2016 Feb

Thomas Edison and Light Bulb

February 9, 2016

Thomas A. Edison

The story of the invention of light bulb exemplifies the importance of perseverance in science. For those of you who are not aware of the story, Thomas Alva Edison invented the light bulb and he did it so after failing thousands of times. When he was asked, “How did it feel to fail 1,000 times?”, he simply said “I didn’t fail 1,000 times. The light bulb was an invention with 1,000 steps”. It would take an extremely uninterested mind not to ask why did the invention take so many attempts and what is the science behind workings of a light bulb?

As described in a previous essay, light bulb works on the concept of Joule heating. A typical light bulb has positive and negative terminals that are connected to a filament enclosed by a medium of inert gases. When current passes through the filament, due to Joule heating the filament is heated to very high temperature and as result visible light is released. But what took 1,000 unsuccessful attempts in making such a simple device? A filament needs to be heated to temperatures of the order of thousands, for it to release visible light. From the Joule heating, we know that the amount heat energy released while passing current is proportional to the amount of current passed, resistance and time for which the current is passed. So ideally the filament should have high resistance and should be able to remain at high temperature for long time. The challenge was in finding the right material for the filament.

Tungsten was chosen because it has high melting point and can conduct electricity. To increase the resistance, a thin tungsten wire is taken and coiled and these coiled wire is further super-coiled to accommodate a very long wire (of around few hundreds of centimeters) within few inches space provided for filament in a typical light bulb. This tungsten filament is vulnerable to react with oxygen in the air and change its chemical form. To avoid this, a covering is used to enclose the filament and all the air is pumped out of the covering (called as bulb). But this results in another complication.

The chapter on thermal physics learnt during our Physics tuition classes explains this complication. When a material is heated in vacuum, due to the absence of any atoms in the space the tendency of the material to undergo evaporation is high. Thus a tungsten filament heated in a vacuum like bulb will not last long. To prolong the life of filament and avoid its evaporation, inert gas is pumped into the bulb. Due to the presence these gas molecules, the evaporation rate is reduced and also since these molecules are inert, any reaction that degrades the quality of the filament is averted. It is now not a wonder to see why it took Edison 1,000 attempts to make it.

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