||This SO/H sodium vapour lamp was manufactured by the General Electric Company of England, under their Osram brand name in 1949. The lamp is still in mint unused condition and was a production sample saved from the old Hammersmith lamp works in London.
This particular lamp represents one of the later designs of SO/H lamp, after it had been subjected to numerous improvements in glass technology and production engineering. The discharge tube lining is manufactured from X91 (Na10) barium-aluminoborate glass, produced at GEC's Lemington Glassworks, which is characterised by a lower susceptibility to staining but still has a high rate of argon gas adsorption and a poor adhesion to liquid sodium. As a result lamp life is still limited by how long it takes before the glass cleans up the argon gas filling, which raises the striking voltage above what the ballast can supply. In addition, if the lamp is not kept absolutely level when alight, the sodium can run down to one end forming large light-blocking mirrors. The high stresses associated with X91 glass also often resulted in cracking during manufacture.
Despite these drawbacks, no effort has been spared in assembling the lamp to a level of very high quality. It also exhibits the later design of electrode construction, in which a small magnesia ceramic tube is sealed into the glass around each wire leading to the electrodes. This prevents liquid sodium from coming into contact with the weak point at the glass-to-metal seal and reduces the risk of the seal failing in service. Another thoughtful touch is the placement of the monogram for the dewar jacket on the inner glass vessel, so that it cannot fade or be accidentally wiped off.