Page:Popular Science Monthly Volume 90.djvu/856

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The World's Most Powerful Searchlight

��It sends forth a beam as brilliant as fiercest sunlight

��IT is ten feet high, its mirror has a diame- ter of five feet, and it weighs three tons. Its beam is as brilhant as the sun at eight o'clock in the morning or four in the afternoon. New York latitude, and you can read a newspaper by its light thirty miles away. The heat of its focused beam is so intense that it will set paper afire at a dis- tance of two hundred and fifty feet. It has a candlepower of more than one and a quarter billion.

These are a few astonishing facts about the Sperry search- light, the invention of Elmer A. Sperry, of Brooklyn, N. Y., who is already known as the inventor of the air- plane stabilizer and ship gyroscope bearing his name and the first elec- tric arc light. When the last big air raid over London was made by Zep- pelins, the Sperry search- lights bathed the big dirigi- bles in beams of light they could not escape. According to some London accounts the Sperry searchlight is the Zeppelin's Nemesis.

One of the most powerful beacons along the coast is the Sandy Hook Lighthouse. But the Sperry searchlight is twenty-two times more bril- liant than that light. Were the Sperry lamp substituted for the lighthouse beacon, a ship passing out to sea could be bathed in light until it disappeared below the horizon. By swinging the light back and forth across the sky it has been made visible one hun- dred and fifty miles away. For Navy use the Sperry lamp illuminates a target ten times more brilliantly than any other pro- jector devised.

Equipped with a carriage that permits the lamp to be turned in a circle and in any direction up to ninety degrees, the giant searchlight is of the greatest value in detecting aircraft. The operator can not control it near at hand; the great heat prevents that. He must stand fifty feet away. At that distance he is able to focus accurately upon any moving object. Because the rays projected by the lamp are

���Elmer A. Sperry, inventor of the greatest searchlight

��nearly parallel, there is no diffusion of light over a wide area. The beam is concentrated. When the searchlight is being operated, the temperature of the arc is nine thousand degrees Fahrenheit- — seven thousand de- grees higher than the melting point of the metal holders of the carbons. Conse- quently, in order to prevent these parts from melting, a current of air is forced, by means of a motor-driven blower, through the carbon supports and discharged through the heat- radiating disks that surround the holders. In the Beck lamp the holders are sprayed with alcohol to prevent thern from melt- ing.

The several factors which combine to make the Sperry lamp so power- ful are the small elec- trodes, the special carbons used, the manner in which they burn and the parabolic mirror. A colored glass peep- sight enables the operator to watch the arc without being blinded by the glare, or the arc is reproduced by lenses upon a ground glass outside the lamp. An iris diaphragm similar to that used on cameras, regulates the light.

Stand in the beam of the Sperry lamp at any distance closer than three hundred feet and your skin will be burned. At that distance the skin peels. The great heat of the arc is due to the fact that it produces a crater which more nearly approximates the mathematical point of light than does that in other searchlights. The candlepower is more than three hundred and twenty thousand per square inch.

Designed for naval and military pur- poses, the Sperry lamp, in addition to locating enemy forces on land and in the air, is useful also in throwing a screen of powerful light in front of the enemy. It is impossible to see through its concentrated beam. Allied field forces have mounted the lamp on armored cars and have found it available for signaling at any distance up to one hundred miles.

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