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Repeated object lessons have demonstrated that nearly all progress in science has resulted in important advances in industry

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G-E Research Laboratory

Schenectady, N. Y.

Among the many products developed by the General
Electric Company's research laboratories the following
are of special interest to manufacturers:

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For further information address Supply Department, Schenectady Office.

General Electric

General Office
Schenectady, NY.

Company

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RAYLEIGH-HILGER

GAS INTERFERENCE REFRACTOMETER

THE TEST

IN OUR STOCK FOR IMMEDIATE SHIPMENT

OF SERVICE

DZE

ADAM HILGER LE LONDON

Interference Refractometer, Rayleigh-Hilger for Gases. This instrument is designed for work on the refractometry of gases, of which the refractive indices differ so little from unity that a very high degree of sensitivity is required in any refractometer to be used with them. Some idea will be obtained as to the sensitivity of the instrument when it is stated that it is possible by means of it to detect the presence of .01% of hydrogen in air, a quantity which causes a change of refractive index of only .000,000,015. The principle underlying the design of the instrument is that of the interference bands formed by two adjacent slits in collimated light, and observed by means of a telescope. The arrangement is, in fact, a diffraction grating with only two openings. The means whereby the system of bands thus obtained is made to indicate the refractive index of a substance is provided by the sideway shift of the whole band system when any change is made in the optical path of one of the interfering beams; part of the optical path being, of course, the substance under examination.

Quantities of Gases in Air Detectable by Rayleigh Interference Refractometer

Minimum observable shift assumed to be 1/40 band.

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Lord Rayleigh..

Ramsay & Travers...

Travers.

J. D. Edwards.

Adams..

Original Arrangement and Laboratory Type

64

Proc. Roy. Soc. (A) vol. 59, p. 201 (1896).
Proc. Roy. Soc. (A) vol. 64, p. 97 (1898).
Proc. Roy. Soc. (A) vol. 62, p. 225 (1897).
Proc. Roy. Soc. (A) vol. 64, p. 190 (1899).
Proc. Roy. Soc. (A) vol. 67, p. 331 (1900).

Study of Gases," Macmillan, 1901.

(Technologic Papers of the Bureau of Standards.

No. 113.

(Jour. Amer. Chem. Soc. 39, p. 2382, 1917.

Jour. Amer. Chem. Soc. 39, p. 1181, 1915.

45940. Refractometer, Interference, Rayleigh-Hilger, for gases, with double gas tube 100 cm. long, but without illuminating lamp.

..654.75 NOTE-By the substitution of a Liquid Cell in place of the Gas Tube, the Rayleigh Interference Refractometer can be converted into an instrument for the investigation of liquids, or can be furnished for this purpose without the Gas Tube. Prices on application.

Prices subject to change without notice

ARTHUR H. THOMAS COMPANY

WHOLESALE, RETAIL AND EXPORT MERCHANTS

LABORATORY APPARATUS AND REAGENTS

WEST WASHINGTON SQUARE

PHILADELPHIA, U. S. A.

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THE ELECTRON THEORY OF

MAGNETISM1

EVER since the time of Faraday it has been known that all varieties of matter can be grouped in three classes on the basis of magnetic behavior, ferromagnetic, paramagnetic and diamagnetic.

It would be far too much to claim that the electron theory has as yet given anything like a complete account of the phenomena witnessed in connection with these three types of magnetism; but it is the only theory proposed which has been in any way satisfactory and which appears to hold out any hope for the future.

In accordance with the plans of this symposium I shall restrict myself to a consideration of the more general aspects of the theory and its simplest applications. For the sake of logical completeness I shall have to refer to many matters well known. The extension of the theory and its application to more special and complex cases, in so far as they can be handled on this occasion, will be treated by my colleagues.

The first electrical theory of ferromagnetism was proposed by Ampère just about one hundred years ago. On the basis of his own experiments on the behavior of electric circuits and magnets, and on the assumption, already justified, that magnetism is a molecular and not a molar phenomenon, he concluded that the molecule of iron is the seat of a permanent electrical whirl and thus essentially a permanent magnet with its axis perpendicular to the whirl. When the iron is fully magnetized, all the whirls are oriented alike, and

1 A paper read as a part of the symposium on recent progress in magnetism held at the joint meeting of the American Association for the Advancement of Science, Section B, and the American Physical Society, December, 1920. Revised, January, 1921.

the magnetic moment of the mass of iron is the sum of the moments of the elementary molecular magnets. Ampère undoubtedly considered that in a neutral mass of iron the molecular magnets are turned indiscriminately in all directions, but he did not enter into any discussion of the process by which their axes are made parallel by the field during magnetization, nor did he consider the nature of the electrical whirls themselves.

