18 October 2014

Tungsten filament

The invention: 

Metal filament used in the incandescent light bulbs
that have long provided most of the world’s electrical lighting.

The people behind the invention:

William David Coolidge (1873-1975), an American electrical
Thomas Alva Edison (1847-1931), an American inventor

The Incandescent Light Bulb

The electric lamp developed along with an understanding of
electricity in the latter half of the nineteenth century. In 1841, the
first patent for an incandescent lamp was granted in Great Britain.A
patent is a legal claim that protects the patent holder for a period of
time from others who might try to copy the invention and make a
profit from it. Although others tried to improve upon the incandescent
lamp, it was not until 1877, when Thomas Alva Edison, the famous
inventor, became interested in developing a successful electric
lamp, that real progress was made. The Edison Electric Light
Company was founded in 1878, and in 1892, it merged with other
companies to form the General Electric Company.
Early electric lamps used platinum wire as a filament. Because
platinum is expensive, alternative filament materials were sought.
After testing many substances, Edison finally decided to use carbon
as a filament material. Although carbon is fragile, making it difficult
to manufacture filaments, it was the best choice available at the time.

The Manufacture of Ductile Tungsten

Edison and others had tested tungsten as a possible material for
lamp filaments but discarded it as unsuitable. Tungsten is a hard,
brittle metal that is difficult to shape and easy to break, but it possesses
properties that are needed for lamp filaments. It has the highest
melting point (3,410 degrees Celsius) of any known metal; therefore,
it can be heated to a very high temperature, giving off a
relatively large amount of radiation without melting (as platinum
does) or decomposing (as carbon does). The radiation it emits when
heated is primarily visible light. Its resistance to the passage of electricity
is relatively high, so it requires little electric current to reach
its operating voltage. It also has a high boiling point (about 5,900 degrees
Celsius) and therefore does not tend to boil away, or vaporize,
when heated. In addition, it is mechanically strong, resisting breaking
caused by mechanical shock.
William David Coolidge, an electrical engineer with the General
Electric Company, was assigned in 1906 the task of transforming
tungsten from its natural state into a form suitable for lamp filaments.
The accepted procedure for producing fine metal wires was
(and still is) to force a wire rod through successively smaller holes in
a hard metal block until a wire of the proper diameter is achieved.
The property that allows a metal to be drawn into a fine wire by
means of this procedure is called “ductility.” Tungsten is not naturally
ductile, and it was Coolidge’s assignment to make it into a ductile
form. Over a period of five years, and after many failures, Coolidge
and his workers achieved their goal. By 1911, General Electric
was selling lamps that contained tungsten filaments.
Originally, Coolidge attempted to mix powdered tungsten with a
suitable substance, form a paste, and squirt that paste through a die
to form the wire. The paste-wire was then sintered (heated at a temperature
slightly below its melting point) in an effort to fuse the
powder into a solid mass. Because of its higher boiling point, the
tungsten would remain after all the other components in the paste
boiled away. At about 300 degrees Celsius, tungsten softens sufficiently
to be hammered into an elongated form. Upon cooling, however,
tungsten again becomes brittle, which prevents it from being
shaped further into filaments. It was suggested that impurities in
the tungsten caused the brittleness, but specially purified tungsten
worked no better than the unpurified form.
Many metals can be reduced from rods to wires if the rods are
passed through a series of rollers that are successively closer together.
Some success was achieved with this method when the rollers
were heated along with the metal, but it was still not possible to
produce sufficiently fine wire. Next, Coolidge tried a procedure
called “swaging,” in which a thick wire is repeatedly and rapidly
struck by a series of rotating hammers as the wire is drawn past
them. After numerous failures, a fine wire was successfully produced
using this procedure. It was still too thick for lamp filaments,
but it was ductile at room temperature.
Microscopic examination of the wire revealed a change in the
crystalline structure of tungsten as a result of the various treatments.
The individual crystals had elongated, taking on a fiber like
appearance. Now the wire could be drawn through a die to achieve
the appropriate thickness. Again, the wire had to be heated, and if
the temperature was too high, the tungsten reverted to a brittle
state. The dies themselves were heated, and the reduction progressed
in stages, each of which reduced the wire’s diameter by a
thousandth of an inch.
Finally, Coolidge had been successful.Pressed tungsten bars
measuring 1/4 x 3/8x6 inches were hammered and rolled into rods 1/8
inch , or 125/1000 inc, in diameter.
The unit 1/1000 inch is often called a “mil.”
These rods were then swaged to approximately 30 mil and
then passed through dies to achieve the filament size of 25 mil or
smaller, depending on the power output of the lamp in which the
filament was to be used. Tungsten wires of 1 mil or smaller are now
readily available.


Ductile tungsten wire filaments are superior in several respects
to platinum, carbon, or sintered tungsten filaments. Ductile filament
lamps can withstand more mechanical shock without breaking.
This means that they can be used in, for example, automobile
headlights, in which jarring frequently occurs. Ductile wire can also
be coiled into compact cylinders within the lamp bulb, which makes
for a more concentrated source of light and easier focusing. Ductile
tungsten filament lamps require less electricity than do carbon filament
lamps, and they also last longer. Because the size of the filament
wire can be carefully controlled, the light output from lamps
of the same power rating is more reproducible. One 60-watt bulb is
therefore exactly like another in terms of light production.
Improved production techniques have greatly reduced the cost
of manufacturing ductile tungsten filaments and of light-bulb man-
ufacturing in general. The modern world is heavily dependent
upon this reliable, inexpensive light source, which turns darkness
into daylight.

See also : Fluorescent lighting; Memory metal; Steelmaking process ,Tungsten filament


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