Siemens Double Cage Tantalum Filament

The emergence of the tantalum lamp resulted from continual efforts to improve the luminous efficacy and life of the incandescent lamp, and its invention is credited to Dr. Werner von Bolton and Dr. Otto Feuerlein of the Siemens & Halske laboratories in Berlin-Moabit. Although their first lamp was created in December 1902, it was not until 1905 that production commenced. The reduced vapour pressure of tantalum, by contrast with carbon, permits higher temperature operation for the same lamp life. This yields an efficacy of about 6 lm/W, double that of the carbon filament.

Siemens' lampmaking had until then been restricted almost exclusively to Germany, but the invention of the Tantalum lamp was to shape in very great measure the future of that company as a powerful international force. At first, Siemens sold a license to GE of America to manufacture its own Tantalum lamps. It proved to be a very lucrative deal, with Siemens profiting strongly from its invention. The increased revenue allowed Siemens to expand its own manufacturing base, and in 1908 it set up a factory at Dalston in London, to compete with the British carbon lamp manufacturers.

Although not marked, this lamp is believed to have been produced by Siemens' British subsidiary. This was not unusual, because owing to the considerably greater efficacy of the tantalum lamp, the Dalston works was soon supplying its lamps to many other competitors under private brand. At first only low voltage tantalum lamps were made. This model is a later design, which has been made suitable for operation on high mains voltages by the unusual construction bearing two separate cages, to support the greater length of filament wire.
Manufacturer: Siemens Brothers
Lamp Power: ~85 Watts
Lamp Voltage: ~230 Volts
Lamp Current: ~0.37 Amperes
Cap Type: B22d Large skirted, vitrite
Bulb Type: SA-76 SA-24 in eighths/inch
Bulb Finish: Clear Soda-lime glass
Filament Type: 2S-1 Double Cage
Atmosphere: Vacuum Red phosphorus getter
Luminous Flux: Approx 640 lm 50 Candle Power
Luminous Efficacy: 7.5 lm/W 1.7 Watts per Candle
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: CRI:
Chromaticity Co-ordinates: CCx: CCy:
Rated Lifetime: Approx. 1000h on DC 300-500h on AC
Burning Position: Vertical
Overall Length: 155.6 mm 6⅛ inches
Light Centre Length: mm inches
Factory: Dalston England
Date of Manufacture: June 1913? Date Code: XIII F
Original / Present Value: 2s6d (1912) GB £10.73 (2013)
References: 1) Siemens Tantalum Lamp Catalogue, UK, 1912
2) Royal Ediswan Tantalum Lamp Catalogue, UK, 1913