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Andre-Marie Ampere: Enlightenment and Electrodynamics by James R. Hofmann Cambridge University Press: 1996. Pp. 406. £40, $49.95

EARLY in April last, while my son, Malcolm Thomson, was operating, in a building of the River Works plant of the General Electric Co., a resistance welder for closing the seams of steel Langmuir mercury vacuum pumps, in which work the current is applied and cut off at about one-half second intervals, there was noticed by one of the working force, Mr. Davis, who happened to be favourably located, a peculiar intermittent illumination of the space near the welder as the current went on and off. My son at once placed himself in a similar position and saw the novel effect, and noted a number of conditions accompanying it, perhaps the most important being that a single-turn loop from the welding transformer to the work and back was carrying about 7000 amperes, and that the luminous effect was spread in the space in which would be located the magnetic field from this loop; that the sunlight was entering the building through high windows and shining across the space in which the field was produced at intervals; that the effect was most conspicuous when one looked towards the shadows and across the sunbeams, and also across the magnetic field.

Two simple experiments are reported here which demonstrate jet-propulsion in the direction of current flow between liquid and solid conductors. The observed effects seem to conform with Ampere's force-law, but no qualitative nor quantitative explanation has so far been found in terms of lorentzian forces and associated magnetohydrodynamics (MHD) phenomena. This research forms part of a wider investigation of the Ampere–Neumann electrodynamics and its relationship to modern science and technology.

As Prof. Daehne (NATURE, June 24, p. 168) has called attention again to the treatment of Ampere's rule in my “Electricity Treated Experimentally,” perhaps you will allow me to point out that the rule given by Ampere is quoted historically only, and for it is substituted a rule, due, I believe, to Clerk-Maxwell, which seems to me preferable to either the original rule of Ampere, or to that quoted by Prof. Daehne, namely, that the movement of a north pole is right-handed to the direction of the current. That is to say, if we assume any right-handed screw to be propelled along the current, the north pole will move in the direction of the twist in the muscles of the wrist in propelling it; and vice versâ, if the north pole move in the direction of propulsion, the current urging it will be in the direction of twist in the muscles of the wrist. In treating the movement of a conductor carrying a current in the magnetic field, I have used a rule identical in character with Ampere's, and that was probably the rule to which J. T. B. referred in his critique, namely, that a figure swimming in the current and looking along the lines of force is carried to his left. I should be glad to find a rule at once as complete and more simple, although after a pretty wide experience, not always with the very brightest of pupils, I have not been sorely pressed with the difficulty J. T. B. seems to have felt, All the required attitudes are pretty familiar to a boy who is accustomed to diving in the water and swimming on his front, side, or back.

THE whole scientific world will join in celebrating Faraday's discovery of electromagnetic induction, and it will certainly not be passed over in Bohemia. I may be excused if I begin with myself. In our middle school (gymnasium) we had a most excellent teacher of physics, Prof. Pokorny. In his clear and beautiful lectures, which would have served as a model to those given at a university, he taught us in 1870 the principal discoveries in the domain of electricity. He spoke of Galvani, Volta, Carlyle and Nicholson, Orsted, Ampere, and others, but before all he praised the discoveries of Faraday.

THIS volume consists of reprints of the author'sarticles on various classic researches in physics and chemistry which appeared inThe Engineer during 1944—48. The field, covered includes the problem of friction (Coulomb, Morin, Tower and Osborne Reynolds), the mechanical equivalent of heat (Rumford, Mayer, Joule and others), electrodynamics (Oersted, Ampere, Faraday), ether-drift (Arago, Michelson, Morley, Miller and Fitzgerald), specific, latent and atomic heat (from Black to Planck), the chemical elements and atoms (from antiquity to Dalton and Avogadro), the classification of the elements (Newlands, Lothar Meyer, Mendeleeff), molecular physics (Newton, Daniel Bernoulli, Joule, Clausius and Clerk Maxwell), the conduction of electricity through liquids (Davy, Faraday, Grotthuss and Kohlrausch), the conduction of electricity through gases (Faraday, Plucker, Hittorf, Crookes, Goldstein, Lenard, Perrin, J. J. Thomson, C. T. R. Wilson and Millikan), X-rays and positive rays and isotopes. Historic Researches Chapters in the History of Physical and Chemical Discovery. By Dr. T. W. Chalmers. Pp. vi + 288. (London : Morgan Brothers (Publishers), Ltd., 1949.) 21s. net.