Ampère was the grandfather of the electron theory of magnetism. Wilhelm Weber was its father. In 1852 Weber2 published a paper in which he developed a theory which, slightly modified by Langevin, is still perhaps the most widely accepted theory of diamagnetism, together with a theory of ferromagnetism which formed the starting point for the wellknown theory of Ewing. Weber adopted the molecular whirls of Ampère, but assumed in addition that these whirls, always present in the molecules of magnetic substances, are also present in the molecules of diamagnetic substances when placed in a magnetic field. Further, he took the very important step of attributing mass or inertia to the electricity in the whirls, and he assumed that the electricity moves as if in fixed circular grooves in the molecule, so that each whirl maintains its diameter and its orientation with respect to the rest of the molecule as if rigidly constrained. According to Weber's conception, a substance is paramagnetic or ferromagnetic when the molecule, or magnetic element, contains a permanent whirl, with a definite magnetic moment, and so tends to set with its axis in the direction of any magnetic field in which it is placed; and a substance is diamagnetic when the molecule contains one or more frictionless grooves, with the mobile electricity at rest before the creation of the magnetic field. Langevin merely substitutes electrons moving in fixed orbits for Weber's electricity in grooves; and assumes that in a diamagnetic substance more than one orbit exists in the molecule and that the orbits are so constituted and grouped that the magnetic

2 W. Weber's Werke, III., p. 555.

8 Ann. chim. phys. (8), 5, 1905, p. 70.

moment of the whole molecule is zero in a neutral field.

μ

In this case, which we shall consider in some detail, the complete molecule will suffer no change of orientation when introduced into a magnetic field, but the speed of the electricity in each orbit or groove will change on account of the electromotive force around the orbit or groove due to the alteration of the extraneous magnetic flux through it. Its magnetic moment will thus increase (algebraically) by an amount Au, which can readily be calculated. If e denotes the charge of electricity circulating in an orbit (whether as a single electron, or a ring of electrons, or a continuous ring), m the mass associated with the moving charge, r the radius of the orbit, H the intensity of the extraneous magnetic field, and the angle between the axis of the orbit and the direction of the field,

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but the agreement is in general far from close. The equation requires that K should be independent of the temperature, unless e, m, r and N depend upon it. As is well known, the susceptibilities of many diamagnetic substances are independent of the temperature over wide ranges, while in other cases there is a marked dependence.

According to this theory also, effects of the same kind must exist in bodies which are ferromagnetic or paramagnetic superposed on effects of opposite sign, the resultant susceptibility being, as Larmor long ago pointed out, the sum of the two. The paramagnetic term may account for the variation of the resultant susceptibility with temperature in many diamagnetic bodies. From Weber's equation it may be shown that when 0=0

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Thomson, Voigt, Lorentz, and others, including very recently H. A. Wilson." If a substance contains electrons either at rest or in plain rectilinear motion due to thermal agitation, and a magnetic field is created, an electrical intensity will evidently be developed with a curl equal to the negative rate of increase of the flux density, which will cause the electrons to move in paths curved in such a way as to produce a magnetic moment opposed to the direction of the applied field; and as the field becomes steady curvature will be maintained by the action of the field on the moving electrons normal to their velocities. Calculation on this hypothesis gives susceptibilities of the same order of magnitude as those given by the Weber-Langevin theory. This form of theory has the advantages over the other of greater freedom from assumptions and of giving, when applied to the optical case, a Zeeman effect with sharp lines. Weber does not attempt to justify his assumption that in a molecule the diameters of his orbital grooves remain constant, and that in a diamagnetic substance the grooves maintain their orientations independent of the applied magnetic intensity. With respect to the diameters, however, Langevin has shown that the magnetic field will produce no alteration provided the law of force is not precisely that of the inverse cube, which is quite improbable.

We shall return to the subject of diamagnetism later.

The first detailed theory of paramagnetism was given for perfect gases by Langevin in 1905.3 Following Langevin, I shall begin with a gravitational analogue. Let us consider an enclosure containing a gas at uniform temperature and let us suppose the gravitational field anulled. The density of the gas will then be uniform throughout the enclosure. If now the uniform gravitational field is brought into action every particle of gas will receive an acceleration downward, 4 Int. cong. phys., 1900, vol. 3, p. 138. Ann. der Phys. (4), 9, 1902, p. 130. 6"The Theory of Electrons," P. 124.

7 Roy. Soc. Proc. A, 97, 1920, p. 321.

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