Meyer thinks that the susceptibilities of the metals praseodymium, neodymium, ytterbium, samarium, gadolinium, and erbium, when obtained in a pure form, will be found to equal or even exceed those of the well-known ferromagnetic metals. Many of their compounds are very strongly magnetic; erbium, for example, in Er2O3being four times as strong as iron in the familiar magnetite or lodestone, Fe2O3. The susceptibilities of some hundreds of inorganic compounds have also been determined by the same investigator (loc. cit.). Among other researches relating to atomic and molecular magnetism are those of O. Liebknecht and A. P. Wills (Ann. d. Phys., 1900, 1, 178), H. du Bois and O. Liebknecht (ibid. p. 189), and Meyer (ibid. p. 668). An excellent summary regarding the magnetic properties of matter, with many tables and references, has been compiled by du Bois (Report to the Congrès Int. de Phys., Paris, 1900, ii. 460).
Meyer thinks that the susceptibilities of the metals praseodymium, neodymium, ytterbium, samarium, gadolinium, and erbium, when obtained in a pure form, will be found to equal or even exceed those of the well-known ferromagnetic metals. Many of their compounds are very strongly magnetic; erbium, for example, in Er2O3being four times as strong as iron in the familiar magnetite or lodestone, Fe2O3. The susceptibilities of some hundreds of inorganic compounds have also been determined by the same investigator (loc. cit.). Among other researches relating to atomic and molecular magnetism are those of O. Liebknecht and A. P. Wills (Ann. d. Phys., 1900, 1, 178), H. du Bois and O. Liebknecht (ibid. p. 189), and Meyer (ibid. p. 668). An excellent summary regarding the magnetic properties of matter, with many tables and references, has been compiled by du Bois (Report to the Congrès Int. de Phys., Paris, 1900, ii. 460).
12. Molecular Theory of Magnetism
According to W. E. Weber’s theory, the molecules of a ferromagnetic metal are small permanent magnets, the axes of which under ordinary conditions are turned indifferently in every direction, so that no magnetic polarity is exhibited by the metal as a whole; a magnetic force acting upon the metal tends to turn the axes of the little magnets in one direction, and thus the entire piece acquires the properties of a magnet. If, however, the molecules could turn with perfect freedom, it is clear that the smallest magnetizing force would be sufficient to develop the highest possible degree of magnetization, which is of course not the case. Weber therefore supposed each molecule to be acted on by a force tending to preserve it in its original direction, the position actually assumed by the axis being in the direction of the resultant of this hypothetical force and the applied magnetizing force. Maxwell (Electricity and Magnetism, § 444), recognizing that the theory in this form gave no account of residual magnetization, made the further assumption that if the deflection of the axis of the molecule exceeded a certain angle, the axis would not return to its original position when the deflecting force was removed, but would retain a permanent set. Although the amended theory as worked out by Maxwell is in rough agreement with certain leading phenomena of magnetization, it fails to account for many others, and is in some cases at variance with observed facts.
J. A. Ewing (Proc. Roy. Soc., 1890, 48, 342) has demonstrated that it is quite unnecessary to assume either the directive force of Weber, the permanent set of Maxwell, or any kind of frictional resistance, the forces by which the molecular magnets are constrained being simply those due to their own mutual attractions and repulsions. The effect of these is beautifully illustrated by a model consisting of a number of little compass needles pivoted on sharp points and grouped near to one another upon a board, which is placed inside a large magnetizing coil. When no current is passing through the coil and the magnetic field is of zero strength, the needles arrange themselves in positions of stable equilibrium under their mutual forces, pointing in many different directions, so that there is no resultant magnetic moment. This represents the condition of the molecules in unmagnetized iron. If now a gradually increasing magnetizing force is applied, the needles at first undergo a stable deflection, giving to the group a small resultant moment which increases uniformly with the force; and if the current is interrupted while the force is still weak, the needles merely return to their initial positions. This illustrates the first stage in the process of magnetization, when the moment is proportional to the field and there is no hysteresis or residual magnetism (seeante). A somewhat stronger field will deflect many of the needles beyond the limits of stability, causing them to turn round and form new stable combinations, in which the direction assumed by most of them approximates to that of the field. The rearrangement is completed within a comparatively small range of magnetizing force, a rapid increase of the resultant moment being thus brought about. When the field is removed, many of the newly formed combinations are but slightly disturbed, and the group may consequently retain a considerable resultant moment. This corresponds to the second stage of magnetization, in which the susceptibility is large and permanent magnetization is set up. A still stronger magnetizing force has little effect except in causing the direction of the needles to approach still more nearly to that of the field; if the force were infinite, every member of the group would have exactly the same direction and the greatest possible resultant moment would be reached; this illustrates “magnetic saturation”—the condition approached in the third stage of magnetization. When the strong magnetizing field is gradually diminished to zero and then reversed, the needles pass from one stable position of rest to another through a condition of instability; and if the field is once more reversed, so that the cycle is completed, the needles again pass through a condition of instability before a position of stable equilibrium is regained. Now the unstable movements of the needles are of a mechanically irreversible character; the energy expended in dissociating the members of a combination and placing them in unstable positions assumes the kinetic form when the needles turn over, and is ultimately frittered down into heat. Hence in performing a cycle there is a waste of energy corresponding to what has been termed hysteresis-loss.
Supposing Ewing’s hypothesis to be correct, it is clear that if the magnetization of a piece of iron were reversed by a strong rotating field instead of by a field alternating through zero, the loss of energy by hysteresis should be little or nothing, for the molecules would rotate with the field and no unstable movements would be possible.91Some experiments by F. G. Baily (Phil. Trans., 1896, 187, 715) show that this is actually the case. With small magnetizing forces the hysteresis was indeed somewhat larger than that obtained in an alternating field, probably on account of the molecular changes being forced to take place in one direction only; but at an induction of about 16,000 units in soft iron and 15,000 in hard steel the hysteresis reached a maximum and afterwards rapidly diminished. In one case the hysteresis loss per cubic centimetre per cycle was 16,100 ergs for B = 15,900, and only 1200 ergs for B = 20,200, the highest induction obtained in the experiment; possibly it would have vanished before B had reached 21,000.92These experiments prove that actual friction must be almost entirely absent, and, as Baily remarks, the agreement of the results with the previously suggested deduction affords a strong verification of Ewing’s form of the molecular theory. Ewing has himself also shown how satisfactorily this theory accords with many other obscure and complicated phenomena, such as those presented by coercive force, differences of magnetic quality, and the effects of vibration, temperature and stress; while as regards simplicity and freedom from arbitrary assumptions it leaves little to be desired.
