||The gas-filled tungsten lamp marked a major improvement over its vacuum predecessor, allowing a considerable boost in luminous efficacy as well as improved lumen maintenance during life. The lamp shown here is representative of the earliest low wattage type that was produced by Philips.
The Philips Half-Watt lamps were first announced on 19th November 1913, available from 600 to 3000 candle power - these large lamps benefiting most from gas filling. They contained an atmosphere of pure nitrogen and were so named because the efficacy could be as high as half a watt per candle (25 lumens per watt). During 1914 they became available in lower ratings such as this, down to 100 candles.
The filament consists of a single coil of tungsten wire, the metal having been extracted via the Battersea process in which tungstic acid is heated in refractory crucibles. This results in the pick-up of K2O, SiO2 and Al2O3 impurities, which influence grain structure and improve sag resistance. It was not understood for many years as to why tungsten made by this process sometimes resulted in a better wire, or why the process yielded such variable results. Some batches would still sag severely, as evidenced in this particular lamp. Filaments of reliable quality were not produced until 1917 when Aldar Pacz at GE perfected the modern generation of AKS doped tungsten with long interlocking grain structure.
Incidentally the bulb shape here is the early spherical form. Soon afterwards it was re-designed to the now standard pear shape. The tapered neck of that bulb encourages evaporated tungsten to be swept right up into the neck area where any blackening has minimal effect on the light output.