Donald M. Bliss.

Having made the electromagnet and circuit closer, as described in the preceding article, and connected with the battery, it will be noticed that whenever a current is sent through the coils, by operating the circuit closer, the soft iron core will become a powerful magnet so long as the current continues to flow through the coils ; but as soon as the circuit is opened or broken, the magnetism will as suddenly disappear. If the iron core is not very soft or thoroughly annealed, the magnetism will not disappear entirely with the loss of current, but will remain indefinitely. The strength and duration of such residual, or permanent magnetism as it is called, depends upon the degree of hardness and quality of iron used for the core. If hard steel be employed, the first instant the current flows through the coils it will become a permanent magnet, the strength of which may last for years. This may be easily demonstrated by removing the soft iron core and placing a piece of hard steel rod, an old file, or knitting needle in one of the coils and closing the circuit for an instant.

Before taking up the study of the principles embodied in this experiment, the following instructions for making permanent magnets with this apparatus may be noted.

If it is desired to magnetize small round or flat steel magnets from 3" to 4" long, leave the soft iron core in the coils, and magnetize by placing the magnets against the poles or ends of the core and closing the circuit for a few seconds. Do not remove the magnetized pieces from the poles while the current is on, but open the circuit first, then pull off the magnets. The reason for this is that if the piece is carelessly pulled off while the core is strongly magnetized, the magnet is liable to slide sideways, and weaken, or possibly reverse, the magnetism in the piece.

If the pieces to be magnetized are quite large or long, the best method is to take out the soft iron core entirely and insert one piece in each coil. Join the ends projecting beyond the coils by. laying a piece of iron rod or strip across the ends. Then turn the current on and off a few times, finally opening the circuit and removing the pieces from the coil, when they will be found to be strongly magnetized. For small magnets, Stubbs steel wire or rod answers very well for some cases. Old files, or any piece of good tool steel, will answer most purposes, but it must be hard, not annealed.

The term "poles " invariably applied to a magnet, whether under the action of a current or permanent, is used because the two ends of such a magnet are always attracted towards the earth's north and south poles. Of course, the magnet, as a whole, cannot move towards such a direction unless it is of the proper form and free to move, as in the case of a compass needle. To demonstrate this, magnetize an ordinary sewing needle, stick it through a bit of cork and float it in water, and it will at once move so as to point north and south. If another needle, magnetized in the same direction, be held near the floating needle, it will at once be noticed that any two like poles will repel each other, attraction only taking place between north and south, or unlike poles ; hence, the important and well-known rule, "Like poles repel, and unlike poles attract each other." The earth is a huge magnet whose poles nearly coincide with the true geographical north and south poles. Magnetism is shown in a natural state in a well-known ore of iron, magnetic or magnetic oxide of iron, discovered centuries ago in Magnesia, a city in Asia Minor, hence the name "magnet." The magnetic properties of this iron ore have no commercial value, as artificial magnets of steel are easily made of any desired strength.

The permanent magnet in its most familiar form is shown in Fig. 14. This is made of a steel bar or strip bent into a horseshoe form, then hardened, magnetized, and provided with a piece of soft iron or keeper for connecting the ends or poles when not in use. This helps to prevent loss of magnetism. A magnet with only one pole cannot be made, and a single magnetic pole does not exist in nature. If a long magnet be broken into any number of small lengths, each piece will show north and south poles, and exhibit all the properties of a perfect magnet. Iron is not the only magnetic metal, though it is by far the most powerful. The following metals are also magnetic : nickel, cobalt, manganese, cerium, and chromium. Oxygen gas is also magnetic. All other known substances are so little influenced by magnetism that they are called non-magnetic.

Studies In Electricity IV Magnetism and Induction 83

Figure 14.

An interesting experiment showing the earth's magnetism may be made as follows : Take a rod of steel or a piece of gas pipe, say two feet long ; hold it in one hand, pointing it due east and west. Strike the rod a sharp blow on one end with a hammer ; test it for magnetism by touching it to iron filings or small tacks. They will not be attracted. Now repeat the operation, But hold the rod due north and south with the north end pointing slightly toward the ground. After striking the rod in this position you will find it has become magnetized quite strongly, and will attract the small bits of iron, while the usual attraction or repulsion of poles will be found on testing it with a compass, and a knife blade may be magnetized by striking it with one end of the bar.

Referring again to our electromagnet, the result of the current flowing through the coils, and its effect on the iron core, is shown by the powerful magnetic display. As there can be no action without reaction, let us see what effect the iron core and magnetizing coils have on the battery and circuit as a whole.

First, disconnect the electromagnet, and touch the battery wires together for an instant. A slight spark will be noticed when the circuit is broken, and if the wire is bare and held by the fingers, a very slight shock may be felt if the skin is not too dry or oily. Now connect the electromagnet in circuit, and the following changes will be noted : The spark on breaking contact will be much brighter and larger, and a much stronger shock may be felt. If the iron core be removed from the coils, the spark will not be so large or the shock so strong, though both effects will be more intense than with the battery wire or leads only in circuit and coils disconnected. As there has been no change in the strength of the battery during these changes, it is evident that the coils and core are responsible for the effects observed, and this brings us to the interesting and important study of electromagnet induction.