The fact being established that magnetism is essentially a molecular phenomenon, the next step is to inquire what is the constitution of a magnetic molecule, and why it is that some molecules are ferromagnetic, others paramagnetic, and others again diamagnetic. The best known of the explanations that have been proposed depend upon the magnetic action of an electric current. It can be shown that if a current i circulates in a small plane circuit of area S, the magnetic action of the circuit for distant points is equivalent to that of a short magnet whose axis is perpendicular to the plane of the circuit and whose moment is iS, the direction of the magnetization being related to that of the circulating current as the thrust of a right-handed screw to its rotation. Ferromagnetism was explained by Ampère on the hypothesis that the magnetization of the molecule is due to an electric current constantly circulating within it. The theory now most in favour is merely a development of Ampère’s hypothesis, and applies not only to ferromagnetics, but to paramagnetics as well. To account for diamagnetism, Weber supposed that there exist within the molecules of diamagnetic substances certain channels around which an electric current can circulate without any resistance. The creation of an external magnetic field H will, in accordance with Lenz’s law, induce in the molecule an electric current so directed that the magnetization of the equivalent magnet is opposed to the direction of the field. The strength of the induced current is −HS cos θ/L, where θ is the inclination of the axis of the circuit to the direction of the field, and L the coefficient of self-induction; the resolved part of the magnetic moment in the direction of the field is equal to −HS² cos² θ/L, and if there are n molecules in a unit of volume, their axes being distributed indifferently in all directions, the magnetization of the substance will be −1⁄3nHS²/L, and its susceptibility -1⁄3S²/L (Maxwell,Electricity and Magnetism, § 838). The susceptibility is therefore constant and independent of the field, while its negative sign indicates that the substance is diamagnetic. There being no resistance, the induced current will continue to circulateround the molecule until the field is withdrawn, when it will be stopped by the action of an electromotive force tending to induce an exactly equal current in the opposite direction. The principle of Weber’s theory, with the modification necessitated by lately acquired knowledge, is the basis of the best modern explanation of diamagnetic phenomena.
There are strong reasons for believing that magnetism is a phenomenon involving rotation, and as early as 1876 Rowland, carrying out an experiment which had been proposed by Maxwell, showed that a revolving electric charge produced the same magnetic effects as a current. Since that date it has more than once been suggested that the molecular currents producing magnetism might be due to the revolution of one or more of the charged atoms or “ions” constituting the molecule. None of the detailed hypotheses which were based on this idea stood the test of criticism, but towards the end of the 19th century the researches of J. J. Thomson and others once more brought the conception of moving electric charges into prominence. Thomson has demonstrated the existence under many different conditions of particles more minute than anything previously known to science. The mass of each is about 1/1700th part of that of a hydrogen atom, and with each is indissolubly associated a charge of negative electricity equal to about 3.1 × 10−10C.G.S. electrostatic unit. These particles, which were termed by their discoverercorpuscles, are more commonly spoken of aselectrons,93the particle thus being identified with the charge which it carries. An electrically neutral atom is believed to be constituted in part, or perhaps entirely, of a definite number of electrons in rapid motion within a “sphere of uniform positive electrification” not yet explained. One or more of the electrons may be detached from the system by a finite force, the number so detachable depending on the valency of the atom; if the atom loses an electron, it becomes positively electrified; if it receives additional electrons, it is negatively electrified. The process of electric conduction in metals consists in the movement of detached electrons, and many other phenomena, both electrical and thermal, can be more or less completely explained by their agency. It has been supposed that certain electrons revolve like satellites in orbits around the atoms with which they are associated, a view which receives strong support from the phenomena of the Zeeman effect, and on this assumption a theory has been worked out by P. Langevin,94which accounts for many of the observed facts of magnetism. As a consequence of the structure of the molecule, which is an aggregation of atoms, the planes of the orbits around the latter may be oriented in various positions, and the direction of revolution may be right-handed or left-handed with respect to the direction of any applied magnetic field. For those orbits whose projection upon a plane perpendicular to the field is right-handed, the period of revolution will be accelerated by the field (since the electron current is negative), and the magnetic moment consequently increased; for those which are left-handed, the period will be retarded and the moment diminished. The effect of the field upon the speed of the revolving electrons, and therefore upon the moments of the equivalent magnets, is necessarily a very small one. If S is the area of the orbit described in time τ by an electron of charge e, the moment of the equivalent magnet is M = eSτ; and the change in the value of M due to an external field H is shown to be ΔM = −He2S/4πm,mbeing the mass of the electron. Whence
According to the best determinations the value ofe/mdoes not exceed 1.8 × 107, and τ is of the order of 10−15second, the period of luminous vibrations; hence ΔM/M must always be less than 10−9H, and therefore the strongest fields yet reached experimentally, which fall considerably short of 105, could not change the magnetic moment M by as much as a ten-thousandth part. If the structure of the molecule is so perfectly symmetrical that, in the absence of any external field, the resultant magnetic moment of the circulating electrons is zero, then the application of a field, by accelerating the right-handed (negative) revolutions, and retarding those which are left-handed, will induce in the substance a resultant magnetization opposite in direction to the field itself; a body composed of such symmetrical molecules is therefore diamagnetic. If however the structure of the molecule is such that the electrons revolving around its atoms do not exactly cancel one another’s effects, the molecule constitutes a little magnet, which under the influence of an external field will tend to set itself with its axis parallel to the field. Ordinarily a substance composed of asymmetrical molecules is paramagnetic, but if the elementary magnets are so conditioned by their strength and concentration that mutual action between them is possible, then the substance is ferromagnetic. In all cases however it is the diamagnetic condition that is initially set up—even iron is diamagnetic—though the diamagnetism may be completely masked by the superposed paramagnetic or ferromagnetic condition. Diamagnetism, in short, is an atomic phenomenon; paramagnetism and ferromagnetism are molecular phenomena. Hence may be deduced an explanation of the fact that, while the susceptibility of all known diamagnetics (except bismuth and antimony) is independent of the temperature, that of paramagnetics varies inversely as the absolute temperature, in accordance with the law of Curie.
13. Historical and Chronological Notes
The most conspicuous property of the lodestone, its attraction for iron, appears to have been familiar to the Greeks at least as early as 800B.C., and is mentioned by Homer, Plato, Aristotle, Theophrastus and others. A passage inDe rerum natura(vi. 910-915) by the Roman poet, Lucretius (96-55B.C.), in which it is stated that the stone can support a chain of little rings, each adhering to the one above it, indicates that in his time the phenomenon of magnetization by induction had also been observed. The property of orientation, in virtue of which a freely suspended magnet points approximately to the geographical north and south, is not referred to by any European writer before the 12th century, though it is said to have been known to the Chinese at a much earlier period. The application of this property to the construction of the mariner’s compass is obvious, and it is in connexion with navigation that the first references to it occur (seeCompass). The needles of the primitive compasses, being made of iron, would require frequent re-magnetization, and a “stone” for the purpose of “touching the needle” was therefore generally included in the navigator’s outfit. With the constant practice of this operation it is hardly possible that the repulsion acting between like poles should have entirely escaped recognition; but though it appears to have been noticed that the lodestone sometimes repelled iron instead of attracting it, no clear statement of the fundamental law that unlike poles attract while like poles repel was recorded before the publication in 1581 of theNew Attractiveby Robert Norman, a pioneer in accurate magnetic work. The same book contains an account of Norman’s discovery and correct measurement of the dip (1576). The downward tendency of the north pole of a magnet pivoted in the usual way had been observed by G. Hartmann of Nüremberg in 1544, but his observation was not published till much later.
