Carbon Filament Configuration and Light Distribution

Collectors of early incandescent lamps sometimes come across a lamp that has an unusual carbon filament configuration. At the beginning of the 20th century some manufacturers put lamps on the market that had filament shapes that deviated from the norm. This write-up concerns a few of those different designs that were made when it was desired to have more light directed in a downward direction when the lamps were burned base-up. It is also of interest to know which design might have approached the condition of equal light output in all directions. One such design appears to have achieved that goal.

In 1901 Francis W. Willcox read a paper before the National Electric Light Association that dealt with the distribution of light in vertical and horizontal planes from various standard incandescent filaments. One printed version of that talk appeared in American Electrician, Vol XIII, Nov 1901, pg 513. Light distributions from that paper are shown below.

One filament configuration, not shown above, was designed by A. A. Chaillet while he worked at the Shelby Electric Company, in Shelby, Ohio; the design is shown to the left. The requirement to design the filament so that much of it projected in a downward direction resulted in a bulb shape that was somewhat "mushroom" shaped. This particular design seems to be a popular one among collectors of incandescent lamps. It is also of especial interest because a lamp of that design has become known as the "Livermore Centennial Light." That particular lamp has been in operation for over 100 years. U. S. Patent No. 701,295, applied for in 1900 and granted in 1902, covers its design. The increased light output in a downward direction was important for some applications. The lamp was manufactured in Shelby, Ohio from 1900 to 1914.

A lamp designed for increased downward light output was put on the market in 1902 by J. H. Bunnell & Co. of New York City. The lamp had a round bulb and a porcelain petticoat reflector. The filament was moulded into several convolutions to direct light downward when burned base-up. It was used for show-windows, display rooms, halls and dining-rooms. It was made for 104, 110, 115 and 118 volts, 16 candle-power and 3.5 watts. The lamp was described in Electrical Review, Vol 41, No 26, July 26, 1902, pg 892.

An example of a Bunnell lamp is shown below where the word "SUNSHINE" is etched in a circle around the exhaust tip. The word "SUNSHINE" carries trade-mark No. 40,324. The application was filed on April 17, 1903. The word was used on lamps since July 16, 1902.

Another lamp that was designed for the same reason, that is, to direct the majority of light downward when the lamp was burned base-up was one manufactured by the Downward Light Electric Company, of New York City. The lamp was made tipless to enhance the downward light output. The word "Downward" was trade-marked (No. 44,037), with the application being filed on Dec 19, 1904; the word was used since Oct, 1903. All but a small percentage of the filament was in a nearly horizontal plane when the lamp burned base-up. The lamp is shown to the left.

It appears that the filament configuration in the first picture above that gives equal light distribution in all directions is that in Fig. 6. Although it was not designed to maximize light output in the downward direction, the design did result in equal light output in all directions; the design, therefore, is in a class by itself. The configuration in Fig. 6 was patented (U. S. No. 708,432) by Glenn Cannon Webster on Sep 2, 1902. The application was filed on Dec 30, 1901. Webster worked for the Sterling Electrical Manufacturing Company, of Warren, Ohio.