Diamagnetism

DIAMAGNETISM Most substances, other than ferromagnetics, are very feebly magnetic under ordinary conditions, and in investigating their behaviour strong fields are necessary. The general methods for determining low susceptibilities have already been described, and the distinguishing characteristics of dia- and paramagnetics men tioned. The greater number of substances belong to the dia magnetic class. Diamagnetics are repelled from a magnetic pole, a force acting on them in the direction in which the square of the field decreases most rapidly. In the inhomogeneous field between the poles of a magnet (if these are of the usual form, giving the strongest field along the axis) a solid isotropic diamagnetic, in the form of a rod, tends to set itself at right angles to the lines of force, while a paramagnetic body tends to set along the lines of force. (The names dia- and paramagnetic were given by Faraday on account of this distinctive orientation of the two classes of substances in a field.) The behaviour of diamagnetics may be formally accounted for by attributing to them a negative sus ceptibility, which means that the magnetization induced is in the opposite direction to that of the inducing field. Shortly of ter the discovery of the "universality" of magnetism by Faraday, Weber developed a theory of dia- and paramagnetism based on the as sumption of "molecular currents," which had been suggested long before by Ampere. In paramagnetics, it was supposed, the mole cules behaved as magnets owing to permanent circulating cur rents, while diamagnetism was attributed to the molecular currents induced by the magnetic field. When an ordinary conducting cir cuit is moved in a magnetic field, or the magnetic field through it is changed, there will be an electromotive force proportional to the rate of change of the magnetic flux acting round the cir cuit, and a current will flow in such a direction as to produce a field tending to oppose the change. When the change is com pleted, owing to the resistance of the circuit, the current will gradually die down, the energy being converted into heat. In the imagined resistanceless molecular circuit, however, the current would be maintained, and the molecule would acquire a polarity in the opposite direction to that of the applied field. The idea of

molecular currents was very speculative, but toward the end of the nineteenth century the electron was discovered and, in 2905, Langevin showed that on the basis of an electronic theory of the constitution of matter, precision could be given to Weber's theory. Meanwhile an extensive research had been carried out by Curie, who had measured the susceptibilities of a large number of sub stances over a wide range of temperature, so that satisfactory magnetic data were available. Before discussing the results, it will be convenient to consider briefly the theory of diamagnetism due in essentials to P. Langevin (Ann. de Chim. et Phys., 2905 ).

Theory.

A large number of experimental facts may be ex plained if it is supposed that an atom consists of a heavy con centrated positively charged nucleus with negative electrons circu lating round it in orbits (see ATOM). According to the classical electron theory, the electrons should lose energy continuously by radiation, but the stability of the atoms shows that they do not do so. The classical theory is inadequate to deal with the problem of atomic structure, and must he supplemented by some form of quantum theory. None the less, the classical theory, which is based on macroscopic experiments, may to a certain extent be applied tentatively to the motion of electrons in atoms, and pro visional conclusions as to their magnetic effects may be drawn.

On the basis of the ordinary theory, it may be shown that an electron moving in an orbit of area S in a periodic time T pro duces at a distance the same mean magnetic field as a small magnet of moment where e being the charge on the electron, and c the velocity of light ; the moment p. being at right angles to the plane of the circuit. The electron moving in its orbit may be regarded as a "molecular cur rent" in a circuit without resistance. The variable field which the classical theory predicts may be left out of consideration for the present purpose and, indeed, since electrons in atoms do not radiate continuously, it may be supposed that a steady magnetic field is associated with any stationary state of the atom.