The foundations of the modern science of magnetism were laid by William Gilbert (q.v.). HisDe magnete magneticisque corporibus et de magno magnete tellure physiologia nova(1600), contains many references to the expositions of earlier writers from Plato down to those of the author’s own age. These show that the very few facts known with certainty were freely supplementedby a number of ill-founded conjectures, and sometimes even by “figments and falsehoods, which in the earliest times, no less than nowadays, used to be put forth by raw smatterers and copyists to be swallowed of men.”95Thus it was taught that “if a lodestone be anointed with garlic, or if a diamond be near, it does not attract iron,” and that “if pickled in the salt of a sucking fish, there is power to pick up gold which has fallen into the deepest wells.” There were said to be “various kinds of magnets, some of which attract gold, others silver, brass, lead; even some which attract flesh, water, fishes;” and stories were told about “mountains in the north of such great powers of attraction that ships are built with wooden pegs, lest the iron nails should be drawn from the timber.” Certain occult powers were also attributed to the stone. It was “of use to thieves by its fume and sheen, being a stone born, as it were, to aid theft,” and even opening bars and locks; it was effective as a love potion, and possessed “the power to reconcile husbands to their wives, and to recall brides to their husbands.” And much more of the same kind, which, as Gilbert says, had come down “even to [his] own day through the writings of a host of men, who, to fill out their volumes to a proper bulk, write and copy out pages upon pages on this, that and the other subject, of which they know almost nothing for certain of their own experience.” Gilbert himself absolutely disregarded authority, and accepted nothing at second-hand. His title to be honoured as the “Father of Magnetic Philosophy” is based even more largely upon the scientific method which he was the first to inculcate and practise than upon the importance of his actual discoveries. Careful experiment and observation, not the inner consciousness, are, he insists, the only foundations of true science. Nothing has been set down in his book “which hath not been explored and many times performed and repeated” by himself. “It is very easy for men of acute intellect, apart from experiment and practice, to slip and err.” The greatest of Gilbert’s discoveries was that the globe of the earth was magnetic and a magnet; the evidence by which he supported this view was derived chiefly from ingenious experiments made with a spherical lodestone orterrella, as he termed it, and from his original observation that an iron bar could be magnetized by the earth’s force. He also carried out some new experiments on the effects of heat, and of screening by magnetic substances, and investigated the influence of shape upon the magnetization of iron. But the bulk of his work consisted in imparting scientific definiteness to what was already vaguely known, and in demolishing the errors of his predecessors.
No material advance upon the knowledge recorded in Gilbert’s book was made until the establishment by Coulomb in 1785 of the law of magnetic action. The difficulties attending the experimental investigation of the forces acting between magnetic poles have already been referred to, and indeed a rigorously exact determination of the mutual action could only be made under conditions which are in practice unattainable. Coulomb,96however, by using long and thin steel rods, symmetrically magnetized, and so arranged that disturbing influences became negligibly small, was enabled to deduce from his experiments with reasonable certainty the law that the force of attraction or repulsion between two poles varies inversely as the square of the distance between them. Several previous attempts had been made to discover the law of force, with various results, some of which correctly indicated the inverse square; in particular the German astronomer, J. Tobias Mayer (Gött. Anzeiger, 1760), and the Alsatian mathematician, J. Heinrich Lambert (Hist. de l’Acad. Roy. Berlin, 1766, p. 22), may fairly be credited with having anticipated the law which was afterwards more satisfactorily established by Coulomb. The accuracy of this law was in 1832 confirmed by Gauss,97who employed an indirect but more perfect method than that of Coulomb, and also, as Maxwell remarks, by all observers in magnetic observatories, who are every day making measurements of magnetic quantities, and who obtain results which would be inconsistent with each other if the law of force had been erroneously assumed.
Coulomb’s researches provided data for the development of a mathematical theory of magnetism, which was indeed initiated by himself, but was first treated in a complete form by Poisson in a series of memoirs published in 1821 and later.98Poisson assumed the existence of two dissimilar magnetic fluids, any element of which acted upon any other distant element in accordance with Coulomb’s law of the inverse square, like repelling and unlike attracting one another. A magnetizable substance was supposed to consist of an indefinite number of spherical particles, each containing equivalent quantities of the two fluids, which could move freely within a particle, but could never pass from one particle to another. When the fluids inside a particle were mixed together, the particle was neutral; when they were more or less completely separated, the particle became magnetized to an intensity depending upon the magnetic force applied; the whole body therefore consisted of a number of little spheres having north and south poles, each of which exerted an elementary action at a distance. On this hypothesis Poisson investigated the forces due to bodies magnetized in any manner, and also originated the mathematical theory of magnetic induction. The general confirmation by experiment of Poisson’s theoretical results created a tendency to regard his hypothetical magnetic fluids as having a real existence; but it was pointed out by W. Thomson (afterwards Lord Kelvin) in 1849 that while no physical evidence could be adduced in support of the hypothesis, certain discoveries, especially in electromagnetism, rendered it extremely improbable (Reprint, p. 344). Regarding it as important that all reasoning with reference to magnetism should be conducted without any uncertain assumptions, he worked out a mathematical theory upon the sole foundation of a few well-known facts and principles. The results were substantially the same as those given by Poisson’s theory, so far as the latter went, the principal additions including a fuller investigation of magnetic distribution, and the theory of magnetic induction in aeolotropic or crystalline substances. The mathematical theory which was constructed by Poisson, and extended and freed from doubtful hypotheses by Kelvin, has been elaborated by other investigators, notably F. E. Neumann, G. R. Kirchhoff, and Maxwell. The valuable work of Gauss on magnetic theory and measurements, especially in relation to terrestrial magnetism, was published in hisIntensitas vis magneticae terrestris, 1833, and in memoirs communicated to theResultate aus den Beobachtungen des magnetischen Vereins, 1838 and 1839, which, with others, are contained in vol. 5 of the collectedWerke. Weber’s molecular theory, which has already been referred to, appeared in 1852.99
An event of the first importance was the discovery made in 1819 by H. C. Oersted100that a magnet placed near a wire carrying an electric current tended to set itself at right angles to the wire, a phenomenon which indicated that the current was surrounded by a magnetic field. This discovery constituted the foundation of electromagnetism, and its publication in 1820 was immediately followed by A. M. Ampère’s experimental and theoretical investigation of the mutual action of electric currents,101and of the equivalence of a closed circuit to a polar magnet, the latter suggesting his celebrated hypothesis that molecular currents were the cause of magnetism. In the same year D. F. Arago102succeeded in magnetizing a piece of iron by the electric current, and in 1825 W. Sturgeon103publicly exhibited an apparatus “actingon the principle of powerful magnetism and feeble galvanism” which is believed to have constituted the first actual electromagnet. Michael Faraday’s researches were begun in 1831 and continued for more than twenty years. Among the most splendid of his achievements was the discovery of the phenomena and laws of magneto-electric induction, the subject of two papers communicated to the Royal Society in 1831 and 1832. Another was the magnetic rotation of the plane of polarization of light, which was effected in 1845, and for the first time established a relation between light and magnetism. This was followed at the close of the same year by the discovery of the magnetic condition of all matter, a discovery which initiated a prolonged and fruitful study of paramagnetic and diamagnetic phenomena, including magnecrystallic action and “magnetic conducting power,” now known as permeability. Throughout his researches Faraday paid special regard to the medium as the true seat of magnetic action, being to a large extent guided by his pregnant conception of “lines of force,” or of induction, which he considered to be “closed curves passing in one part of the course through the magnet to which they belong, and in the other part through space,” always tending to shorten themselves, and repelling one another when they were side by side (Exp. Res.§§ 3266-8, 3271). In 1873 James Clerk Maxwell published his classicalTreatise on Electricity and Magnetism, in which Faraday’s ideas were translated into a mathematical form. Maxwell explained electric and magnetic forces, not by the action at a distance assumed by the earlier mathematicians, but by stresses in a medium filling all space, and possessing qualities like those attributed to the old luminiferous ether. In particular, he found that the calculated velocity with which it transmitted electromagnetic disturbances was equal to the observed velocity of light; hence he was led to believe, not only that his medium and the ether were one and the same, but, further, that light itself was an electromagnetic phenomenon. Since the experimental confirmation of Maxwell’s views by H. R. Hertz in 1888 (Weid. Ann., 1888, 34, 155, 551, 609; and later vols.) they have commanded universal assent, and his methods are adopted in all modern work on electricity and magnetism.
