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266. Magnetic materials. Iron and steel are the only substances which exhibit magnetic properties to any marked degree. Nickel and cobalt are also attracted appreciably by strong magnets. Bismuth, antimony, and a number of other substances are actually repelled instead of attracted, but the effect is very small. It has recently been found possible to make quite strongly magnetic alloys out of certain nonmagnetic materials. For example, a mixture of 65% copper, 27% manganese, and 8% aluminium is quite strongly magnetic. These are called Heusler alloys. For practical purposes, however, iron and steel may be considered as the only magnetic materials.
267. Magnetic induction. If a small unmagnetized nail is suspended from one end of a bar magnet, it is found that a second nail may be suspended from this first nail, which itself acts like a magnet, a third from the second, etc., as shown in Fig. 209. But if the bar magnet is carefully pulled away from the first nail, the others will instantly fall away from each other, thus showing that the nails were strong magnets only so long as they were in contact with the bar magnet. Any piece of soft iron may be thus magnetized temporarily by holding it in contact with a permanent magnet. Indeed, it is not necessary that there be actual contact, for if a nail is simply brought near to the permanent magnet it is found to become a magnet. This may be proved by FIG. 210. Magnetpresenting some iron filings to one end of
ism induced without contact
FIG. 209. Magnetism induced by contact
a nail held near a magnet in the manner shown in Fig. 210. Even inserting a plate of glass, or of copper, or of any other material except iron between S and N will not change appreciably the number of filings which cling
to the end of S', a fact which shows that nonmagnetic materials are transparent to magnetic forces. But as soon as the permanent magnet is removed, most of the filings will fall. Magnetism produced by the mere presence of adjacent magnets, with or without contact, is called induced magnetism. If the induced magnetism of the nail in Fig. 210 is tested with a compass needle, it is found that the remote induced pole is of the same kind as the inducing pole, while the near pole is of unlike kind. This is the general law of magnetic induction.
Magnetic induction explains the fact that a magnet attracts an unmagnetized piece of iron, for it first magnetizes it by induction, so that the near pole is unlike the inducing pole, and the remote pole like the inducing pole; and then, since the two unlike poles are closer together than the like poles, the attraction overbalances the repulsion and the iron is drawn toward the magnet. Magnetic induction also explains the formation of the tufts of iron filings shown in Fig. 207, each little filing becoming a temporary magnet such that the end which points toward the inducing pole is unlike this pole, and the end which points away from it is like this pole. The bushlike appearance is due to the repulsive action which the outside free poles exert upon each other.
268. Retentivity and permeability. A piece of soft iron will very easily become a strong temporary magnet, but when removed from the influence of the magnet it loses practically all of its magnetism. On the other hand, a piece of steel will not be so strongly magnetized as the soft iron, but it will retain a much larger fraction of its magnetism after it is removed from the influence of the permanent magnet. This quality of resisting either magnetization or demagnetization is called retentivity. Thus steel has a much greater retentivity than wrought iron, and, in general, the harder the steel the greater its retentivity.
A substance which has the property of becoming strongly magnetic under the influence of a permanent magnet, whether it has a high retentivity or not, is said to possess permeability in large degree. Thus iron is much more permeable than nickel.
269. Magnetic lines of force. If we could separate the N and S poles of a small magnet so as to get an independent N pole, and were to place this N pole near the N pole of a bar magnet, it would move over to the S pole along some curved path similar to that shown in
Fig. 211. The reason it would FIG. 211. A line of force set up by the magnet AB move in a curved path is that it would be simultaneously repelled by the N pole of the bar magnet and attracted by its S pole, and the relative strengths of these two forces would continually change as the relative distances of the moving pole from these two poles changed.
To verify this conclusion let a strongly magnetized sewing needle be floated in a small cork in a shallow dish of water, and let a bar or horseshoe magnet be placed just above or just beneath the dish (see Fig. 212). The cork and needle will then move as would an independent pole, since the remote pole of the needle is so much farther from the magnet than the near pole that its influence on the motion is very small.
The cork will actually be found to move in a curved path from N to S.
FIG. 212. Showing direction of motion of an isolated pole near a magnet
Any path which an independent N pole would take in going from N to S is called a line of force. The simplest way of finding the direction of this path at any point near a magnet is to hold a short compass needle at the point considered. The needle sets itself along the line in which its poles would move if independent, that is, along the line of force which passes through the given point (see C, Fig. 211).
270. Fields of force. The region about a magnet in which its magnetic forces can be detected is called its field of force. The easiest way of gaining an idea of the way in which the
lines of force are arranged in the magnetic field about any magnet is to sift iron filings upon a piece of paper placed immediately over the magnet. Each little filing becomes a temporary magnet by induction, and therefore, like the compass needle, sets itself in the direction of the line of force at the point where it is. Fig. 213 shows how the filings arrange themselves about a bar magnet. Fig. 214 is the corresponding ideal diagram showing the lines of force emerging from the N pole and passing about in curved paths to the S pole. It is customary to imagine these lines as returning through the magnet from S to N in the manner shown, so that each line is thought of as a closed curve. This convention was introduced by Faraday, and has been found of great assistance in correlating the facts of magnetism.
FIG. 215. The strength of a magnetic field is represented by the number of lines of force per square centimeter
A magnetic field of unit strength is defined as a field in which a unit magnet pole experiences 1 dyne of force. It is customary
to represent graphically such a field by drawing one line per square centimeter through a surface such as ABCD (Fig. 215) taken at right angles to the lines of force. If a unit N pole between N and S (Fig. 215) were pushed toward S with a force of 1000 dynes, the strength of the field would be 1000 units and it would be represented by 1000 lines per square centimeter.
271. Molecular nature of magnetism. If a small test tube full of iron filings be stroked from end to end with a magnet, it will be found to have become itself a magnet; but it will lose its magnetism as soon as the filings are shaken up. If a magnetized knitting needle is heated red-hot, it will be found to have lost its magnetism completely. Again, if such a needle is jarred, or hammered, or twisted, the strength of its poles, as measured by their ability to pick up tacks or iron filings, will be found to be greatly diminished.
These facts point to the conclusion that magnetism has something to do with the arrangement of the molecules, since causes which violently disturb the molecules of a magnet weaken its magnetism. Again, if a magnetized needle is broken, each part will be FIG. 216. Effect of breaking a magnet found to be a complete mag
net; that is, two new poles will appear at the point of breaking, a new N pole on the part which has the original S pole, and a new S pole on the part which has the original N pole. The subdivision may be continued indefinitely, but always with the same result, as indicated in Fig. 216. This suggests that the molecules of a magnetized bar may themselves be little magnets arranged in rows with their opposite poles in contact.
If an unmagnetized piece of hard steel is pounded vigorously while it lies between the poles of a magnet, or if it is heated to redness and then allowed to cool in this position, it will be found to have become magnetized. This suggests that the