=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. [Illustration: 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. Important Topics 1. Magnet; natural, artificial, bar, horseshoe. 2. Magnetic poles; north seeking, south seeking. 3. Law of action, magnetoscope, retentivity, induced magnet. Exercises 1. Make a summary of the facts of magnetism presented in this lesson. 2. Is magnetism matter, force, or energy? How do you decide? To what other phenomenon that we have studied is it similar? How? 3. Make a simple magnetoscope for yourself by suspending a thin steel needle or rod 5 to 10 cm. long, with a light thread or silk fiber at its center, so that it will hang level. Then magnetize the needle, and keep the magnetoscope in your book. 4. Name three uses for magnets or magnetism. 5. Mention three uses for a magnetoscope. 6. Are all magnets produced by induction? Explain. 7. In what magnetic devices is a high retentivity desirable? (2) THE THEORY OF MAGNETISM AND MAGNETIC FIELDS