the ends of the arc tube, hence its efficacy is slightly lower than for later lamps. Also the resistor in series with the auxiliary starting electrode is grossly oversized by comparison with more modern MA lamps. It takes the form of a wirewound resistor rather than the much more compact ceramic resistors used today. The lettering written around the end of the arc tube indicates the date of manufacture along with the precise volt drop across the arc tube in coded fashion. The purpose of the white line drawn around each end of the arc tube is not known, it is perhaps just some form of identification.