[emeritus]

Fascinating, thanks so much for posting the link! I knew they existed, but it was great to see one in operation. The Nernst lamp is described and (line) illustrated in an article in the 1910 Encyclopaedia Brittannica about illumination, and a 1970 Science Museum booklet (Lighting: 3: Other than in the home) has a colour photo of a different design of lamp from those in the video but without the glass globe or the brass cover for the barretter. The Science Museum booklet caption says:

" In 1897 Count Auer von Welsbach, of incandescent gas mantle fame, introduced a new kind of electric filament lamp which required no vacuum. The filament was a rod composed mainly of magnesia which decreased rapidly in resistance after a critical temperature was reached, but was virtually non-conducting at room temperature. Up to about 1900 a spirit lamp was used to heat the filament initially, but after that a platinum wire wound round a ceramic spiral was used as a heating element and cut out by a simple electro-magnetic relay as soon as the main filament current rose sufficiently to indicate that the lamp was well alight. Because there would have been difficulty in operating the relay from an alternating current source the lamps were normally restricted to direct current supplies.

Resistance to the filament decreased so rapidly with temperature that it was necessary to put into the circuit an iron wire barretter resistance ( an evacuated bulb on top of the lamp) which increased in resistance as the current rose. Efficiency was about 6 - 8 lumens per watt, and the filaments were normally enclosed by a glass globe for protection while the barretter was enclosed by a metal tube (not shown). As the glass globe was not essential these lamps later became a source of radiation for infra-red spectroscopy. "

N.B. The Science Museum booklet photo seems to be upside-down, as the barretter is depicted below the filament assembly.