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. An 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 Park lab. 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.

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