Page:How and Why Library 466.jpg

 must have the oxygen of the air to feed the flame. But in an electric lamp there is no opening for air. The light is made in a vacuum, or closed place, with no air in it. And the light is not made by a flame at all. It is made by an electric current passing along a loop of carbon filament as fine as a hair. It looks like pure magic, just as when Aladdin rubbed his lamp and did such wonderful things. A boy named Alva just rubbed his head, and did some wonderful thinking to make the electric lamp.

Of course he knew about the electric arc-lamp that was invented long before he was born, and that was in use to light our streets when he was a young man. The arc-lamp has two thick carbon pencils with the points nearly meeting. An electric current running along a wire, and to the end of one of the pencils jumps to the other pencil carrying some carbon dust along for a bridge. This electric bridge, in burning carbon dust and oxygen of the air, gives a dazzling white light. But it burns the oxygen rapidly, and is, in many ways, unfit for indoor use.

It seemed to our boy that as an electric flash is not a flame, a light should be made to pass over a carbon filament bridge in an air-tight lamp, so as not to burn up the oxygen we need to breathe. The trouble was to find a material for a small lamp, to take the place of the carbon pencils in the arc-light. He tried a hundred different things—platinum, cotton, bamboo and many others—and treated them in many ways. Some were too expensive, burning away too fast, even in a vacuum; some too frail. At last the perfect filament was made. Then how was the electric current to be carried to the filament inside an air-tight lamp?

The filament ends were set in platinum wires into the solid neck of the lamp, that was made to screw into a socket. The snapping on and off of a button on the socket, connected and disconnected the platinum wires with the copper wires that carry the electric current into, the house. But don’t you wonder how the filament is put into the closed glass bulb? When it is made the bulb is left open at the tip, where there is always a little knob or point. When the wires and the filament ends are set in the neck, the end of the bulb is heated, until the glass is soft. Then the air is sucked out making a vacuum, the tip is squeezed together, and there is the magic loop of light shut up in an airless, crystal prison. (See, .)