Fig. 172.—Iron filings attracted to the poles of a magnet.
201. Law of Magnetic Action.—The north pole of a magnet is usually marked. If a marked bar magnet be held in the hand and its north-seeking pole be brought near the north-seeking pole of a freely suspended bar magnet, the two poles will be found to repel each other, as will also two south-seeking poles, while a north-seeking and a south-seeking pole attract each other. (See Fig. 173.) This action leads to the statement of the Law of Magnetic Action: Like poles repel, while unlike poles attract each other. The force of attraction or repulsion lessens as the distance increases. The force of the action between magnetic poles is inversely proportional to the square of the distance between them. Compare this with the law of gravitation (Art. 88).
Fig. 173.—Like poles of two magnets repel.
Fig. 174.—A magnetoscope.
202. Magnetic Substances and Properties.—It is found that if an iron or steel magnet is heated red hot that its magnetic properties disappear. Accordingly one method of demagnetizing a magnet is to raise it to a red heat. If a magnet that has been heated red hot and then cooled is brought near a suspended bar magnet, it is found to attract either end, showing that it has regained magnetic properties even though it has lost its magnetic polarity. A suspended bar magnet used to test the magnetic properties of a body is called a magnetoscope. (See Fig. 174.) The needle of a magnetic compass serves very well as a magnetoscope. Magnetic properties are most strongly exhibited by iron and steel, though nickel and cobalt show some magnetic effects. There is a peculiar alloy of copper, aluminum, and manganese, known as Heusler's Alloy, that is also magnetic. However, of all substances, iron and steel show the strongest magnetic effects.
203. Magnetic Induction.—Let the north-seeking pole of a bar magnet support an iron nail by its head. (See Fig. 175.) Test the point of the nail for polarity. See whether a second nail can be attached by its head to the point of the first. Test the polarity of the point of this nail. Find by trial how many nails can be suspended in succession from the magnet. Test in each case for polarity. Withdraw carefully the magnet from the first nail—the string of nails will fall apart. Repeat the test with a thickness of paper between the magnet and the first nail. Results similar to those secured at first will be found, though probably fewer nails will be supported. The presence of paper between the magnet and nails simply weakens the action. Test the action of the magnet upon the nail when there is between them a piece of glass, one's thumb, thin pieces of wood, copper, zinc, etc. The magnetizing of a piece of iron or steel by a magnet near or touching it is called magnetic induction. This action takes place through all substances except large bodies of iron or steel hence these substances are often used as magnetic screens. The pole of the new induced magnet adjacent to the bar magnet is just opposite to the pole used. Thus the N.-pole of the magnet used will produce a S.-pole at the near end of the nail and a N.-pole at the end farther away. (See Fig. 175.) On removing the magnet, the nails are found to retain a part of their induced magnetism.
Fig. 175.—Nails magnetized by induction.
204. Retentivity.—In several of the foregoing paragraphs it has been seen that a piece of iron or steel when once magnetized does not entirely lose its magnetism when the magnetizing force is removed. Different pieces of iron and steel vary greatly in this respect, some remaining strongly magnetized, others losing much of their magnetism. This property of retaining magnetism is called retentivity. Hardened steel has a high degree of retentivity, while soft iron retains but little magnetism.