A Bit of Trivia About Light Bulbs . . . .
Why Light Bulbs Wear Out
As a young engineer a "few" years ago, I did some investigating on light bulb failures as part of a project for aircraft cockpit warning lights. As one might guess, the lights needed to be rugged and long lasting. My focus was on understanding the characteristics that determine the way that incandescent lights wear and eventually fail.
It turns out that the failure mechanism was the same for regular tungsten light bulbs that are filled with an inert gas and similar tungsten light bulbs that also have a small amount of gas in the halogen series (bromine or iodine, for example), and also the same as the filament wearing out in antique evacuated light bulbs which have a hard vacuum. There are always some water molecules that are chemically bonded to the tungsten filament. When sufficient electrical current is passed through the tungsten filament, it gets hot enough to glow ... and the more current the brighter it gets. Light bulbs are heated to a bright white while some of the antique vacuum light bulbs were heated to a bright yellow. In either case the water molecules that were trapped in the tungsten boils off and becomes water gas molecules. In the process, some of the super heated tungsten molecules are carried off as they become attached to the water molecules.
When a light bulb is turned off, it cools down and the water molecules mostly are absorbed back into the tungsten filament, but some condenses on the interior of the glass. When a water molecule with an attached tungsten molecule deposits on the glass, it eventually reaches a point where you begin to see a silvery black color, mostly near the neck of the bulb where it is the coolest. This phenomenon is known as the "water cycle". The
water cycle is responsible for the filament wearing out as tungsten material gradually erodes away from the filament and onto the interior of the glass surface.
By the way, it was discovered many years ago that adding a small amount of gas in the halogen series of the periodic table slows down the process of the filament eroding away by causing more of the tungsten to be redeposited back onto the filament wire when the light is turned off.
If a filament is examined under a microscope as it wears out, it can be seen that the original smooth surface begins to take on a saw-tooth appearance. The saw-tooth shape of the filament means that there will be hot spots where the filament is the thinnest so this is sort of an avalanche effect that once started, can't be stopped. The filament reaches the point where it becomes very fragile at these narrow points and the thermal shock when the light is turned on will eventually cause the filament to fail ... usually going out in a blaze of glory, so to speak.
The culprit turns out to be the least expected suspect ... water ... plain old simple H
[SUB]2[/SUB]O. The same old seemingly benign H
[SUB]2[/SUB]O that created the Grand Canyon and is a participant in causing wood to split when it evaporates.
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