||This SO/H sodium vapour lamp was manufactured in 1950 by the British Thomson-Houston Co. Ltd, under their Mazda brand name. Still in mint condition with its original packaging for both the dewar and the arc tube (which were always sold separately) , it is interesting to note the extra efforts made by BTH to ensure that their sodium lamps were the best quality that money could buy.
Most noticeable is the small nickel cap, thoughtfully attached to the end of the dewar jacket to protect the delicate concealed tip-off which holds the vacuum. A spring supports the long inner vessel which helped to minimise breakages during transportation or as a result of vibration when in service. To prolong the effective life of the vacuum and ensure that lamp efficacy was always maintained as high as possible, a barium getter is also present in the dewar jacket. All three of these constructional features were unique to BTH-Mazda sodium lamps.
The inner discharge tube is also manufactured from a very special glass, BTH's Chesterfield C64 and C40 hard glasses (similar to Pyrex) and these reduced the tendency to crack compared to the earlier C10/C19 soft glasses. In addition it has better adhesion to liquid sodium, preventing it flowing down to one end of the lamp where it can form large light-blocking mirrors.
But the C64 lining unfortunately exhibits greater tendency to adsorb argon, which increases lamp striking voltage during life. Lamps made in hard glass are easily identifiable by the green-grey colour of the "Fernico" glass-metal seals where the lead wires pass through the inner tube to the electrodes.