The practice of measuring magnetic induction and permeability with scientific accuracy was introduced in 1873 by H. A. Rowland,104whose careful experiments led to general recognition of the fact previously ignored by nearly all investigators, that magnetic susceptibility and permeability are by no means constants (at least in the case of the ferromagnetic metals) but functions of the magnetizing force. New light was thrown upon many important details of magnetic science by J. A. Ewing’sExperimental Researchesof 1885; throughout the whole of his work special attention was directed to that curious lagging action to which the author applied the now familiar term “hysteresis.”105His well-known modification106of Weber’s molecular theory, published in 1890, presented for the first time a simple and sufficient explanation of hysteresis and many other complexities of magnetic quality. The amazing discoveries made by J. J. Thomson in 1897 and 1898107resulted in the establishment of the electron theory, which has already effected developments of an almost revolutionary character in more than one branch of science. The application of the theory by P. Langevin to the case of molecular magnetism has been noticed above, and there can be little doubt that in the near future it will contribute to the solution of other problems which are still obscure.
See W. Gilbert,De magnete(London, 1600; trans. by P. F. Mottelay, New York, 1893, and for the Gilbert Club, London, 1900); M. Faraday,Experimental Researches in Electricity, 3 vols. (London, 1839, 1844 and 1855); W. Thomson (Lord Kelvin),Reprint of Papers on Electrostatics and Magnetism(London, 1884, containing papers on magnetic theory originally published between 1844 and 1855, with additions); J. C. Maxwell,Treatise on Electricity and Magnetism(3rd ed., Oxford, 1892); E. Mascart and J. Joubert,Leçons sur l’électricité et le magnétisme(2nd ed., Paris, 1896-1897; trans., not free from errors, by E. Atkinson, London, 1883); J. A. Ewing,Magnetic Induction in Iron and other Metals(3rd ed., London, 1900); J. J. Thomson,Recent Researches in Electricity and Magnetism(Oxford, 1893);Elements of Mathematical Theory of Electricity and Magnetism(3rd ed., Cambridge, 1904); H. du Bois,The Magnetic Circuit(trans. by E. Atkinson, London, 1896); A. Gray,Treatise on Magnetism and Electricity, vol. i. (London, 1898); J. A. Fleming,Magnets and Electric Currents(London, 1898); C. Maurain,Le magnétisme du fer(Paris, 1899; a lucid summary of the principal facts and laws, with special regard to their practical application);Rapports présentés au Congrès international de physique, vol. ii. (Paris, 1900); G. C. Foster and A. W. Porter,Treatise on Electricity and Magnetism(London, 1903); A. Winkelmann,Handbuch der Physik, vol. v. part i. (2nd ed., Leipzig, 1905; the most exhaustive compendium of magnetic science yet published, containing references to all important works and papers on every branch of the subject).
See W. Gilbert,De magnete(London, 1600; trans. by P. F. Mottelay, New York, 1893, and for the Gilbert Club, London, 1900); M. Faraday,Experimental Researches in Electricity, 3 vols. (London, 1839, 1844 and 1855); W. Thomson (Lord Kelvin),Reprint of Papers on Electrostatics and Magnetism(London, 1884, containing papers on magnetic theory originally published between 1844 and 1855, with additions); J. C. Maxwell,Treatise on Electricity and Magnetism(3rd ed., Oxford, 1892); E. Mascart and J. Joubert,Leçons sur l’électricité et le magnétisme(2nd ed., Paris, 1896-1897; trans., not free from errors, by E. Atkinson, London, 1883); J. A. Ewing,Magnetic Induction in Iron and other Metals(3rd ed., London, 1900); J. J. Thomson,Recent Researches in Electricity and Magnetism(Oxford, 1893);Elements of Mathematical Theory of Electricity and Magnetism(3rd ed., Cambridge, 1904); H. du Bois,The Magnetic Circuit(trans. by E. Atkinson, London, 1896); A. Gray,Treatise on Magnetism and Electricity, vol. i. (London, 1898); J. A. Fleming,Magnets and Electric Currents(London, 1898); C. Maurain,Le magnétisme du fer(Paris, 1899; a lucid summary of the principal facts and laws, with special regard to their practical application);Rapports présentés au Congrès international de physique, vol. ii. (Paris, 1900); G. C. Foster and A. W. Porter,Treatise on Electricity and Magnetism(London, 1903); A. Winkelmann,Handbuch der Physik, vol. v. part i. (2nd ed., Leipzig, 1905; the most exhaustive compendium of magnetic science yet published, containing references to all important works and papers on every branch of the subject).
(S. Bi.)
