996-7835 Rapid Nigrescence Masthead Signal
The transmission of morse signals over short distances can be achieved simply by the rapid switching of a small incandescent lamp. As ranges increase, signalling speeds must decreased owing to a limitation of incandescent lamp technology. More powerful lamps are naturally required, and these contain larger filaments. The greater mass of metal takes longer to bring up to incandescent temperatures and longer to cool down, which causes pulses to blur together.

During the second world war there was an increased requirement for long-range signalling and faster lamps were required. A satisfactory solution was eventually achieved with this so-called rapid nigrescence incandescent lamp which could achieve 12-14 words per minute. Rather than using one large filament, fourteen small filaments are employed. Due to the smaller wire diameter, their thermal inertia is low and the light-up time is fast. In order to decrease nigrescence (cooling) time, the bulb is filled with a helium-nitrogen mixture. On account of helium's high thermal conductivity the lamp is inefficient and short-lived.

The filaments are mounted in a series of vertical rings on a complex wire frame, to form a relatively compact source. This is located at the centre of a spherical bulb, one side of which is mirroed so as to project a beam of high luminous intensity. It is equipped with with a pre-focus cap for accurate alignment in the optical system. Interestingly, the hand-made glass stem assembly employs the old-fashioned Jaeger construction, in which the exhaust tube pierces one side of the stem rather being integrated into the pinch-seal. Today these lamps have been superseded by mechanical shutters to interrupt the beam of a standard lamp.
Manufacturer: Thorn Electrical Industries Type 85-6901
Lamp Power: 3000 Watts
Lamp Voltage: 210 Volts
Lamp Current: 14.29 Amperes
Cap Type: P40s/45 Brass & porcelain
Bulb Type: GS-170 GS-54 in eighths/inch
Bulb Finish: Side Mirror Monax borosilicate glass
Filament Type: 14C-9 14-part Cylindrical
Atmosphere: Helium-Nitrogen Aluminium-zirconium getter
Luminous Flux: 38,000 Lumens
Luminous Efficacy: 12.67 Lumens per Watt
Beam Intensity:
Beam Distribution:
Colour Temperature & CRI: CCT: 3000 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.452 CCy: 0.409
Rated Lifetime: 100 hours
Burning Position: Vertical cap down ± 30°
Overall Length: 285 mm 111/4 inches
Light Centre Length: 160 mm 61/3 inches
Factory: Leicester England
Date of Manufacture: November 1975 Date Code: 0123
Original / Present Value: Unknown Unknown
References: 1) United Kingdom Ministry of Defence Directorate of Standardisation, DefStan 62-6, Nato Stock Nr. 996-7835
2) AEI Mazda Manufacturing Specification for Lamp 85-6901.
3) Ship Lighting, J.T.Grundy, AEI Engineering July/AugustS 1962 pp.196-201.