Incandescent - Tantalum Filament

The tantalum filament lamp was the first widely successful replacement for its carbon predecessors. Its invention is credited to Drs. Werner von Bolton and Otto Feuerlein of the Siemens & Halske laboratories at Berlin-Moabit, Germany. 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, together with a much whiter colour light.

The mechanical properties of metal 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 the commercialisation of tantalum lamps.

One of the primary limitations of tantalum lamps is their unsuitability for operation on AC circuits. The rapid expansion and contraction of the filament as it follows the sinusoidal current flow leads to structural changes within the filament. The wire increases in length, becomes strongly facetted, and shows a tendency to twist which can lead to short-circuiting of adjacent pieces of filament and premature failure. Nevertheless, their advantage in efficacy over the carbon lamp was so profound that in many cases they were still employed on AC circuits, with a consequent reduction in useful life.

The success of tantalum lamps was relatively short-lived, because in the same decade as their invention they were usurped by the still more efficient tungsten filament. Siemens & Halske at first denied the superiority of the tungsten lamp and persisted with the manufacture of tantalum models until about 1913, but elsewhere, tantalum lamps were made obsolete around 1910. On account of their brief period of manufacture, there are very few surviving examples of tantalum lamps.



British-made with spherical shape bulb



British-made with double cage filament mount



American-made for use in Mirroscope



American-made with unusual filament mount



German-made with "Focus" type filament