1In London in 1910 the needle pointed about 16° W. of the geographical north. (SeeTerrestrial Magnetism.)2For the relations between magnetism and light seeMagneto-Optics.3Clerk Maxwell employed German capitals to denote vector quantities. J. A. Fleming first recommended the use of blockletters as being more convenient both to printers and readers.4The C.G.S. unit of current = 10 amperes.5The principal theoretical investigations are summarized in Mascart and Joubert’sElectricity and Magnetism, i. 391-398 and ii. 646-657. The case of a long iron bar has been experimentally studied with great care by C. G. Lamb,Proc. Phys. Soc., 1899, 16, 509.6Wied. Ann., 1884, 22, 411.7See C. G. Lamb,loc. cit.p. 518.8Hopkinson specified the retentiveness by the numerical value of the “residual induction” (= 4πI).9For all except ferromagnetic substances the coefficient is sensibly equal to κ.10See W. Thomson’sReprint, §§ 615, 634-651.11Ibid. §§ 646, 684.12Faraday,Exp. Res.xxi.13J. J. Thomson,Electricity and Magnetism, § 205.14Maxwell,Electricity and Magnetism, § 431.15H. du Bois,Electrician, 1898, 40, 317.16M. Faraday,Exp. Res.xxii., xxiii.; W. Thomson,Reprint, § 604; J. C. Maxwell,Treatise, § 435; E. Mascart and J. Joubert,Electricity and Magnetism, §§ 384, 396, 1226; A. Winkelmann,Physik, v. 287.17See A. Winkelmann,Physik, v. 69-94; Mascart and Joubert.Electricity and Magnetism, ii. 617.18Sci. Abs.A, 1906, 9, 225.19See C. G. Lamb,Proc. Phys. Soc., 1899, 16, 517.20Soc. Franc. Phys. Séances, 1904, 1, 27.21E. G. Warburg,Wied. Ann.1881, 13, 141; Ewing,Phil. Trans., 1885, 176, 549; Hopkinson,Phil. Trans.1885, 176, 466. For a simple proof, see Ewing,Magnetic Induction(1900), p. 99. Hopkinson pointed out that the greatest dissipation of energy which can be caused by a to-and-fro reversal is approximately represented byCoercive force×maximum induction/π.22Magnetic Induction, 1900, 378.23Phil. Trans., 1902, 198, 33.24Phil. Mag., 1903, 5, 117.25Some experiments by F. G. Baily showed that hysteresis ceased to increase when B was carried beyond 23,000. This value of B corresponds to I = 1640, the saturation point for soft iron.—Brit. Assoc. Rep., 1895, p. 636.26Tokyo Phys.-Math. Soc., 1904, 2, No. 14.27Phil. Mag., 1873, 46, 140.28S. Bidwell,Proc. Roy. Soc., 1886, 40, 495.29Since in most practicable experiments H³ is negligible in comparison with B², the force may be taken as B²/8π without sensible error.30The same phenomenon is exhibited in a less marked degree when soft iron is magnetized in stronger fields (Ewing,Phil. Trans., 1885, 176, 569).31Principal publications: J. P. Joule,Scientific Papers, pp. 46, 235; A. M. Meyer,Phil. Mag., 1873, 46, 177; W. F. Barrett,Nature, 1882, 26, 585; S. Bidwell,Phil. Trans., 1888, 179A, 205;Proc. Roy. Soc., 1886, 40, 109 and 257; 1888, 43, 406; 1890, 47, 469; 1892, 51, 495; 1894, 55, 228; 1894, 56, 94; 1904, 74, 60;Nature, 1899, 60, 222; M. Cantone,Mem. d. Acc d. Lincei, 1889, 6, 487;Rend. d. Acc. d. Lincei, 1890, 6, 252; A. Berget,C.R., 1892, 115, 722; S. J. Lochner,Phil. Mag., 1893, 36, 498; H. Nagaoka,Phil. Mag., 1894, 37, 131;Wied. Ann., 1894, 53, 487; C. G. Knott,Proc. Roy. Soc. Ed., 1891, 18, 315;Phil. Mag., 1894, 37, 141;Trans. Roy. Soc. Ed., 1896, 38, 527; 1898, 39, 457; C. G. Knott and A. Shand,Proc. Roy. Soc. Ed., 1892, 19, 85 and 249; 1894, 20, 295; L. T. More,Phil. Mag., 1895, 40, 345; G. Klingenberg,Rostock Univ. Thesis, Berlin, 1897; E. T. Jones,Phil. Trans., 1897, 189A, 189; B. B. Brackett,Phys. Rev., 1897, 5, 257; H. Nagaoka and K. Honda,Phil. Mag., 1898, 46, 261; 1900, 49, 329;Journ. Coll. Sci. Tokyo, 1900, 13, 57; 1903, 19, art. 11; J. S. Stevens,Phys. Rev., 1898, 7, 19; E. Rhoads,Phys. Rev., 1898, 7, 5;Phil. Mag., 1901, 2, 463; G. A. Shakespear,Phil. Mag., 1899, 17, 539; K. Honda,Journ. Coll. Sci. Tokyo, 1900, 13, 77; L. W. Austin,Phys. Rev., 1900, 10, 180;Deutsch. Phys. Gesell. Verh., 1904, 6, 4, 211; K. Honda and S. Shimizu,Phil. Mag., 1902, 4, 338; 1905, 10, 548.32The loads were successively applied in decreasing order of magnitude. They are indicated in fig. 25 as kilos per sq. cm.33Joule believed that the volume was unchanged.34For a discussion of theories of magnetic stress, with copious references, see Nagaoka,Rap. du Congrès International de Physique(Paris, 1900), ii. 545. Also Nagaoka and Jones,Phil. Mag., 1896, 41, 454.35S. Bidwell,Phil. Trans., 1888, 179a, 321.36Phil. Mag., 1895, 40, 345.37J. C. Maxwell,Treatise, § 643.38See correspondence inNature, 1896, 53, pp. 269, 316, 365, 462, 533; 1906, 74, pp. 317, 539; B. B. Brackett,loc. cit., quotes the opinion of H. A. Rowland in support of compressive stress.39J. A. Ewing,Phil. Trans., 1885, 176, 580; 1888, 179, 333;Magnetic Induction, 1900, ch. ix.; J. A. Ewing and G. C. Cowan,Phil. Trans., 1888, 179a, 325; C. G. Knott,Trans. Roy. Soc. Ed., 1882-1883, 32, 193; 1889, 35, 377; 1891, 36, 485;Proc. Roy. Soc. Ed., 1899, 586; H. Nagaoka,Phil. Mag., 1889, 27, 117; 1890, 29, 123; H. Nagaoka and K. Honda,Journ. Coll. Sci. Tokyo, 1900, 13, 263; 1902, 16, art. 8;Phil. Mag., 1898, 46, 261; 1902, 4, 45; K. Honda and S. Shimizu,Ann. d. Phys., 1904, 14, 791;Tokyo Physico-Math. Soc. Rep., 1904, 2, No. 13; K. Honda and T. Terada,Journ. Coll. Sci. Tokyo, 1906, 21, art. 4.40H. Tomlinson found a critical point in the “temporary magnetization” of nickel (Proc. Phys. Soc., 1890, 10, 367, 445), but this does not correspond to a Villari reversal. Its nature is made clear by Ewing and Cowan’s curves (Phil. Trans., 1888, 179, plates 15, 16).41Wied. Ann., 1894, 52, 462;Electrician, 1894, 34, 143.42Phil. Trans., 1890, 131, 329.43Magnetic Induction, 1900, 222.44Phys. Rev., 1904, 18, 432.45Phil. Mag., 1886, 22, 50.46Ibid.251.47Phil. Mag., 1891, 32, 383.48C.R., 1896, 122, 1192; 1898, 126, 463.49Phil. Mag., 1889, 27, 117.50Journ. Coll. Sci. Tokyo, 1904, 19, art. 9.51Phil. Mag., 1905, 10, 548;Tokyo Phys.