Mazda MBM 40W Mercury for Coal Mines

Coal mines are often exposed to the dangers of fire damp - a mixture of flammable gases. This requires especially strict regulations on the use of electricity. Explosions were occasionally produced by sparks within electrical switches, or poor contacts at lampholder terminals. The challenges are especially severe at the coal face, where high light levels are required to facilitate the mining operations.

A safer and higher efficacy form of lighting was pioneered by the British Thomson-Houston company during the 1940s, in which lamps were powered by compressed air generators. These could be switched on and off by operating an air valve without risk of sparking. A small high pressure mercury lamp was developed because of its high efficacy, which allowed the generators to be kept small. Their windings had an inherent inductance which served a ballast for the discharge.

Unlike other high pressure mercury lamps this small 40W model does not contain the usual third auxiliary ignition electrode. The generator produced a 500Hz AC supply and at such high frequency, a capacitive antenna consisting of a few turns of wire around the arc tube proved satisfactory.

The new lamp was named MBM, i.e. a type MB for Mining applications. This original model employs an ST-shape radio valve bulb, the domed top allowing secure mounting of the arc tube to survive the rough-service application. Later examples adopted a standard ellipsoidal shape outer bulb and a much more resilient arc tube mounting structure. Such lamps were widely used in British coal mines until the late 1980s, when they were made obsolete by more efficient fluorescent lamps in special explosion-proof luminaires.

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 1947	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.