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Name: Melonie S.
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: January 2004


Question:
What determines how long a light bulb will last before it burns out? Is it the wattage? Is it the number of times the bulb is turned on and off?



Replies:
Melonie -

I do not know of any research done specifically on light bulbs. (Though I am sure if you contacted the General Electric PR people, they would have such.) But, I am aware of research that was done on eclectic filaments in vacuum tubes a number of years ago. The factor most predictive of the life expectancy of a tube was the number of cycles... the number of times it heated and cooled. I would suspect this might relate to light bulbs also.

Larry Krengel


There are many causes for a light bulb to fail. Assuming you mean a standard incandescent light, failure results when the circuit is opened. This usually occurs when the filament (usually tungsten) breaks. What can cause it to break are numerous. Evaporation, if too much current overloads the capacity of the filament, vibration, turning the light on and off frequently can also be damaging because of the temperature change that occurs every time that happens, the size and construction of the filament. If you look at some light bulbs closely e.g. a flashlight bulb you will see that the filament is wound like a spring to give it greater flexibility. These are just a few of the things that can affect the life of a light bulb.

Vince Calder


Up-date: July 2008
"It is well known that the act of turning on an incandescent lamp often destroys the lamp due to "inrush" current in a cold filament. However, this is not a wear-out mechanism. Inrush current doesn't in itself reduce lamp life, it's merely the thing that finally kills it. The lamp's life is affected mostly by time. Over time, the tungsten vaporizes off the filament (which is why it often redeposits as a black "smudge" on the inside of the glass). When the filament develops a thin spot in the filament, it makes it more vulnerable to the inrush current at turn-on time, as that spot will get hotter than other spots (because it's thinner) and ultimately melt apart.

One factor which can decrease lamp life dramatically is increased operating voltage. In most cases this is tightly controlled by the power company (or battery, in the case of a car), so it's not under our control, but under lower voltage conditions it's easy to get an incandescent bulb to last for decades (albeit at lower brightness).

P. Bridges



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