60 Watt (equivalent of 35 Watt SOX)
The slight discolouration of the arc tube, and blackening around the electrodes, suggests that the lamp has seen quite a few hours' use. An outer jacket was not included with this arc tube.
Date code 66 = June 1963 (Hamilton, Scotland)
The other side of the cap features a small chart that would have allowed engineers to write the column number of the lantern that the lamp was fitted in, along with the date it was installed and the date it was removed - this is presumably for guarantee purposes in the event of premature failure. Given that this lamp was 48 years old when it arrived in the Collection, I think the guarantee may no longer be valid if the lamp doesn't work!
140 Watt (equivalent of 90 Watt SOX)
Thanks to John Evans for donating this lamp to the Collection.
The arc tube carries a rare "Property of H.M. Government" frank in the space above the electrodes; a close-up of this is shown below:
The other side of the lamp carries the Philips frank. This particular example was made at the Philips lamp factory in Hamilton, Scotland, although SO/H lamps were also produced at the company's factory in Eindhoven, The Netherlands. This arc tube does not feature the characteristic dimples of modern Philips SOX lamps; the reason being that the dimples were sometimes found to break as the arc tube was inserted into the outer Dewar jacket.
The arc tube carries the code '37' as a date of manufacture - this could either be the January of 1937, 1945, 1953 or 1961. Out of this choice, either of the latter two years are more likely to have been the lamp's year of manufacture, and from those two, 1953 seems (for some reason) to be the likeliest year.
A small steel strip provides support for the fragile arc tube. Over the years, the fork joint has rusted, resulting in the staining of the surrounding glass.
Given that the arc tube of this lamp looked new and unused, I decided to see whether it would work if power was applied. Using a 90 Watt SOX transformer, I connected up and the circuit and switched on. Thankfully, the lamp fired up immediately. The starting colour was a far deeper red than the starting colour of SOX lamps is - the lamp, at this stage, therefore resembled an uncoated neon lamp.
The sodium vaporised rapidly in the arc tube; resulting in the appearance of the familiar colour attributed to warmed-up low-pressure sodium lamps.
The brightness increased throughout the next few minutes, although the portions of the arc tube that immediately surrounded the electrodes were still to fully undergo the colour transition.
After approximately twenty minutes, the lamp appeared to be operating at virtually full output. An interesting feature of this lamp was that, despite the presence of the Dewar jacket surrounding the arc tube, the heat losses were still considerable - by the time that the lamp had reached this level of operation, the surface of the outer bulb was too hot to touch for more than even half a second.
The progress of the lamp warm-up was also captured in a video, which can be accessed by clicking on the YouTube window below. For this production, the lamp was fitted in the GEC Z9464, dating from 1956.