-Math. Soc. Rep., 1904, 2, No. 14;Journ. Coll. Sci. Tokyo, 1905, 20, art. 6.52C.R., 1888, 106, 129.53Proc. Phys. Soc., 1888, 9, 181.54C.R., 1892, 115, 805; 1894, 118, 796 and 859.55Elekt. Zeits., 1894, 15, 194.56Phil. Mag., 1900, 50, 1.57Phil. Trans., 1903, 201, 1.58Phil. Mag., 1904, 8, 179.59A. M. Thiessen (Phys., 1899, 8, 65) and G. Claude (C. R., 1899, 129, 409) found that for considerable inductions (B = 15,000) the permeability and hysteresis-loss remained nearly constant down to -186°; for weak inductions both notably diminished with temperature.60Proc. Roy. Soc., 1898, 62, 210.61C.R., 1895, 120, 263.62Amer. Journ. Sci., 1898, 5, 245.63Phys. Rev., 1901, 14, 181.64C.R., 1897, 124, 176 and 1515; 1897, 125, 235; 1898, 126, 738.65Ibid., 1898, 126, 741.66Ibid., 1899, 128, 304 and 1395.67See also J. Hopkinson,Journ. Inst. Elect. Eng., 1890, 19, 20, and J. A. Ewing,Phil. Trans., 1889, 180, 239.68Many of the figures which, through an error, were inaccurately stated in the first paper are corrected in the second.69The marked effect of silicon in increasing the permeability of cast iron has also been noticed by F. C. Caldwell,Elect. World, 1898, 32, 619.70Trans. Roy. Dub. Soc., 1902-4, 8, 1 and 123.71J. Trowbridge and S. Sheldon,Phil. Mag., 1890, 29, 136; W. H. Preece,Journ. Inst. Elec. Eng., 1890, 19, 62;Electrician, 1890, 25, 546; I. Klemençiç,Wien. Ber., 1896, 105, IIa, 635; B. O. Peirce,Am. Journ. Sci., 1896, 2, 347; A. Abt,Wied. Ann., 1898, 66, 116; F. Osmond,C. R., 1899, 128, 1513.72Deutsch. phys. Gesell. Verh., 1903, 5, 220 and 224.73Exp. Res., iii. 440.74No record can be found of experiments with manganese at the temperature of liquid air or hydrogen; probably, however, negative results would not be published.75The critical temperature of iron, for instance, is raised more than 100° by the addition of a little carbon and tungsten.76Bull. Soc. Int. des Électriciens, 1906, 6, 301.77Proc. Roy. Soc., 1905, 76A, 271.78E. H. Hall,Phil. Mag., 1880, 9, 225; 1880, 10, 301; 1881, 12, 157; 1883, 15, 341; 1885, 19, 419.79The large Hall effect in bismuth was discovered by Righi,Journ. de Phys., 1884, 3, 127.80References.—(2) A. von Ettinghausen,Wied. Ann., 1887, 31, 737.—(4) H. W. Nernst, ibid., 784.—(i.) and (iv.); A. von Ettinghausen and H. W. Nernst,Wied. Ann., 1886, 29, 343.—(ii.) and (iii.); A. Righi,Rend. Acc. Linc., 1887, 3 II, 6 and I, 481; and A. Leduc,Journ. de Phys., 1887, 6, 78. Additional authorities are quoted by Lloyd,loc. cit.81P. Drude,Ann. d. Phys., 1900, 1, 566; 1900, 3, 369; 1902, 7, 687. See also E. van Everdingen,Arch. Néerlandaises, 1901, 4, 371; G. Barlow,Ann. d. Phys., 1903, 12, 897; H. Zahn, ibid. 1904, 14, 886; 1905, 16, 148.82Phil. Trans., 1856, p. 722. According to the nomenclature adopted by the best modern authorities, a metal A is said to be thermo-electrically positive to another metal B when the thermo-current passes from A to B through the cold junction, and from B to A through the hot (seeThermo-Electricity).83C.R., 1893, 116, 997.84Journ. de Phys., 1896, 5, 53.85Phil. Trans., 1887, 177, 373.86Proc. Roy. Soc., 1885, 39, 513.87Phys. Rev., 1902, 15, 321. The sign of the thermo-electric effect for nickel, as given by Rhoads, is incorrect.88Proc. Roy. Soc., 1904, 73, 413.89C.R., 1903, 136, 1131.90Journ. Coll. Sci. Tokyo, 1906, 21, art. 4. The paper contains 40 tables and 85 figures.91This deduction from Ewing’s theory appears to have been first suggested by J. Swinburne. SeeIndustries, 1890, 289.92R. Beattie (Phil. Mag., 1901, 1, 642) has found similar effects in nickel and cobalt.93The charge associated with a corpuscle is the same as that carried by a hydrogen atom. G. J. Stoney in 1881 (Phil. Mag., 1881, 11, 387) pointed out that this latter constituted the indivisible “atom of electricity” or natural unit charge. Later he proposed (Trans. Roy. Dub. Soc., 1891, 4, 583) that such unit charge should be called an “electron.” The application of this term to Thomson’s corpuscle implies, rightly or wrongly, that notwithstanding its apparent mass, the corpuscle is in fact nothing more than an atom of electricity. The question whether a corpuscle actually has a material gravitating nucleus is undecided, but there are strong reasons for believing that its mass is entirely due to the electric charge.94Jour. de Phys., 1905, 4, 678; translated inElectrician, 1905, 56, 108 and 141.95The quotations are from the translation published by the Gilbert Club, London, 1900.96C. A. Coulomb,Mem. Acad. Roy. Paris, 1785, p. 578.97Intensitas vis magneticae, § 21, C. F. Gauss’sWerke, 5, 79. See also J. J. Thomson,Electricity and Magnetism, § 132.98S. D. Poisson,Mém. de l’Institut, 1821 and 1822, 5, 247, 488; 1823, 6, 441; 1838, 16, 479.99For outlines of the mathematical theory of magnetism and references see H. du Bois,Magnetic Circuit, chs. iii. and iv.100Gilbert’sAnn. d. phys., 1820, 6, 295.101Ann. de chim. et de phys., 1820, 15, 59, 170;Recueil d’observations électrodynamiques, 1822;Théories des phénomènes électrodynamiques, 1826.102Ann. de chim. et de phys., 1820, 15, 93.103Trans. Soc. Arts, 1825, 43, 38.104Phil. Mag., 1873, 46, 140; 1874, 48, 321.105Phil. Trans., 1885, 176, 523;Magnetic Induction, 1900.106Proc. Roy. Soc., 1890, 48, 342.107Phil. Mag., 1897, 44, 293; 1898, 46, 528.
1In London in 1910 the needle pointed about 16° W. of the geographical north. (SeeTerrestrial Magnetism.)
2For the relations between magnetism and light seeMagneto-Optics.
3Clerk Maxwell employed German capitals to denote vector quantities. J. A. Fleming first recommended the use of blockletters as being more convenient both to printers and readers.
4The C.G.S. unit of current = 10 amperes.
5The principal theoretical investigations are summarized in Mascart and Joubert’sElectricity and Magnetism, i. 391-398 and ii. 646-657. The case of a long iron bar has been experimentally studied with great care by C. G. Lamb,Proc. Phys. Soc., 1899, 16, 509.
6Wied. Ann., 1884, 22, 411.
7See C. G. Lamb,loc. cit.p. 518.
