Tungsten For Incandescent Bulbs

Table of contents

  • The filament
    • Brief history of electric lighting
    • The interest of tungsten
  • The inside of the bulb
    • The role of rare gas
    • Halogen bulbs
  • The future of this type of light bulbs…
  • For more information on the different lighting
Reminder of the questions that we asked:

Tungsten is a metal, steel-gray color to white, chemical W symbol, and number Atomic Z = 74. Can you explain the choice of the metal for the filaments in incandescent bulbs?

In addition, inside the bulbs, we find a rare gas, often krypton or argon. How is the presence of this gas improves the performance of the light bulb?

The Filament

Brief History of Electric Lighting

In the middle of the 19th century, we still lights the candle or gas. Knowledge of electrical phenomena is progressing, but does not provide more enforcement to lighting. The main difficulty to get a stable incandescent is the fact that in the presence of oxygen a door high temperature filament burns instantly.

The chemist Sir Joseph Swan (1828-1914, United Kingdom) was the first to try to make a lamp. In 1860 he succeeded in making blush a piece of paper covered with carbon and maintained between two electrodes in an empty air bulb to keep it from burning. But it is facing difficulties to the vacuum in the bulb and make sure the electrical contact between the electrodes and the paper. In the 1870s, it has pumps capable of achieving a sufficient vacuum quality. In 1878, Swan gave a demonstration of future electric light with the incandescent carbon wire in a light bulb under vacuum.

The following year, in 1879, the great inventor Thomas Edison (1847-1931, U.S.) which has already imagined the phonograph and improved microphone, that challenge too. He directed hundreds of tests, for example using a bamboo bent u and connected to two sons of Platinum. With a filament of carbonized cotton, he gets a lamp that illuminates for about 48 hours. Edison protected all his discoveries by patent.

In 1883, after a legal dispute over the precedence of their inventions, Swan and Edison founded the EDISON & SWAN UNITED ELECTRIC LIGHT Co. In 1909, the tungsten filament replaces the filament of carbon, which definitely improves the longevity of incandescent lamps.

The Interest of Tungsten

Today, the filament inside the bulb is made of tungsten, because it is the metal which has the highest melting temperature (3410 ° C according to the www.techniques-ingenieur.fr website) or 3422 ° C according to www.webelements.com. A filament of tungsten can be heated at high temperature, remaining solid and rigid up to 2700 K about, without too quickly evaporate.

The radiation from a black body at T = 2900 K is maximum for the wavelength λm = 1 μm (law of “displacement” of Wien: λ)m T = 2898 μmK). A large part of the radiation of filament of tungsten filaments is always issued in the infra-red radiation that heats without light.

Ideally, he would reach the temperature of 5800 K to reproduce a spectrum white, close to that of light solar and fits the spectral sensitivity of the eye. By increasing the temperature of the filament incandescent, you move the infrared to visible light radiation and improved the efficiency of lighting.

The Inside of the Bulb

The Role of Rare Gas

If the bulb was filled with air, the filament in high temperature burn in the presence of oxygen, and life would not exceed the second. A solution is, as we have seen, to the vacuum in the bulb. In these ordinary lamps, tungsten of the filament evaporates gradually and is deposited on the inner walls more cold bulb. It is darkening, illuminates less and the filament will eventually break (after about a thousand hours anyway…).

The presence of krypton or argon (which are inert gases) to slow down the deposit of vaporized Tungsten on the walls of the bulb through the shocks with the inert gas atoms. The advantage is to increase the temperature, and thus the effectiveness, without decreasing the operating time of the bulb.

To see a filament glow in nitrogen and burn in air, you can see the article n fun, and more particularly the part of the video called “nitrogen and light.”

Halogen Bulbs

Halogen bulbs are very intense lighting, like that of the “quartz-iodine” lamps used in work practices. What is the principle?

Gas halogen (iodine) added to the inert gas in the bulb is used to recycle tungsten, which vaporizes: iodine reacts chemically with the vaporized tungsten atoms, formed WI tungsten iodide molecules dissociate themselves in contact with the hot filament, the metal deposits, which regenerates the filament. This allows here again to increase the temperature of the filament (can reach approximately 3100 K), and therefore the efficiency, shorten the lifetime of the bulb. The “quartz-iodine” bulbs are quartz to withstand this high temperature.

The Future of This Type of Light Bulbs…

Incandescent bulbs illuminate, but especially heat: a small fraction (about 5%) of the electrical energy consumed is converted into visible light radiation, the majority being dissipated as heat by Joule effect. Light efficiency is usually 10 to 25 lm/W (the power ratio “photometric” of the emitted radiation, lumen, electric power consumption in watt).

This type of classic lamp is rivaled by the more efficient fluorescent lamps and discharge lamps. Some CFLs (miniature fluorescent lamps) are sold under the label “low power” their efficiency is 60 to 70 lm/W. concretely, we get the same illumination with a lamp 40 W (to 60 lm/W) CFLs with a classic lamp 100 W (efficiency 25 lm/W). Another advantage of CFLs, they have a life expectancy of 6 to 10 times longer than incandescent lamps. They have the disadvantage of being more expensive to buy and still producing a bluish, less flattering light for the interior lighting.

Since the beginning of 2007, several countries set the goal to replace by more efficient bulbs like LED bulbs featured on aparentingblog.com, in order to combat the greenhouse gas missions. On 20 February 2007, the Australian Government announced its intention to ban by 2010. Indeed, it was calculated that such a change, done by all Australian households, would reduce emissions of CO of 800,000 tonnes per year2. In Canada, the same announcement took place on 25 April. In the United States, the Governor of California, Arnold Schwarzenegger, announced an identical bill for a total suppression in 2012. After the newspaper Le Monde of 24 March 2007, Europe, driven by the United Kingdom and especially the Germany, is considering such measures to save energy, but with a very progressive application: a personal decision could be taken in 2009.

 

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