||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.
The mechanical properties of tantalum filaments are quite different than carbon, and do not show the same degree of rigidity and elasticity. Whereas carbon filaments can be formed into a given shape and this does not change when they are brought to incandescent temperatures, metallic filaments quickly sag under the influence of gravity. A complex mount structure which supports the filament at frequent intervals is required, and the classic 'squirrel-cage' construction was first introduced with tantalum lamps.
This lamp has an unusual filament formation which is a modification of the squirrel cage geometry. The upper set of carriers are of reduced diameter so as to achieve a conically-shaped filament, and unusually, there is a third intermediate set of carriers to achieve a 'cage within a cage' type construction. These lamps, known as the "Focus" type were intended to increase the downward luminous flux when lamps were hung vertically from the ceiling. They were of value in shop window lighting to increase the luminous intensity of the displays while allowing the lamps to be mounted out of view, without need for external reflectors.