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Only a few substances are noticeably attracted by magnets, the most important being iron, steel, nickel, and cobalt.

341. Magnetic forces act through most substances. It is very easily shown that small pieces of iron-tacks, iron filings, etc.-are attracted by a magnet, even though a sheet of glass or a board is placed between the magnet and the pieces of iron. Trial with other substances shows that the magnetic attraction takes place through all substances, with no appreciable change except through

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FIG. 209. Iron filings clinging to a magnet

those that are attracted by the magnet. A sheet of iron acts as a partial screen; a thin sheet produces a small effect, but thick pieces, especially when they completely inclose a region, greatly diminish the magnetic attraction.

342. Poles. A magnet exerts the greatest attractive force at points near the ends. The part halfway between the ends shows very little attractive force, often none. This is very clearly shown if a magnet is thrust into iron filings and then lifted out. The filings will be found sticking to the magnet in large clusters around the ends (Fig. 209). On this account it was once thought that the "seat" of attraction was in or near the ends. These apparent seats of attraction were called poles. Every bar magnet has at least two poles, usually one near each end.

343. Poles always exist in pairs. It has not been found possible to make a magnet which has only one pole. If one takes a piece of tool steel (such as a knitting-needle or a piece of hack-saw blade) that has been hardened until it is brittle, and magnetizes it, he will find on breaking it up into pieces that each piece will be a complete magnet with two poles.

344. Attractive forces have a

limited range. The attractive or repulsive force exerted by one magnet on another or by a magnet on iron is never large unless the two are close together.* Even the powerful lifting-magnets used in handling large masses of iron exert little effect until the iron to be lifted is very close to the poles. 345. The magnetic compass. If a bar magnet is suspended so that it is free to rotate about a vertical axis, it will finally come to rest, pointing in a definite direction. If there are no other magnets or masses of iron near, it will point approximately north and south. The magnetic compass usually consists of a light magnetized steel needle mounted on a sharp point, about which it can turn with very little friction. No one knows who discovered and first utilized a magnetic compass. The Chinese knew about it as long ago as A.D. 121, but how this knowledge was brought to Europe we do not know. Sometime during the tenth or eleventh century the tendency of a magnet to point north and south became known, and in the twelfth century, or perhaps earlier, it was used in navigation.

The pole that points north is called the north-seeking pole, and that pointing south the south-seeking pole.

346. Two kinds of poles. It is always the same pole of a magnet that tends to point northward. This shows that the two poles of a magnet are not alike. Further confirmation of this difference can be obtained by the following simple experiment: A bar magnet in a stirrup is suspended by a thread from some suitable support, or a moderately long compass needle may be used. When a second magnet is brought near the suspended one, the forces of attraction and repulsion are easily observed.

* The force between two magnets at distances much larger than the lengths of the magnets varies inversely as the fourth power of the distance, a very rapid decrease in force as the distance increases.

1. One of the poles of the second magnet will repel one of the

poles of the suspended magnet, but will attract the other. This shows that the two poles of the suspended magnet are not alike.

2. If the other pole of the second magnet is now used, it also will attract one pole of the suspended magnet and repel the other. But the pole now attracted is the one that was repelled in the first case, and the pole now repelled is the one that was attracted in the first case, This shows that the two poles of the second magnet (the one held in the hand) are not alike.

If the poles of other magnets are brought near the suspended magnet, it will be found that one of the poles of each magnet attracts the north-seeking pole and repels the south-seeking pole of the suspended magnet, while the other pole attracts the southseeking pole and repels the north-seeking pole of the suspended magnet. In this way it can be shown that there are two kinds of poles and that every magnet always has one of each kind.

The following conclusion can be drawn from experiments similar to those just described: Like poles repel and unlike attract.

R 347. The magnetic field. The region around a magnet where a force would act on a small piece of iron or on another magnet brought into that region is usually called the field of that magnet. If a

N

S

B

short compass is brought into this field,

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B

it will at each place point in a definite direction. This direction can be approximately predicted by the following

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FIG. 210

method: In Fig. 210 the vector OA is drawn* to represent the force produced by the north-seeking pole of the magnet on the north-seeking pole of the compass. The vector OB represents the attractive force the south-seeking pole of the magnet has for the north-seeking pole of the compass. The direction of the resultant force is given by OR. The force acting on the south-seeking pole of the compass will be just opposite to OR. The direction of the line OR will be the direction that the compass needle points.

* Diagrams are drawn at different places to show that the result is different in different parts of the field.

The direction in which the north-seeking pole of the compass needle points is called the direction of the magnetic field at that place.

It is a very instructive experiment to map out, with the aid of a small compass, the direction of the magnetic field at a large number of points near a magnet.

348. Magnetic lines. The following experiment should be seen by each student. A sheet of glass or smooth cardboard is laid on a bar magnet, and iron filings are sprinkled evenly over the glass. If the glass is tapped or slightly jarred, the filings will arrange themselves in lines which run from one pole around to the other. This is shown in the photographic reproduction given in Fig. 211.

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FIG. 211

It is an interesting fact that these lines always take at each point the direction of the magnetic field. Hence a magnetic field can be mapped out either by observing at a large number of places the direction a compass points or by using the lines formed by iron filings. Fig. 212 shows the lines traced out by filings in a field between two unlike poles; Fig. 213 is for the case of two like poles.

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FIG. 212

If a series of lines are drawn through the field in such a way that at each point of the field the direction of the line through that point is in the direction a small compass points, they are called magnetic lines.

It is customary to say that magnetic lines always run from a north-seeking pole to a south-seeking one, in the direction a compass points. Hence magnetic lines give at each point the direction of the force that would act on a north-seeking pole if such a pole were placed at that point.

The attraction or repulsion between magnetic poles can be explained if we imagine the magnetic lines to have the following properties: (1) they are under tension and therefore tend to contract; (2) they repel each other sidewise.

If the lines are under tension and trying to shorten, the attraction between the unlike poles in Fig. 212 would be explained. As shown in the figures, the lines often take an indirect path from one pole to the other. This bulging of

the lines is imagined to be due to side repulsion of the lines for each other. Any line, because of its tension, tries to move in between the poles and take the shortest path. But the repulsion of the lines inside holds it out, in the position shown. The distortion of the lines in Fig. 213 is also due to this repulsion. This gives a sort of visual explanation of how the two poles of Fig. 213 repel each other.

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FIG. 213

It should be understood that these magnetic lines are purely imaginary. But they are so very useful for purposes of study that many electricians are in the habit of referring to them as if they actually existed in the space around every magnet.

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349. The earth a great magnet. It seems that an Englishman, William Gilbert, in the year 1600, was the first to point out that the earth is a great magnet, and that this is the reason why a magnet always tends to point in an approximately north-andsouth direction. The magnetic poles of the earth do not coincide with the geographical poles. If we call the line joining the two poles of a magnet the magnetic axis of the magnet, the magnetic axis of the earth does not coincide with its axis of rotation. If the magnetic axis is prolonged until it cuts through the surface, it emerges in the Northern Hemisphere about one thousand miles from the north geographical pole, just inside the arctic circle and

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