Welcome to You Ask Andy

Joe Descans, age 11, of San Diego, Calif., for his question:

HOW DO LIGHT BULBS LIGHT?

Long., long ago people told stories of magical genies that lived in lamps. If you were lucky enough to come across one of these lamps, and rubbed it just so, the genie would pop out and give you three wishes. In our modern times we, too, have a magical genie  we call him electricity. Like the genies of old, he also lives in a lamp, but we call his home a light bulb, and he can turn night into day.

There are many different kinds of electric light bulbs. A flashbulb give us lots of light for a brief moment. The mercury vapor lamp that lines exist and entrances of many streets and freeways is another lamp that serves a specific purpose. The lights that turn night into day at a football or baseball game are filament flood lamps.

Since the kind of light bulbs we are most familiar with are the ones that light up our homes, let's spend our time talking about them. And to make them easier to understand let's first see if we can compare them to something we really know something about  water in a hose.

Electrical current is something like water in a garden hose. The force of the water flowing through the hose is called water pressure. Electricity moves through a wire, and the force of its flow is called voltage. If you put a nozzle on the hose, or use a smaller hose, you increase the water pressure. To increase the flow of electricity, then, you use a thinner wire.

As everyone knows, water comes into a hose from a faucet and flows out the open end  electricity does not. Electricity flows continuously only in a complete circuit. It is doomed to travel in circles, from a positive to a negative pole. If you break the circuit, the electrical current stops flowing.

Every time you flip the light switch you complete an electrical circuit and electricity flows. In a light bulb, the electric current flows into the bulb from a heavier wire. Inside the bulb it is forced through a much thinner wire, then out in another heavy wire. The thin wire inside the bulb is called the filament. As electricity passes through the filament the pressure causes the wire to get hot. The fact is that the thin wire becomes white hot and its glow gives us light.    

If the flowing filament were exposed to ordinary air, it would burn up in a second. The trick then, is to get all the air out of a light bulb before it is sealed. This is done with the aid of a vacuum pump, and the ordinary air is replaced with a nonactive gas such as nitrogen or argon.

 

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