Mazda SO/H Sodium Vapour Lamp & Dewar

This SO/H sodium vapour lamp was manufactured in 1953 by the British Thomson-Houston Co. Ltd, under its Mazda brand name. Still in mint condition with its original packaging, 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 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 breakage in transit, and it also features a barium getter to prolong the useful life of the vacuum jacket and maximise lamp efficacy.

A pair of sachets of silica gel held in nickel gauze are clipped to the U-bend of the discharge tube. This eliminates the deleterious films of condensation which can otherwise occur after a cold night's street lighting service. Moisture films can make starting difficult, and later lamps were supplied with a superior water-repellent silicone film on the discharge tube.

Whereas other sodium lamps employed soft glass discharge tubes, BTH was unique in producing hard glass lamps for several years. Its Chesterfield C64 and C40 grades showed reduced tendency to crack compared to earlier C10/C19 soft glasses. It also shows better adhesion to liquid sodium, limiting sodium movement during life. Unfortunately the C64 glass has a high affinity for argon, which increases 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.
Manufacturer: The British Thomson-Houston Co. Ltd.
Lamp Power: 140 Watts
Lamp Current: 0.9 Amps 0.81A Starting Current
Lamp Voltage: 155-180 Volts 450V ignition
Cap Type: By22d Porcelain
Bulb Type: Inner: TU-17 (TU-4½") Dewar: TA-65 (TA-20½")
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% 11torr Outer: Vacuum
Luminous Flux: 10,220 lm (@ 100 hrs) 9,100 lm (@ 4000hrs)
Luminous Efficacy: 73.0 lm/W (@ 100 hrs) 65.0 lm/W (@ 4000hrs)
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: 20 minutes Instantaneous
Burning Position: Horizontal +20° -5°
Overall Length: 518 mm 20½ inches
Light Centre Length: 280 mm 11 inches
Factory: Leicester England
Date of Manufacture: Inner: June 1953 Dewar: August 1953
Original / Present Value: £5 7s 6d (1954) £98.62 (2005)
References: 1) BTH-Mazda Lamp Catalogue, UK, 1954, p.38