Inventions and Inventors Volume 1 - Online Public Access Catalog

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 fiberlike
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 × 3 8 × 6 inches were hammered and rolled into rods 1 8
inch, or 125 1000 inch, 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.
Impact
Tungsten filament / 797
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-