8Hopkinson specified the retentiveness by the numerical value of the “residual induction” (= 4πI).
9For all except ferromagnetic substances the coefficient is sensibly equal to κ.
10See W. Thomson’sReprint, §§ 615, 634-651.
11Ibid. §§ 646, 684.
12Faraday,Exp. Res.xxi.
13J. J. Thomson,Electricity and Magnetism, § 205.
14Maxwell,Electricity and Magnetism, § 431.
15H. du Bois,Electrician, 1898, 40, 317.
16M. Faraday,Exp. Res.xxii., xxiii.; W. Thomson,Reprint, § 604; J. C. Maxwell,Treatise, § 435; E. Mascart and J. Joubert,Electricity and Magnetism, §§ 384, 396, 1226; A. Winkelmann,Physik, v. 287.
17See A. Winkelmann,Physik, v. 69-94; Mascart and Joubert.Electricity and Magnetism, ii. 617.
18Sci. Abs.A, 1906, 9, 225.
19See C. G. Lamb,Proc. Phys. Soc., 1899, 16, 517.
20Soc. Franc. Phys. Séances, 1904, 1, 27.
21E. G. Warburg,Wied. Ann.1881, 13, 141; Ewing,Phil. Trans., 1885, 176, 549; Hopkinson,Phil. Trans.1885, 176, 466. For a simple proof, see Ewing,Magnetic Induction(1900), p. 99. Hopkinson pointed out that the greatest dissipation of energy which can be caused by a to-and-fro reversal is approximately represented byCoercive force×maximum induction/π.
22Magnetic Induction, 1900, 378.
23Phil. Trans., 1902, 198, 33.
24Phil. Mag., 1903, 5, 117.
25Some experiments by F. G. Baily showed that hysteresis ceased to increase when B was carried beyond 23,000. This value of B corresponds to I = 1640, the saturation point for soft iron.—Brit. Assoc. Rep., 1895, p. 636.
26Tokyo Phys.-Math. Soc., 1904, 2, No. 14.
27Phil. Mag., 1873, 46, 140.
28S. Bidwell,Proc. Roy. Soc., 1886, 40, 495.
29Since in most practicable experiments H³ is negligible in comparison with B², the force may be taken as B²/8π without sensible error.
30The same phenomenon is exhibited in a less marked degree when soft iron is magnetized in stronger fields (Ewing,Phil. Trans., 1885, 176, 569).
31Principal publications: J. P. Joule,Scientific Papers, pp. 46, 235; A. M. Meyer,Phil. Mag., 1873, 46, 177; W. F. Barrett,Nature, 1882, 26, 585; S. Bidwell,Phil. Trans., 1888, 179A, 205;Proc. Roy. Soc., 1886, 40, 109 and 257; 1888, 43, 406; 1890, 47, 469; 1892, 51, 495; 1894, 55, 228; 1894, 56, 94; 1904, 74, 60;Nature, 1899, 60, 222; M. Cantone,Mem. d. Acc d. Lincei, 1889, 6, 487;Rend. d. Acc. d. Lincei, 1890, 6, 252; A. Berget,C.R., 1892, 115, 722; S. J. Lochner,Phil. Mag., 1893, 36, 498; H. Nagaoka,Phil. Mag., 1894, 37, 131;Wied. Ann., 1894, 53, 487; C. G. Knott,Proc. Roy. Soc. Ed., 1891, 18, 315;Phil. Mag., 1894, 37, 141;Trans. Roy. Soc. Ed., 1896, 38, 527; 1898, 39, 457; C. G. Knott and A. Shand,Proc. Roy. Soc. Ed., 1892, 19, 85 and 249; 1894, 20, 295; L. T. More,Phil. Mag., 1895, 40, 345; G. Klingenberg,Rostock Univ. Thesis, Berlin, 1897; E. T. Jones,Phil. Trans., 1897, 189A, 189; B. B. Brackett,Phys. Rev., 1897, 5, 257; H. Nagaoka and K. Honda,Phil. Mag., 1898, 46, 261; 1900, 49, 329;Journ. Coll. Sci. Tokyo, 1900, 13, 57; 1903, 19, art. 11; J. S. Stevens,Phys. Rev., 1898, 7, 19; E. Rhoads,Phys. Rev., 1898, 7, 5;Phil. Mag., 1901, 2, 463; G. A. Shakespear,Phil. Mag., 1899, 17, 539; K. Honda,Journ. Coll. Sci. Tokyo, 1900, 13, 77; L. W. Austin,Phys. Rev., 1900, 10, 180;Deutsch. Phys. Gesell. Verh., 1904, 6, 4, 211; K. Honda and S. Shimizu,Phil. Mag., 1902, 4, 338; 1905, 10, 548.
32The loads were successively applied in decreasing order of magnitude. They are indicated in fig. 25 as kilos per sq. cm.
33Joule believed that the volume was unchanged.
34For a discussion of theories of magnetic stress, with copious references, see Nagaoka,Rap. du Congrès International de Physique(Paris, 1900), ii. 545. Also Nagaoka and Jones,Phil. Mag., 1896, 41, 454.
35S. Bidwell,Phil. Trans., 1888, 179a, 321.
36Phil. Mag., 1895, 40, 345.
37J. C. Maxwell,Treatise, § 643.
38See correspondence inNature, 1896, 53, pp. 269, 316, 365, 462, 533; 1906, 74, pp. 317, 539; B. B. Brackett,loc. cit., quotes the opinion of H. A. Rowland in support of compressive stress.
39J. A. Ewing,Phil. Trans., 1885, 176, 580; 1888, 179, 333;Magnetic Induction, 1900, ch. ix.; J. A. Ewing and G. C. Cowan,Phil. Trans., 1888, 179a, 325; C. G. Knott,Trans. Roy. Soc. Ed., 1882-1883, 32, 193; 1889, 35, 377; 1891, 36, 485;Proc. Roy. Soc. Ed., 1899, 586; H. Nagaoka,Phil. Mag., 1889, 27, 117; 1890, 29, 123; H. Nagaoka and K. Honda,Journ. Coll. Sci. Tokyo, 1900, 13, 263; 1902, 16, art. 8;Phil. Mag., 1898, 46, 261; 1902, 4, 45; K. Honda and S. Shimizu,Ann. d. Phys., 1904, 14, 791;Tokyo Physico-Math. Soc. Rep., 1904, 2, No. 13; K. Honda and T. Terada,Journ. Coll. Sci. Tokyo, 1906, 21, art. 4.
40H. Tomlinson found a critical point in the “temporary magnetization” of nickel (Proc. Phys. Soc., 1890, 10, 367, 445), but this does not correspond to a Villari reversal. Its nature is made clear by Ewing and Cowan’s curves (Phil. Trans., 1888, 179, plates 15, 16).
41Wied. Ann., 1894, 52, 462;Electrician, 1894, 34, 143.
42Phil. Trans., 1890, 131, 329.
43Magnetic Induction, 1900, 222.
44Phys. Rev., 1904, 18, 432.
45Phil. Mag., 1886, 22, 50.
46Ibid.251.
