||During the 1950s and 60s Thorn and its predecessors fell behind their competitors in the field of single-ended low pressure sodium lamps. The earliest SO/H types were of course made by BTH and AEI-Mazda, but when the Integral SOI and the IR-Coated SOX ranges were introduced in 1955 and 1964 respectively, the Thorn/Mazda did not follow the lead of their competitors at Philips and Osram-GEC. Possibly this is because the company pioneered its own low pressure sodium concept in the form of the SLI Linear lamps in 1959, and believed in the superiority of that lamp type to the exclusion of the traditional single-ended ranges.
By the late 1960s it became clear that this was a mistake, and Thorn sourced in both SOI and SOX lamps from its competitors at Osram and Philips. It was not until 1972 that Thorn finally realised that it could not afford to ignore the single-ended concept, and set up its own SOX production.
This lamp is representative of Thorn's 2nd generation SOX. The earliest lamps were based on the Philips design, employing sodium-retaining dimples in the discharge tube wall. After a few years the superiority of the Osram-GEC design was recognised, which employed a graded film thickness along lamp length which allowed the elimination of the dimples, to result in a cheaper and stronger lamp. The discharge tube is supported at the U-bend by a small steatite ceramic tube held in a steel support. This avoids electrical contact between the discharge tube and the electrified IR film, which would otherwise lead to sodium loss. The other end of the discharge tube is supported by a mica disc. The cap is made from a phenolic resin based polymer moulded into a brass shell.