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MAGNETISM

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MAGNETISM

to America, it was thought that the magnetic declination was a constant quantity. But on the voyage mentioned Columbus found that the declination varies from one point to another on the earth's surface. This was a discovery of the utmost importance to navigators. It might be called the second important discovery in magnetism. [In 1436 Andrea Bianco had marked this magnetic declination for different parts of the ocean on his atlas. What Columbus really discovered was a line of no variation. See COLUMBUS, COMPASS and Humboldt's Cosmos.}

Hartmann, who lived at a time intermediate between Columbus and Galileo, found that if a needle be so suspended as to lie horizontal before magnetization, it does not remain horizontal after magnetization; on the contrary, the north end "dips" down as if it had become heavier than the south end. This phenomenon is known as magnetic dip; and its discovery may be called the third important one in magnetism.

The first profound student of magnetism was Dr. William Gilbert (1540-1603), who was the leading man of science in England dining the reign of Queen Elizabeth. He was led to conclude from the manner in which small magnets behave at various points on the earth's surface that the earth itself is a gigantic magnet. But how the earth became magnetized is a problem which no one has yet been able to explain. Gilbert showed also that when an iron magnet is heated to red heat, it not only ceases to be magnetized, but loses all ability to become magnetized. Red-hot iron behaves, therefore, not as cold unmagnetized iron, but as cold brass or glass or zinc. This discovery is described by saying that iron loses magnetic quality at red heat. Gilbert also made it highly probable, by his experiments on broken magnets and on heated magnets, that magnetism is a molecular phenomenon, a prime discovery which all subsequent experiments have confirmed.

If, to the phenomena already described, we add that of magnetic induction, we shall have a fairly complete summary of the fundamental facts of magnetism. A piece of unmagnetized iron, when brought near a magnet, immediately acquires magnetic poles, i. e., it exhibits magnetization as well as magnetic quality. A wire nail held near a strong magnet not only is attracted itself, but will attract other iron nails or iron filings. These wire nails are «aid to be magnetized by induction. The most useful and most interesting case of induction met with in modern science is that of the electromagnet. Here an electric current is made to flow through a coil of wire in which is placed an iron core. This combination is called an electromagnet, and is a fundamental part of the dynamo, motor, telephone ana telegraph instrument.

The lifting power of the electromagnet is now used effectively in modern structural iron and boiler shops, foundries, shipyards and machine shops, and a great saving of labor is thus accomplished. Go into one of these great establishments, and you may see an electromagnet hooked to the end of a hoisting-chain carried by a crane. Suddenly it will be let down into contact with a pile of pig iron and ascend with a dozen iron pigs hanging to it. The crane moves along the overhead runway to the furnace platform, when the current is switched off and the pigs are dropped on the platform. At a trial

MAGNET LIFTING   PIG-IRON

made a steel gondola-car containing 109,000 pounds of pig iron was unloaded in two hours and five minutes, one man, the crane operator, doing the work. Scrap iron, which is difficult and tedious to handle by hand, is easily and rapidly picked up or unloaded from cars and placed where wanted. A safe weighing seven or eight tons is picked up and with a crane carried from one shop to another. In a hundred similar ways the electromagnet is made to accomplish feats which seem little less than marvelous, and the result is- large economy of labor.

Faraday made one of the great advances of modern times when he introduced into the study of magnetism the method of lines of force and the idea of the magnetic field. For these and other more advanced considera-