Mazda MBM Mercury Lamp for Coal Mines

On account of the frequent build up of fire damp (explosive gases) in coal mines there have always been very strict regulations on the use of electricity. The tiny spark occurring when a switch is opened or if there is poor contact at lamp terminals can easily trigger an explosion.

The provision of electric light at the coal face where fire damp can be encountered therefore poses a number of challenges. Many of these were overcome in a BTH system developed in which lamps were powered by compressed air generators, and the light could be turned on and off simply by operating an air valve with no sparking whatsoever.

A small mercury lamp was desired on account of its good efficacy which meant that generators could be kept small. This 40W lamp was designed and first made available in the mid 1940's. It operated on a 500Hz AC supply from the compressed air generator, and at such high frequency ignition can reliably be effected by means of a few turns of wire around the outside of the arc tube. The windings of the generator also have an inherent reactance such that they operate as the lamp ballast after ignition.

The new lamp was named MBM, i.e. a type MB for Mining applications. The outer bulb was originally made in this ST shape borrowed from the valve industry, because the domed top allows the arc tube to be very securely mounted, a requirement in the rough service application the lamp was created for. During the mid 1980's, Thorn Lighting changed the glass the elliptical shape as used on the 50W MB to facilitate production. Today, explosion proof switches and luminaires are the general preference in mines.
Manufacturer: The British Thomson-Houston Co. Ltd.
Lamp Power: 40 Watts
Lamp Current: 0.59 - 0.45 Amps
Lamp Voltage: 80-105 Volts
Cap Type: B22d-3/30x35 Ni plated brass + vitrite
Bulb Finish: Clear Soda-lime glass
Bulb Type: ST-54 ST-17 in eighths/inch
Overall Length: 120 mm 4¾ inches
Light Centre Length: 90 mm 3½ inches
Arc Length 11 mm 0.43 inches
Electrodes: Backwound thoriated Double carbonate emitter
Inner / Outer Atmosphere: Inner: Ar | Hg (12 atm.) Outer: Nitrogen low pres.
Luminous Flux: 1,360lm(100hrs)  1000lm(750hrs)    800lm(1500hrs)
Luminous Efficacy: 34lm/W (100hrs)  25lm/W (750hrs)  20lm/W(1500hrs)
Colour Temperature & CRI: CCT: 6000K CRI: Ra 16
Chromaticity Co-ordinates: CCx: 0.315 CCx: 0.380
Burning Position: Vertical cap down
Rated Lifetime: 1,500 hours (to 50% survival)
Warm-up / Re-strike Time: 5 minutes 5 minutes
Factory: Leicester England
Date of Manufacture: June 1950 Date Code: 66
Original / Present Value: Unknown Unknown
References: 1) Bourne, H.K.,Discharge Lamps for Photography & Projection,Chapman & Hall, London, 1948.
2) Cotton, H.,Electric Discharge Lamps,Chapman & Hall, London, 1946.
3) Bourne, H.K.,Characteristics of Electric Discharge Lamps for Projection,Journal of British Kinematograph Society, London, January 1941.