Beverly Littlewood, age 10, of Chippawa, Ontario, Canada, for her question:
How does a light bulb burn out?
In North America, this small tragedy occurs perhaps three million times every calendar day. And electric light is shedding its warm radiance, when suddenly it stops working without a flicker of warning. If you examine the burned out bulb you may or may not notice a fine sooty film on the inside of the glass. This could have warned you that the bright eyed little bulb was just about ready to give up the ghost.
Electricity is a fierce force of nature and to tame it we must keep it safely imprisoned within shielded copper wires. Experts in the know are sure to approach it with care. So let's follow their procedure and study our light bulb when it is not screwed into a socket. Besides, it's easier that way to peek inside. The best specimen for this job is made of clear glass. Notice the wire inside the little glass bubble. There are two rather sturdy prongs coming up from the base. The tops of the prongs are joined with a fine fine filament of wire. That is, if your bulb is in working good health. After it burns out, the fine filament is in horrible condition and maybe you can see that it is broken.
The metal base of the bulb is made to screw into an electric socket. This socket is a cunning outlet that can tap the electric current in the wires. The current is a stream of zillions of moving electrons, those minuscule particles that usually orbit around inside atoms. The electric current must travel in a circuit from a generator and back again. We make it travel through the house in a circuit of carefully shielded double copper wires. Each socket is a depot where we can latch onto some of the power in the electric circuit. 11
When the bulb is screwed into a socket, its metal base contacts the current. Its built in wires lead the power up through the wires sealed inside the glass bubble. When we turn on the switch, streams of teeming electrons zoom through the rods in the bulb and across the fine filament at the top. In the upright rods, the swarming electrons had a fair amount of room for their antics. But this changes when they crowd into that fine filament. Suddenly their huge wide highway becomes a single traffic lane.
They are all crowded together, traveling at breakneck speeds. You cap guess what happens. Electrons collic ,millions of times a second. As these traffic accidents cause heat, enough heat to make the filament glow with light electric light. However, the fine filament can take only so much of this glowing heat. After so many hours of duty, its mistreated atoms begin to separate and the filament fails. If this fine wire, or any other part of the electric circuit is broken, the electric current is disconnected. So when the filament burns out, the electric light must give up the ghost.
What we need, of course, is a super tough metal for making that thin filament in our light bulbs. The first ones were made of carbon but this substance could not withstand those energetic electrons for very long. Several stronger materials were used later. Nowadays most light bulbs have filaments of tungsten or molybdenum. But even these tough metals give up after so many hours of lighting duty. Some modern bulbs are supposed to last for years, but they give somewhat less light. So far, nobody has invented a super bright light bulb with a filament that lasts and lasts.