Light bulbs


The Huntington has a cool new history of science exhibit. One of the displays consists of about a hundred historical light bulbs, including three test bulbs labeled by Edison himself. They constructed a case filled with Nitrogen (in case the seals on the bulbs were broken) and put a few watts through some of the old bulbs. It’s interesting that Edison’s and many others’ bulbs were designed to maximize the length of filament that was inside the bulb, by looping the filament up and down in a circle. Modern day light bulbs are made with very thin filaments in tightly packed coils. That would seem to concentrate the heat, leading to faster sublimation and weakening of the tungsten. So my question to the blogosphere is, why doesn’t this happen, or why is the modern design better?

Historical light bulbs on display at the Huntington Library

Historical light bulbs on display at the Huntington Library

One Response to “Light bulbs”

  1. 1 gregv

    Sorry, I couldn’t wait for the blogosphere. Wikipedia suggests, in broken English, a solution. The filament is a “coiled coil”, that is one loop made up of a tightly packed tungsten coil. Supposedly this coil sublimates at a rate proportional to the surface area of the envelope coil, instead of the surface area of the “entire coil”, that is, the surface area of the tungsten if you completely unwound it. I guess the benefit of this effect (which is plausible though I don’t completely understand) must outweigh the harm of concentrating the heat and speeding up sublimation.

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