Robertson Infrared Carbon Radiator CV311
The first electric heaters were based around high power single-ended carbon filament lamps, commonly known as the Dowsing Radiator. They had to be used vertically to avoid problems of filament sag. Such lamps came under pressure after the 1905 invention of chromel / nichrome, a nickel-chromium alloy which resists oxidation up to dull red heat without requiring an evacuated glass bulb. The wire was typically coiled around a ceramic former which held the coils in place, and was itself heated and served as an auxiliary heat radiator. The resulting elements were much smaller than the old carbon lamps, and a new generation of portable electric fires was introduced with a horizontal bar heater at the focus of a polished metal reflector.

This lamp appears to have been introduced in an attempt to maintain the carbon heater business. Its double-ended design features a single zig-zag filament to accommodate its thermal expansion, supported at its centre by a helical wire carried on a glass stalk fused to the bulb side. This permits horizontal operation similar to the resistance bar elements, but it appears not to have become successful.

During WWII this lamp found another application as a 300-ohm power resistor, being known by its Air Ministry references 5L/18 and 5L/444, and valve code CV311. Carbon lamps make convenient resistors due to their high power dissipation capability. Banks of four or six lamps were used vertically as a dummy load for testing the output of radio transmitters. Lamps made for this service are characterised by a barium mirror getter coating at one end. That was possibly added to avoid radio frequency interference, which is a common problem from lamps having imperfect vacuum.
Manufacturer: The General Electric Company of England
Lamp Power: 250 Watts
Lamp Voltage: 230-250 Volts
Lamp Current: 1.04 Amperes
Cap Type: S22/19x26 Brass
Bulb Type: T-57 T-18 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Filament Type: Carbon Axial zig-zag
Atmosphere: Vacuum Barium getter
Luminous Flux: 135 lumens
Luminous Efficacy: 0.54 lm/W
Colour Temperature: 1800K
Colour Rendering: Ra 100
Chromaticity Co-ordinates: CCx: 0.544 CCy: 0.401
Lifetime: Approx. 1000 hours
Burning Position: Universal
Overall Length: 285 mm 111/2 inches
Lighted Length: 152 mm 6 inches
Factory: Hammersmith England
Date of Manufacture: 1947 June Date Code: 66 & CF
Original Value: GB £0 32s 6d 1960
References: 1) Photometric Measurement.
2) Osram-GEC Catalogue, 1960, UK, p.18.
3) Osram-GEC Catalogue, 1962, Australia, p.27.
4) Photoelectric measurement.
5) Lamp Collection of Giorgio Basile, Nivelles, Belgium.
6) Virtual Valve Museum, UK.
7) Kilokat's Bulbcollector Forum.
8) The History of Electric Heating, Electric Radiators Direct, UK.