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Thorn MBM 40W Mercury Lamp for Coal Mines |
The lighting of mines is especially challenging due to the presence of fire damp, a mixture of flammable gases. This requires strict regulations on the use of electricity to avoid sparks and ignition at switches or poor contacts. A much safer form of lighting was pioneered by distributing power via compressed air, driving a small generator in each light fitting which could be turned on by operating an air valve. At first incandescent lamps were employed, but were not optimal due to their limited output and fragile nature.
The development of this 40W type MBM mercury lamp enabled a major advance in both output and robustness. In a novel arrangement it was electrically ballasted by slightly modifying the windings of the AC generator. Unlike other mercury lamps this 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, greatly simplifying lamp construction.
The original model employed an ST-shape bulb, the domed top allowing secure mounting of the inner structure. 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 1990s, 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 | |
