=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). [Illustration: FIG. 173.--Like poles of two magnets repel.] [Illustration: 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.