eRittenhouse
The Nernst Lamp
ELECTRICAL CONDUCTIVITY IN NON-METALLIC MATERIALS
Allan Mills
Retired from Dept. of Physics, University of Leicester, U.K.
[email protected]
ABSTRACT
The use of carbon and tungsten filaments mounted in vacuum as early incandescent lamps is
well-known, but it is less often realised that an early competitor was Nernst’s ceramic lamp.
This made use of the property that , although good insulators at room temperature, many
oxides become partially conductive when strongly heated. This is due to the migration of ions
rather than electrons, and was commonly considered a deleterious property. Nernst realised
however that it might permit the construction of a sturdy incandescent lamp that did not
require a vacuum. His early lamps embodied a thin rod of magnesium oxide, initially heated
with a coil of platinum wire. Later, he used a modified composition based on zirconia.
However, these ‘ceramic’ lamps were unable to compete with a new generation of ‘coiledcoil’ tungsten lamps with filaments mounted in low-pressure argon.
Keywords:
Nernst, Incandescent lamps, Ceramic lamps
Thomas Edison introduced the first commercially-successful electric incandescent lamp in
the early 1880s, employing a high-resistance filament of carbonised bamboo enclosed in a
high vacuum within a glass envelope. These lamps lasted over 1200 hours. Later versions
using filaments of carbonised viscose lasted longer, but their luminous efficiency was still
< 5%. All these carbon-based filaments must have contained a percentage of graphitized
carbon in order to achieve their electrical conductivity, which was therefore due to the
migration of electrons – just as in metals. Therefore, like metals, these filaments showed a
positive resistance/temperature characteristic: their resistance increased with temperature.
This made it possible to choose a length and diameter that stabilized at a maximum desirable
temperature when supplied with electricity at a given voltage. (Thus, a modern nominally
60W tungsten ‘coiled coil’ lamp showed a resistance of 111 ohms when cold, rising to 1065
ohms and passing 0.23A when operating from a supply at 245 V, i.e 56W).
The high vacuum was essential to prevent burning of the carbon by reaction with oxygen, and
thin filaments of considerable length were mandatory to give a high resistance permitting
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operation of lamps in parallel. This resulted in a susceptibility to breakage, particularly when
relatively thinner spots were developed on ageing. It was therefore inevitable that Edison’s
competitors would seek some stronger refractory material that possessed sufficient electrical
conductivity to be heated to brilliant incandescence by the passage of an electric current, yet
was sufficiently inert not to require enclosure in vacuum.
Walther Nernst
Walther Hermann Nernst (1864-1941) was a brilliant physical chemist working at Göttingen
University (Ref. 1). He was destined to win a Nobel prize in 1920, but well before then, in
the early years of the century, he had become widely known for his invention of a ceramic
lamp.
Nernst knew that an alternative form of electrical conductivity existed in electrolytes, with
the current being carried by charged ions. The electrolysis of water is very familiar in this
context, but Nernst’s broad knowledge led him to wonder if a refractory solid electrolyte –
with a sufficient mobility of ions at less than a very high melting point – might replace
carbon as a filament to give a rugged incandescent lamp. An oxide-based ceramic (i.e.
already fully oxidized) should allow operation in air, obviating the requirement for vacuum.
The Nernst lamp
Many researchers of the period knew of refractories that
were good insulators at ambient temperatures, but tended to
become more conductive as their temperature increased.
This was generally regarded as a deleterious property, but
Nernst realised that it might form the basis of a ceramicbased lamp (Refs. 2, 3). Possessing more commercial
acumen than most academics, he patented the idea (Ref. 4).
Initial research pointed to magnesium oxide (magnesia,
MgO) as a possible candidate, but it was necessary to heat it
externally above a red heat before it would conduct
sufficiently by the migration of ions to maintain itself at a
high temperature. This he accomplished in a practical lamp
by surrounding a thin rod of the oxide with a platinum
heating spiral (Fig. 1). Once sufficient current was flowing
in the magnesia rod, it tripped a magnetic relay that
disconnected the heating spiral.
Fig. 1 MgO filament of an
early, ca. 1907, Nernst lamp
with its platinum heating
spiral.
Now, this non-metallic oxide conducted electricity by the
migration of ions (O--), and like aqueous electrolytes (and gas
discharges) possessed a negative resistance/temperature characteristic. This meant that as it
heated-up more current flowed – so it got even hotter – so still more current flowed until
(hopefully) a fuse ruptured before something else failed. It was therefore necessary to
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include a metallic ‘ballast’ resistor in series in the
circuit, with oppositely matching characteristics. This
is shown in Fig. 2 as the uppermost tubular
component, and consists of an iron filament within a
hydrogen-filled glass bulb. It operated at a dull red
heat to limit the current flowing through the
magnesium oxide rod.
The Nernst lamp was sold as a complete assembly
(Fig. 3), and was a bulky and expensive object. An
obvious improvement was to make the magnesia rod –
which tended to fail first – as a replaceable unit along
with its associated heater (Fig. 4). Nernst probably
also introduced trace elements into the magnesia to
increase its luminous efficiency and spectrum: he
would have known for example that cerium in the
thoria-based Welsbach gas mantle exerted a beneficial
effect on its emission. Then, in collaboration with
Fig. 2 Interior of a Nernst lamp,
Westinghouse, he improved it still further by using
ca. 1895-1905. This model used a
yttria-stabilised zirconia for the filament (Refs. 5, 6). At
bimetal strip to disconnect the
around 8% the final design was more efficient that the
heater.
carbon filament lamp (Ref. 7), but for domestic and
commercial lighting it could not really compete with the
much simpler tungsten filament lamp filled with argon that was introduced in 1904. In
Fig. 3 (left) Complete Nernst lamp (1901).
