Incandescent light bulb#Reducing filament evaporation

The Langmuir layer refers to a region around the filament or the filament takes place in practically no convection ( flow ) of the filling gas. Its discovery dates back to observations by Irving Langmuir at General Electric in 1912 and was an important finding to reduce the energy loss due to heat dissipation. By coiling of the filament and the introduction of an inert gas, the light yield and the lifetime of the bulb could be increased.

Background research by Irving Langmuir

In an incandescent lamp emits a current -carrying of tungsten - filament light. Modern lamps that are operated with mains voltage of 230 V, typically have a very long, thin filament - finer than a human hair and with a length of about 1150 mm. In the first incandescent lamps the filament was loosely hung or clamped in a zigzag pattern.

To prevent oxidation of the wire by oxygen was in the first light bulbs within the glass envelope is a vacuum. However, the vacuum has the disadvantage that the tungsten atoms of the wire freely evaporate and condense on the inside of the glass bulb as a dark lining. The light transmittance of the glass is reduced. At the same time the wire by the material will burn out quickly and limits the life of the lamp, in particular at higher radiation power.

Langmuir's task was initially to study the cause of the formation of the dark covering and to find potential factors for this. Then he experimented with numerous available to him filling gases by varying the filling pressure. He found out that particular increase traces of water in the residual gas deposit formation significantly. By the heat of the water is dissociated in the vicinity of the filament into oxygen and hydrogen. The tungsten reacts with the oxygen. The resulting molecules are accelerated outwards and deposited as a thin tungsten oxide layer on the colder inside the glass bulb from. The free hydrogen reduces the oxide to dark -appearing, metallic tungsten, again forming water, which represents a new reaction at the filament available. Even with a good vacuum, the effect was only slightly reduce but not prevent.

On the other hand, resulted in the experiments Langmuir that the evaporation of the tungsten filament at significantly reduced when the flask is filled with nitrogen. This inert gas itself does not react with the tungsten. Rather, many of the votes from the filament tungsten atoms collide with the gas atoms and are reflected to the wire. The lifetime of the bulb increases. On the other hand, passes the gas filling clearly in contrast to the vacuum heat. To bring the filament to the same brightness as in a vacuum, a much higher energy input is required. The efficiency of the incandescent lamp is reduced. Unlike in vacuum, this effect can be influenced by the arrangement of the filament in the lamp.

The Langmuir layer

The filament loses heat to the surrounding gas layer him. This layer in turn heats the next layer. In this way the heat is continuously conducted to the outside.

With increase in the temperature, the viscosity of the gas increases. The viscosity effect in gases caused by the growing momentum exchange between the gas due to increased molecular motion. In the prevailing high temperatures at the hot filament can be expected from a static to a first approximation gas layer of 1-2 mm thickness, which wraps around the filament in the form of a cylindrical shell. This case is named after its discoverer Langmuir layer.

In the Langmuir- layer takes place no heat transmission by convection ( flow ) due to the high viscosity of the gas. This is the heat conduction - in addition to the dominant but inevitable heat radiation - the mechanism through which to which the Langmuir layer gives off heat the filament surrounding the filling gas. The heat loss can be simplified approximated by the following formula:

Wherein a constant length of the wire, the thickness of the wire and the thickness of the Langmuir- layer group.

To keep the heat loss low, hence the surface of the cylindrical shell must be as small as possible and thus in particular be as low as possible its length. The filament - like hitherto usual - long and thin, it is discharged through the entire length of the heat to the outside. The thickness of the light wire, however, has a significantly lower impact on the size of the Langmuir layer. Replacing the incandescent filament by means of a fine, spiral-shaped filament, as the Langmuir- layers of the individual wires and the entire arrangement of a wire cylinder with the outer diameter of the spiral overlay may be simplified adopted. In this way, great thread lengths can be realized at low heat loss.

To enhance the effect, is formed especially in light bulbs with higher voltage, like Operation at mains voltage 230 V, the filament in the form of a double helix. The transition from a tungsten filament onto a single double helix results in a gain in light output by 20% and was implemented from 1932. By using krypton instead of argon as the filling gas of the heat conduction loss can be reduced by the higher atomic mass further, so that in addition can be realized 7% more light output for the same service life.

Influence on the Incandescents

Langmuir filed on April 19, 1913 in America a patent entitled Incandescent electric lamp ( electric bulb ) one, which was issued April 18, 1916 patent number 1,180,159. It comprises incandescent lamps with a tungsten filament, which are filled with a gas that has a relatively poor thermal conductivity.

The first tungsten lamps with a gas filling of nitrogen at approximately atmospheric pressure came in 1913 as a 1000 W and 750 W versions on the market. To distinguish them from the vacuum lamps they received in America the name Mazda C. Other variants with a capacity down to 200 W followed in 1914. Starting around 1918, a portion of the gas filling replaced with argon and smaller bulbs with an output of 40-50 W as gas-filled lamps offered.

Increasing the efficiency of the bulbs was critically dependent on the performance. As for the gas-filled high power lamps with a service life of about 1000 hours, the light yield of about 20 lm / W ( lumens per watt ) over a vacuum lamp approximately doubled, settled for a 100 W lamp in 1917 an increase of only about 10 lm / W can be realized to 12.5 lm / W.

A problem of that time was the sagging of the tungsten wire during operation. This limited the minimum distance between the windings of the coil. It was not until 1917 - also by an employee of General Electric - was developed a new alloy for the wire, prevented a change in crystal structure in metal a mutual displacement within the material and thus provided for stability. This discovery was made possible only developing a long -term stable double helix in 1926. Starting in 1936, standard incandescent bulbs could be mass-produced with a double helix. This change from a single to a double helix spiral, the temperature of the wire could be increased with the same lifetime. The light output rose at a 60 - W lamp of 12.5 lm / W 13.8 lm / W, which a 100 W lamp of 15.3 lm / W 16.0 lm / W.