Welcome to You Ask Andy

Jim Brady, age 13, of Visalia, Calif, for his question:

What is used to make a light bulb?

The materials used to make an ordinary light bulb are glass., one or two gases, metal and metal alloys ‑ but the ingredients must be put together just right. Nowadays, this job is done entirely by machinery, and machines in the U.S. produce about 700 million ordinary light bulbs every year. Besides a lot of special bulbs,  big and little, they also produce about 500 million flash bulbs for taking photographs and some 120 million Christmas tree bulbs.

The glass shells are made when the glass is hot and plastic. A set of rollers presses a series of hollows into a ribbon of hot glass and sends it along to a conveyor belt. Here each hollow fits into a hole. A mold rises up and a puff of air descends from a little nozzle. The round top of each glass shell is now formed. The sheet of half‑formed bulbs now goes to another conveyor belt where each one is given a neck. Sometimes the insides of the shells are frosted with a mixture of hydrofluoric acid.

A frosted bulb gives a gentle light, soft to the eyes; but it does not help us to study the working parts inside the light bulb. We need a bulb of clear glass for this. Notice there are two metal prongs sticking up like a couple of miniature flag poles. As a rule, these prongs are made from an alloy of nickel and coated with copper. Their job is to connect the inside of the bulb to the electric wiring system. Look closely and you will see that the two prongs are connected with a thread of wire not much thicker than a hair on your head.

This thread is the all‑important filament, the fine wire which glows and gives off the light. It is made of a durable metal called tungsten. Peering inside, you might suppose that the light bulb is filled with ordinary air ‑ but this is never so.   Air  contains oxygen, the gas which feeds fire, and we do not want that hot  tungsten to burn away. For this reason, the air is pumped out by the  bulb‑making machine and at one time the inside of a bulb was left as a near vacuum.

Nowadays, the inside is usually filled with nitrogen or argon gas or maybe with a mixture of both. These are lazy gases which usually refuse to help anything to burn. The tungsten filament can remain hot for a long, long time before it finally breaks down and the bulb burns out. The final fob is done when the shell is sealed with the metal base we screw into the light socket.

When we turn the switch, the electric current flows into the bulb through the prongs and the filament. The current is really a stream of moving electrons and, when they reach the filament, they are in a bottleneck which makes them become very agitated. They glow and give forth light. It takes about three billions billion of these agitated electrons to keep your reading lamp burning for one second.

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