The Osmium Filament Lamp

The osmium filament lamp is one that wasn't utilized to any extent in the United States, in part, because of the advent of the tungsten filament lamp. It was manufactured in Austria and Germany and was marketed in Germany and Great Britian. By 1905 the Vienna firm stopped producing the lamp. The trade name under which the lamp was sold in England was "Osmi", in Germany the name was "Auer-Os" and in Austria it was called "Osmin". It was the invention of Carl Ritter Auer von Welsbach.

The English 50-volt 25-cp lamp, marketed by the General Electric Company, Limited, of London, had a filament length of approximately 15 inches and consisted of three separate hairpin shape filaments connected in series. Lamps were also available at 2.5 cp and 4 volts, 5.5 cp at 8 volts, 10 cp at 25 and 33 volts, 25 cp at 45 to 60 volts and 32 cp at 70 to 75 volts.

Because osmium was a rare and expensive metal the lamps in 1902 were rented and not sold, so that the user would eventually return the lamps to the manufacturer. In 1903, when a lamp cost $1.25, the user could recoup $0.19 if the burned out, but unopened, lamp was returned within 18 months. In that case the filament could be treated and reused.

Of especial interest are the support wires that retained the filament loops. In general the lamps could be burned only in the vertical position with the bend down. This was necessary because the filaments would sag otherwise and possibly touch the bulb wall or each other. The supports were glass rods fused to the bulb. In the lamps first produced, metal loops were attached to the ends of the glass rods and the filaments passed through them. The supports in such a lamp are shown below.

It was found, however, that the filament would burn out at that location. Later in time lamps utilized an improved support system. On May 31, 1901 von Welsbach applied for a patent that was granted as U.S. No 814,632 on Mar 6, 1906. The patent concerned the combination of an osmium filament and a support that was composed of sintered refractory oxides. The support was non-adherent to the filament as well as chemically non-reactive. The material used consisted of a mixture of ten parts by weight of thorium oxide and one part by weight of magnesia. The oxides were mixed as powders and put into a viscous binding solution of sugar. A paste resulted that was shaped into filaments, dried and then heated in air to get rid of the organic material. The filaments were then subjected to a very high temperature until the particles were sintered together. The oxide material was white, as can be seen in the photograph below.

Although the osmium lamp was more efficacious than the incandescent lamps that preceded it, it probably would never have had widespread use. The lamp could best be made for low voltage circuits, even though one was eventually made for 110 volts. In order to use osmium lamps on 110-volt or 220-volt circuits in England, adapters were sold for connecting to a single socket, two three or more lamps in series.