Page:Elementary Text-book of Physics (Anthony, 1897).djvu/369

§ 299] Such an arrangement is called a solenoid. The poles of a solenoid of very small cross-section are situated at its ends, and it is equivalent to a bar magnet uniformly magnetized.

If a bar of soft iron be introduced into the magnetic field within a solenoid it will become magnetized by induction. This combination is called an electromagnet. Since the strength of the magnetic field varies with the strength of the current in the solenoid, aud with the number of layers of wire wrapped around the iron core, the magnetization of auy bar of iron whatever may be raised to its maximum by increasing the current and the number of turns of wire.

299. Ampère's Theory of Magnetism.—Ampère based upon these facts a theory of magnetism which bears his name. He assumed that around every molecule of iron there circulates an electrical current, and that to such molecular currents are due all magnetic phenomena. He made no hypothesis with regard to the origin or the permanency of these currents. The theory agrees with Weber's hypothesis that magnetization consists in an arrangement of magnetic molecules.

Ampère's theory admits of an explanation of diamagnetism, which was given by Weber. He assumes that all diamagnetic molecules are capable of carrying molecular currents, but that those currents, under ordinary conditions, do not exist in them. When, however, a diamagnetic body is moved up to a magnet an induced current due to the motion (§ 306) is set up in each molecule, and in such a direction that the molecules become elementary magnets, with their poles so directed towards the magnet in the field that there is repulsion between them. If this theory be true, it ought to be possible, as suggested by Maxwell, to lessen the intensity of magnetization of a body magnetized by induction, by increasing the strength of the field beyond a certain limit. No such effect has as yet been observed.

We may state the facts of magnetism in a way which is more in accordance with our view that the current is the result of actions in the medium by saying that each magnetic molecule is the origin of a certain definite number of tubes of induction. The existence