47Phil. Mag., 1891, 32, 383.
48C.R., 1896, 122, 1192; 1898, 126, 463.
49Phil. Mag., 1889, 27, 117.
50Journ. Coll. Sci. Tokyo, 1904, 19, art. 9.
51Phil. Mag., 1905, 10, 548;Tokyo Phys.-Math. Soc. Rep., 1904, 2, No. 14;Journ. Coll. Sci. Tokyo, 1905, 20, art. 6.
52C.R., 1888, 106, 129.
53Proc. Phys. Soc., 1888, 9, 181.
54C.R., 1892, 115, 805; 1894, 118, 796 and 859.
55Elekt. Zeits., 1894, 15, 194.
56Phil. Mag., 1900, 50, 1.
57Phil. Trans., 1903, 201, 1.
58Phil. Mag., 1904, 8, 179.
59A. M. Thiessen (Phys., 1899, 8, 65) and G. Claude (C. R., 1899, 129, 409) found that for considerable inductions (B = 15,000) the permeability and hysteresis-loss remained nearly constant down to -186°; for weak inductions both notably diminished with temperature.
60Proc. Roy. Soc., 1898, 62, 210.
61C.R., 1895, 120, 263.
62Amer. Journ. Sci., 1898, 5, 245.
63Phys. Rev., 1901, 14, 181.
64C.R., 1897, 124, 176 and 1515; 1897, 125, 235; 1898, 126, 738.
65Ibid., 1898, 126, 741.
66Ibid., 1899, 128, 304 and 1395.
67See also J. Hopkinson,Journ. Inst. Elect. Eng., 1890, 19, 20, and J. A. Ewing,Phil. Trans., 1889, 180, 239.
68Many of the figures which, through an error, were inaccurately stated in the first paper are corrected in the second.
69The marked effect of silicon in increasing the permeability of cast iron has also been noticed by F. C. Caldwell,Elect. World, 1898, 32, 619.
70Trans. Roy. Dub. Soc., 1902-4, 8, 1 and 123.
71J. Trowbridge and S. Sheldon,Phil. Mag., 1890, 29, 136; W. H. Preece,Journ. Inst. Elec. Eng., 1890, 19, 62;Electrician, 1890, 25, 546; I. Klemençiç,Wien. Ber., 1896, 105, IIa, 635; B. O. Peirce,Am. Journ. Sci., 1896, 2, 347; A. Abt,Wied. Ann., 1898, 66, 116; F. Osmond,C. R., 1899, 128, 1513.
72Deutsch. phys. Gesell. Verh., 1903, 5, 220 and 224.
73Exp. Res., iii. 440.
74No record can be found of experiments with manganese at the temperature of liquid air or hydrogen; probably, however, negative results would not be published.
75The critical temperature of iron, for instance, is raised more than 100° by the addition of a little carbon and tungsten.
76Bull. Soc. Int. des Électriciens, 1906, 6, 301.
77Proc. Roy. Soc., 1905, 76A, 271.
78E. H. Hall,Phil. Mag., 1880, 9, 225; 1880, 10, 301; 1881, 12, 157; 1883, 15, 341; 1885, 19, 419.
79The large Hall effect in bismuth was discovered by Righi,Journ. de Phys., 1884, 3, 127.
80References.—(2) A. von Ettinghausen,Wied. Ann., 1887, 31, 737.—(4) H. W. Nernst, ibid., 784.—(i.) and (iv.); A. von Ettinghausen and H. W. Nernst,Wied. Ann., 1886, 29, 343.—(ii.) and (iii.); A. Righi,Rend. Acc. Linc., 1887, 3 II, 6 and I, 481; and A. Leduc,Journ. de Phys., 1887, 6, 78. Additional authorities are quoted by Lloyd,loc. cit.
81P. Drude,Ann. d. Phys., 1900, 1, 566; 1900, 3, 369; 1902, 7, 687. See also E. van Everdingen,Arch. Néerlandaises, 1901, 4, 371; G. Barlow,Ann. d. Phys., 1903, 12, 897; H. Zahn, ibid. 1904, 14, 886; 1905, 16, 148.
82Phil. Trans., 1856, p. 722. According to the nomenclature adopted by the best modern authorities, a metal A is said to be thermo-electrically positive to another metal B when the thermo-current passes from A to B through the cold junction, and from B to A through the hot (seeThermo-Electricity).
83C.R., 1893, 116, 997.
84Journ. de Phys., 1896, 5, 53.
85Phil. Trans., 1887, 177, 373.
86Proc. Roy. Soc., 1885, 39, 513.
87Phys. Rev., 1902, 15, 321. The sign of the thermo-electric effect for nickel, as given by Rhoads, is incorrect.
88Proc. Roy. Soc., 1904, 73, 413.
89C.R., 1903, 136, 1131.
90Journ. Coll. Sci. Tokyo, 1906, 21, art. 4. The paper contains 40 tables and 85 figures.
91This deduction from Ewing’s theory appears to have been first suggested by J. Swinburne. SeeIndustries, 1890, 289.
92R. Beattie (Phil. Mag., 1901, 1, 642) has found similar effects in nickel and cobalt.
93The charge associated with a corpuscle is the same as that carried by a hydrogen atom. G. J. Stoney in 1881 (Phil. Mag., 1881, 11, 387) pointed out that this latter constituted the indivisible “atom of electricity” or natural unit charge. Later he proposed (Trans. Roy. Dub. Soc., 1891, 4, 583) that such unit charge should be called an “electron.” The application of this term to Thomson’s corpuscle implies, rightly or wrongly, that notwithstanding its apparent mass, the corpuscle is in fact nothing more than an atom of electricity. The question whether a corpuscle actually has a material gravitating nucleus is undecided, but there are strong reasons for believing that its mass is entirely due to the electric charge.
94Jour. de Phys., 1905, 4, 678; translated inElectrician, 1905, 56, 108 and 141.
95The quotations are from the translation published by the Gilbert Club, London, 1900.
96C. A. Coulomb,Mem. Acad. Roy. Paris, 1785, p. 578.
97Intensitas vis magneticae, § 21, C. F. Gauss’sWerke, 5, 79. See also J. J. Thomson,Electricity and Magnetism, § 132.
98S. D. Poisson,Mém. de l’Institut, 1821 and 1822, 5, 247, 488; 1823, 6, 441; 1838, 16, 479.
99For outlines of the mathematical theory of magnetism and references see H. du Bois,Magnetic Circuit, chs. iii. and iv.
100Gilbert’sAnn. d. phys., 1820, 6, 295.
101Ann. de chim. et de phys., 1820, 15, 59, 170;Recueil d’observations électrodynamiques, 1822;Théories des phénomènes électrodynamiques, 1826.
102Ann. de chim. et de phys., 1820, 15, 93.
103Trans. Soc. Arts, 1825, 43, 38.
104Phil. Mag., 1873, 46, 140; 1874, 48, 321.
105Phil. Trans., 1885, 176, 523;Magnetic Induction, 1900.
106Proc. Roy. Soc., 1890, 48, 342.
107Phil. Mag., 1897, 44, 293; 1898, 46, 528.