Thorn 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. The original model employed an ST-shape bulb, the domed top allowing secure mounting of the arc tube to withstand vibrations in this rough service application. This later example employs heavy steel wires to support the arc tube, and was able to adopt the simpler ellipsoidal bulb. 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:	Thorn EMI
Lamp Power:	40 Watts
Lamp Current:	0.53 Amps
Lamp Voltage:	80-105 Volts
Cap Type:	B22d-3/25x26	Ni plated brass + vitrite
Bulb Type:	E-55	E-17 in eighths/inch
Bulb Finish:	Clear	Soda-lime glass
Electrodes:	Hollow coil	Double carbonate emitter
Arc Length:	11 mm	0.43 inches
Atmosphere:	Inner: Hg \| Ar (12 atm)	Outer: Nitrogen low pressure
Luminous Flux:	1,340 lm @ 100 hours	1,070 lm @ 2000 hours
Luminous Efficacy:	33.5 lm/W @ 100 hours	26.8 lm/w @ 2000 hours
Colour Temperature & CRI:	CCT: 6000K	CRI: Ra 16
Chromaticity Co-ordinates:	CCx: 0.315	CCx: 0.380
Lifetime:	2000 hours	to 50% survival
Warm-up & Re-strike Time:	5 minutes	5 minutes
Burning Position:	Vertical cap down
Overall Length:	125 ± 4 mm	5 inches
Light Centre Length:	90 mm	3½ inches
Factory:	Leicester	England
Date of Manufacture:	1988 November	Date Code: 83
Original Value:	Unknown


References:	1) Thorn Specification Sheet 91-9055
	2) Bourne, H.K.,Discharge Lamps for Photography & Projection,Chapman & Hall, London, 1948.
	3) Cotton, H.,Electric Discharge Lamps,Chapman & Hall, London, 1946.
	4) Bourne, H.K.,Characteristics of Electric Discharge Lamps for Projection,Journal of British Kinematograph Society, London, January 1941.