U.S. Patent 223,898
Thomas Edison's Incandescent Lamp
"To all whom it may concern: Be it known that I,
Thomas Alva Edison, of Menlo Park, in the State of New Jersey, United States of
America, have invented an improvement on Electric Lamps, and in the method of
manufacturing the same, (Case No. 186,) of which the following is a
specification. The object of this invention is to produce electric lamps giving
light by incandescence, which lamps shall have high resistance, so as to allow
of the practical subdivision of the electric light."
This opening paragraph from Edison's patent application
formally presented his light bulb invention to both the U.S. government and the
world. The application was filed on 4 November 1879 and the patent was quickly granted on 27 January 1880.
interesting aspect of the above drawing is the coiled filament depicted in
figures 1 and 3 ("a" on the drawing). Not only did Edison's patent
drawing show spiral filaments but the application repeatedly referred to them.
This rather small detail provides a glimpse into the pace of events at the Menlo
Edison's laboratory notebooks indicate that significant experiments took
place in October 1879 with many filament materials. As Edison noted in the
patent, "I have carbonized and used cotton and linen thread, wood splints,
papers coiled in various ways, also lamp black, plumbago, and carbon in various
forms, mixed with tar and rolled out into wires of various lengths and
diameters." Most of these materials could be coiled prior to baking. Having
found measured success with carbon and knowing that other inventors were seeking
to make a lamp, Edison wanted patent protection quickly. So he hurriedly filed
an application based on the state of experiments in late October.
However, he departed from this experimental path even before the patent was
granted. His demonstration lamps of late December used bristol-board filaments
cut in a single arch, horse-shoe shape. The bamboo filaments used in commercial
lamps from 1880 to 1893 also featured a single arch. Filaments with a tight
spiral did not become common in commercial lamps until Irving Langmuir developed
the gas-filled tungsten lamp in 1913.
The image above has been enhanced electronically.