How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 566

Years ago I heard or read that some incandescent bulbs in Thomas Edison's house are still burning after being turned on back early in the 20th century. Is this true? What are they made of?
From comments that I've received over the web, this story is apparently true. However those bulbs must be operating at reduced power levels and are glowing dimly as a result. There is no magic filament material that can operate indefinitely at yellow-white heat. The life of a filament is determined by how quickly its atoms evaporate (actually sublime) from its surface. Modern tungsten filaments operate at about 2500° C. At that temperature, the filament loses atoms slowly enough that it lives for about 1000 hours. If you were to operate the filament several hundred degrees colder, it would live much, much longer but it wouldn't emit nearly as much light and what light it did emit would be relatively reddish. The design of incandescent bulbs is a trade-off of energy efficiency and operating life. Long-life bulbs are substantially less energy efficient than normal bulbs—you don't have to replace them as often but they cost more to operate. Getting back to Edison's bulbs: they can only live long lives by operating at less than normal temperatures. In that case, they may live a hundred years but have very poor energy efficiencies.
         

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