4. First case study – the subdivision of the light - HM Treasury

F05 Ch4 Subdivision of the light.doc
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Technological roulette – a multi-disciplinary study of the dynamics of innovation in electrical, electronic
and communications engineering.
4. First case study – the subdivision of the light
Bright 1949, p127
advance of electric lighting.
In 1900, there were ten million Welsbach mantles in use in the United States,
compared with about 20m incandescent electric lamps. The gas mantle persisted until the
middle of the twentieth century before it finally was completelysuperseded.
4.5.3 The Nernst lamp
In 1897, Walter Nernst, the chemistry professor at the University of Göttingen,
while he was investigating the theory of light emission from the Welsbach mantle,
devised a novel lamp in which rare earth oxides were heated electrically, achieving a
specific efficiency of 2 watt and an operating life of 400 hours. Fürst 1926 The Nernst
radiator was a small rod about 25mm long and 0.75mm in diameter. As in the Auer
mantle, it was a mixture of oxides of metals such as magnesium, calcium, and the rare
earths. Many combinations were possible. One early mixture was composed of
85 per cent zirconia and 15 per cent yttria. They were powdered, made into a paste with
an organic binder, extruded through dies, and dried.
Later a mixture of the oxides of thorium, zirconium,
yttrium, and cerium was used. These materials,
which are insulators at room temperature, have a high
negative temperature coefficient of resistance. As
they have a high melting point and are also heat-
resistant in free air, they can be raised to white heat
and thus possess ideal properties as radiation sources.
The radiator rod was connected between the
ends of the current supply electrodes and did not need
to be placed in an air-free enclosure, which greatly
simplified manufacture. In operation, however, the
lamp suffered the drawback that the radiator rod needed to be warmed to around 700°C
before it would permit a current to pass. With the earliest Nernst lamps this pre-heating
was performed with matches or specially-constructed spirit burners. This was a major
drawback for the users, the majority of whom knew nothing of its economic advantages
and thus reacted with surprise at a lamp which required ignition like an oil lamp.
75
Fürst 1926
Fig. 4.13 Construction of
Nernst’s lamp