So much were some of the ancients impressed with the property of loadstones for attracting iron that one of them suggested building a great arch of this material in a temple so that the iron statue of the goddess would remain suspended in the air without resting upon any support. There is an old legend that the iron coffin of Mohamet rose and remained near the ceiling of the mosque in which it was buried.
It was early discovered that when pieces of steel were rubbed on a loadstone they took on the properties of the loadstone and became magnets. In the experiments with magnets, it was found that like poles repelled and unlike poles attracted, and that iron or steel in contact with a magnet becomes magnetized. Iron and steel are practically the only substances attracted by a magnet, although nickel and cobalt and a few other substances have a little attraction. Thus steel and iron are always used for magnets.
Experiment 14. -Wind twenty feet of No. 20 insulated copper wire around the nail used in Experiment 13 as you would wind thread on a spool.
Attach one end of this wire to each
pole of a dry cell. Bring the nail thus arranged toward a suspended magnet. Reverse the ends of the nail. Does the nail act as it did before it was placed within the coil of wire connected to the battery?
Bring another nail in contact with
its ends. What happens? What has Fig.14.
the nail as arranged become? Disconnect one of the wires from the battery and try the test again. Does the nail act as it did when the battery was connected?
MAGNETIC CRANE.
The electromagnet is lifting tons of scrap iron.
We found that if a nail is placed in a coil of wire connected with an electric battery it becomes magnetic, but only as long as the connection is maintained. Magnets of this kind are called electromagnets. If the nail had been hard steel and the battery exceedingly strong, the steel would have remained a magnet after being taken out of the coil.
Magnets are at the present time ordinarily made by electrical action and not by rubbing on other magnets. Magnetized steel bars are called permanent magnets. Electromagnets have become of almost inestimable use in modern life. The telegraph, telephone, magnetic crane, electric motor, and almost innumerable other mechanical devices are dependent largely upon the principle of electromagnetism for their usefulness.
19.The Magnetic Field of Force.
Experiment 15. -Place a plate of window glass about the size of a sheet of writing paper above a bar magnet and carefully sprinkle iron filings over it. Describe the behavior of the filings. Sketch on a piece of paper their arrangement. Move a small compass about above the glass plate and note the directions the needle assumes. How do the actions of the needle and of the filings compare? If feasible make blue print.
Holding the small compass two or three inches above the magnet move itparallel with the magnet from end to end. Gently tap the compass occasionally so that the needle will move freely. How does the needle act when it is over the ends of the magnet? How does the direction of the compass needle compare with the direction of the bar magnet?
In the above experiment we found that when iron filings were sprinkled above the magnet they arranged themselves in definite lines. The small compass needle also arranged itself along these lines when brought under the influence of the magnet. There is, then, around a magnet a magnetic field of force which affects magnets and magnetic substances brought within it. It is found that magnetic intensity, like the intensity of sound and light, varies inversely as the square of the distance.
When the compass was placed above the ends of the bar magnet one of the ends of the needle was pulled down toward the magnet, or it might be said to dip toward the magnet. When moved near the middle of the magnet it assumed a horizontal position, and when it approached the opposite end of the magnet the opposite end of the needle dipped. This same action is found when a magnetic needle is carried from north to south upon the earth. If a needle is carefully balanced and then magnetized, it will be found no longer to assume a horizontal position.
In the northern hemisphere the north end will dip and in the southern hemisphere the south end. In the northern hemisphere it is customary to make thesouth end of the needle a little heavier so that it will stay in a horizontal position. At the magnetic pole the needle would stand vertical. If a needle is accurately balanced on a horizontal axis and then magnetized, it will show the angle of dip in anylocality. Such a needle is called a dipping needle(Fig. 15).
Fig. 15.
20.The Mariner"s Compass. -In the ordinary mariner"s compass a magnetic needle is arranged so that it will swing freely in a horizon- tal plane. A circular card is divided into four equal parts the dividing lines of which are marked with the cardinal points of the compass, the intervening spaces being divided into eight equal divisions. The card is attached to the needle and inclosed in a box called the binnacle.
Fig. 16.
This box is arranged so that it will al- ways remain horizontal. A fixed line on the binnacle shows the direction of the keel of the ship. The card being attached to the needle always has its "north" pointing toward the north. To determine the direction of the ship it is only necessary to notice on the card in what direction the keel line is pointing. The mariner of course must know the declination at the place where he is andmake the proper corrections. The different governments furnish tables and charts showing these corrections.
