Thorn M2 Miniature Ultraviolet Mercury Lamp

The M1 and M2 miniature mercury lamps are small sources of long wave ultraviolet and actinic radiation. They consist of a low pressure discharge in mercury vapour, and are optimised for DC or AC supplies respectively.

The original M1 lamp was developed by Siemens of Preston at the outset of WW2, and was intended for the excitation of fluorescent markings on the instrument dials of aircraft cockpits at low illumination levels. In that application they replaced the very inefficient, hot burning and short-lived miniature blacklight incandescent lamps. Since they are intended for use on 24V DC supplies, their ignition is facilitated by momentarily heating a filamentary cathode to dull incandescence, before striking a rather high current glow discharge to an adjacent anode wire.

After the war, the M1 lamp found a multitude of other applications, where operation from the AC mains supply was more convenient. To avoid the need for complex control gear, the M2 version illustrated here was developed for operation on 200/250V mains supplies with a series ballast. As such it requires no filament, and has only a pair of closely-spaced thermionic electrodes.

For many years these kind of lamps found numerous special applications on account of their small size. They were favoured in time markers for recording camera equipment, as a source of the mercury spectral lines in laboratory instruments, and for various biological and entomological purposes such as the light source in smaller insect traps. However, developments in miniature fluorescent UV lamps led to their gradual obsolescence by the early 1980s.
Manufacturer: Thorn Lighting Ltd. (Lamp Ref. 98-9002)
Lamp Power: 4½ Watts
Lamp Current: 0.75 Amps (range 0.5-1.5 Amps)
Lamp Voltage: Approx 6-10 Volts
Cap: BA15d (Nickel Plated Brass)
Bulb Finish: Clear UV-transmitting
Bulb Type: T-15
Overall Length: 38 mm
Light Centre Length: 25 ±S 2 mm
Electrodes: Coiled-Coil tungsten Double carbonate emitter
Atmosphere: Pure Argon
Luminous Flux: N/A
Luminous Efficacy: N/A
Colour Temperature & CRI: N/A
Chromaticity Co-ordinates: N/A
Rated Life: 200 hours
Factory: Leicester England
Date of Manufacture: 1975 July
Original Value: £3 18s 0d (1969)
References: 1) Thorn Lighting Comprehensive Catalogue 1975 UK p.350
2) Thorn Lighting Comprehensive Catalogue 1971/72 UK pp. 7:18, 10:33, 10:44, 2:46
3) Siemens Datasheet - Sieray M1 & M2 Miniature Mercury Discharge Tubes, c.1950