Mazda SO/H Sodium Vapour Lamp & Dewar

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.
Manufacturer: The British Thomson-Houston Co. Ltd.
Lamp Power: 60 Watts
Lamp Current: 0.6 Amps 0.54A Starting Current
Lamp Voltage: 95-125 Volts 450V ignition
Cap Type: By22d Porcelain
Bulb Type: Inner: TU-14 (TU-4½") Dewar: TA-50 (TA-16")
Bulb Finish: Inner: Borate Hard Glass Dewar: Clear Soft Glass
Electrodes: CCC-6 Transverse Black W + BCT Emitter
Arc Length:
Atmosphere: Inner: Na | NeAr0.8% Outer: Vacuum
Luminous Flux: 4,020 lm (@ 100 hrs)
Luminous Efficacy: 67.0 lm/W (@ 100 hrs)
Colour Temperature & CRI: CCT: 1700K CRI: Ra -44
Chromaticity Co-ordinates: CCx: 0.574 CCy: 0.425
Rated Lifetime: 6,000 hours Guaranteed to 4,000 hrs
Warm-up & Re-strike time: 11 minutes Instantaneous
Burning Position: Vertical Cap Up ± 95°
Overall Length: 300 mm 11¾ inches
Light Centre Length: 170 mm 6¾ inches
Factory: Leicester England
Date of Manufacture: Inner: Sept. 1950 Dewar: Nov. 1950
Original / Present Value: £5 3s 0d (1954) £94.49 (2005)
References: 1) BTH-Mazda Lamp Catalogue, UK, 1950