Fig. 4 Replaceable filament and heater used in a later pattern of lamp.
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particular, the latter did not suffer from the annoying half-minute delay after switching on – a
defect shared with the modern coiled fluorescent lamps that we are now expected to use.
The ‘Nernst glower’ hung on for a while as a source of infra-red radiation in IR spectrometers
and similar apparatus, but the need for preheating was always a nuisance. Silicon carbide8
then began to be made with sufficient conductivity at room temperature not to need
preheating, and soon replaced other materials in IR sources and high temperature furnaces.
(The ‘carborundum’ of commerce is a dark blue-grey, but this is due primarily to iron as an
impurity. Pure SiC is a colourless, transparent, highly refractive and very hard crystalline
solid, that may be cut and polished into diamond-like jewellery (Ref. 8)).
The Nernst lamp is now recognized as employing the first commercially-produced solid
electrolyte, and as such is the progenitor of modern developments in solid oxide fuel cells
(Ref. 9). However, the non-aqueous electrolysis of alumina dissolved in a molten eutectic of
synthetic cryolite (Na3AlF6) and calcium fluoride (CaF2) has long been of great economic
importance for the production of aluminium.
The conductivity of hot glass
Fig. 5 (left) Bead of soda glass held between nichrome electrodes.
Fig. 6 Circuit for demonstrating conductivity of hot soda glass:
1. Soda-glass bead; 2. Nichrome electrodes; 3. Brass connectors; 4. Glass fibre
insulated rigid wires; 5. AC meters; 6. 60 W incandescent lamp; 7. Fuse. 3A
8 Switch; 9. ‘Safebloc’ connector to 240 V AC mains.
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The zirconia-based ceramic used in the later Nernst lamps is not readily available in small
quantities, but the principle may be demonstrated with ordinary soda glass. At ambient
temperatures this is a very viscous supercooled liquid, but when heated to a red heat the
viscosity decreases greatly and the contained sodium ions (Na+) become sufficiently mobile
for the matrix to display substantial electrical conductivity (Ref. 10). (Borosilicate glass –
‘Pyrex’ –is not suitable.) A fragment of soda glass (e.g. from a broken bottle) is held between
two 4 mm loops of nichrome wire spaced about 5 mm apart upon rigid conductors of copper
wire insulated with glass fibre. (Brass clamps removed from plastic connecting blocks serve
very well for these connections.) The assembly is held between two pieces of kaolin-based
heatproof board, and the glass melted with a butane torch (Fig. 5).
When cool, the glass bead mounted on its electrodes is inserted in the circuit diagrammed in
Fig. 6, the 60 watt incandescent lamp acting as a suitable ballast resistor. Wearing insulating
rubber gloves and a Perspex/Plexiglass face shield, the glass bead is carefully heated once
Fig. 7 Glowing bead of hot soda glass photographed by its own light.
again. It will begin to conduct at around a red heat, and the tungsten lamp will light up. The
hotter it gets, the more current will flow, and the lamp will become brighter. The butane
torch may then be removed, the heat generated by resistive heating in the glass bead being
sufficient to keep it molten. The yellow radiation characteristic of sodium is emitted (Fig. 7),
and peculiar twinkling points may be seen around the electrode wires. My set-up, employing
a 60 W incandescent lamp as the ballast resistor, operated with 64 V across the bead and a
current of 0.2 A flowing. This is equivalent to 13 W dissipated in the hot glass, with a
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resistance of 320 ohms. This ‘solid state sodium lamp’ operated for > 10 minutes, but
eventually the hot glass sagged and broke the circuit.
It will be obvious that this demonstration should only be conducted by experienced
laboratory personnel with adequate safety precautions.
References
1.
K. Mendelssohn, The World of Walther Nernst: The Rise and Fall of German Science,
Macmillan, 1973. A chronology of Nernst’s life may be found here: Chronology
2.
H. Monmouth Smith, ‘The Nernst lamp’, Science, 11 (1898) 689-690.
3.
Anon., ‘The Nernst light’, Scientific American, 80 (1899) 150.
4.
W. Nernst, Verfahren zur Erzeugung von elektrischen Glülicht, German patent DRP
104872, filed 1897.
5.
W. Nernst, Material for electric-lamp glowers, American patent USP 685730, filed
1899.
6.
Yttria-stabilized zirconia. Wikipedia.
7.
L.R. Ingersoll, ‘On the radiant efficiency of the Nernst lamp’, Phys. Rev., 17 (1903),
371-7.
8.
Silicon carbide. Wikipedia.
9.
H.-H. Möbius, ‘On the history of solid electrolyte fuel cells’, J. Solid State
Electrochem., 1 (1997), 2-16.
10.
W. Thomson, ‘Electrolytic conduction in solids – first example, hot glass’, Proc. Roy.
Soc., 23 (1874-5), 463-4.
Image Credits:
Fig. 1 Dr. Ulrich Schmitt, Physicochemical institute, University of Göttingen, Germany.
www.nernst.de
Fig. 2 Science Museum/Science & Society Picture Library, Image # 10276181, art. #
1966-267.
Fig. 3 Technoseum, Museumsverein für Technik und Arbeit, Mannheim, Germany art. #.
1980/0014-066
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