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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 its sodium lamps were the best quality that money could buy.
Most noticeable is the small nickel cap attached to the end of the dewar jacket, to protect the delicate concealed glass exhaustion tip which holds the vacuum. A spring supports the long inner vessel of the dewar, 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 is 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.
Whereas all other sodium lamps employed soft glass discharge tubes, BTH was unique in producing hard glass lamps for several years, based on its Chesterfield C64 and C40 grades. These showed reduced tendency to crack compared to the earlier C10/C19 soft glasses. It also shows better adhesion to liquid sodium, limiting sodium movement in non-horizontally burned lamps. Unfortunately the C64 lining glass has a high affinity for argon adsorption, which increases lamp striking voltage and limits life. BTH eventually returned to soft glasses once these problems had been overcome. Hard glass lamps are identifiable by the green-grey colour of their "Fernico" Iron-Nickel-Cobalt glass-to-metal seals where the leadwires enter the discharge tube. |