ANDRE MARIE AMPERE was born in Lyons on January 20, 1775, and died at Marseilles on June 7, 1836. His early childhood was spent in the country near his birthplace and his first studies were directed by his parents. A childish pastime of carrying out complicated arithmetical calculations with little pebbles was prophetic of his future devotion to mathematical studies, a devotion that was evidenced again when his father, a retired merchant, began to teach him Latin; for the young Ampere quickly showed his great preference for mathematics, whereupon his father wisely allowed natural inclination to take its own course, providing the necessary introductory works from his own small library. But when these had been mastered, more advanced reading was necessary; and it is recorded that, at twelve years of age, Ampere, accompanied by his father, went to ask in his piping boyish voice for the loan of the works of Euler and Bernoulli from the College Library at Lyons. He appears to have mastered these classics also; and he read widely in the literary, historical, scientific and philosophical authors of his country. In fact, like a recent Lord Chancellor of England, he turned to the current encyclopaedia, in his case that of Diderot and d'Alembert, to equip himself with the accumulated knowledge of the ages; and a half-century later he was able to recite from memory whole passages from the famous “Encyclopedic” that expressed the genius of eighteenth century France.

SEVERAL, experiments have been carried out to check whether the resistance of a bulk metal in the superconducting state is truly zero. The best known is that carried out by Collins1, in which a persistent current of several hundred amperes was maintained in a lead ring for two and a half years. At the end of this time there was no measurable decrease in the magnitude of the current. The resistance of the ring is quoted by Crowe1 as less than 10−21 ohm, but no figure for the resistivity of the lead is given.

IN a lecture last January to the Christiania Academy, Prof. Birkeland 1 gave an interesting summary of his recent researches on solar and planetary electrical phenomena. He describes how in a study intended to elucidate the evolution of celestial bodies he examined the nature of the electric discharge taking place in vacuo in a large discharge vessel from a magnetisable globe serving as kathode. The experiments, which were made under widely differing conditions, were on a scale more ambitious than anything hitherto attempted. Two vessels of 300 and 1000 litres' capacity respectively were employed. In the larger of these the globe used was of 36 cm. diameter, and discharges up to nearly half an ampere were obtained. Some of the published photographs are very remarkable. One of them showing the electric corona and streamers round the magnetised globe might easily be mistaken for a genuine photograph of a typical solar eclipse. Many of the phenomena of sunspots are also very strikingly imitated in the experiments.

I AM glad to see from Mr. Rollo Appleyard's letter in NATURE of December 31 that he is satisfied with my reply to his query concerning the place of magnetism in fundamental electrical theory. Of course there can be no question of “annihilating magnetic notions” in view of the existence and properties of permanent magnets; but, as regards the understanding of fundamental physical phenomena, magnetism appears to have played a part similar to that of money in economics—a useful link for facilitating exchange, but a source of unnecessary complexity and confusion in discussion. Since we can formulate the forces acting on moving electric charges directly from Ampere's experiments without introducing magnetism, and thereby dispense with the two stages of reasoning involved in Maxwell's curl equations, it seems decidedly advantageous to do so; and if a magnetic field is merely a translationally moving electric field, as the flux-cutting principle suggests, it seems superfluous to regard it as a separate entity.

The weak-field NPL apparatus for determining the gyromagnetic ratio of the proton, and for checking the ampere as maintained by the Laboratory standard resistors and Weston cells, is working satisfactorily. The principle of the method is shown in Fig 1. The field-forming coils are vertical, and, when carrying 1.018A, produce at the centre a flux density of about 1.2mT, corresponding to a precessional frequency of about 50 kHz, uniform to 0.5 in 106 over the volume of a sphere 40 mm in diamet er. The current is provided by a 110V battery of large accumulators, and is controlled by a specially made controller of the galvanometer and photo-cell type, so that only very slight hand adjustment, if any, is required to keep the current constant at the value determined by the standard resistor and Weston cell.

THE following method of demonstrating the Peltier and Thomson effects may be of interest. In Fig. 1 the current passes through an Sb-Bi-Sb bar, the points of contact being amalgamated to reduce the resistance. Two coils of No. 36 covered copper wire are wound on the bismuth one near each junction, and by means of the leads A and B are placed in the gaps of a metre bridge, and a balance produced. On passing a current of 1 ampere through the bars, one junction is heated and the other cooled, which is indicated by a galvanometer deflection of about 40 mm. due to the change in resistance of the copper coils. The direction indicates a heating where the current flows from Sb to Bi, and vice versa.

THE decision of the International Electrotechnical Commission to recommend the use of the M.K.S. system of electrical, mechanical and magnetic units, as reported in NATURE of July 6, may have far-reaching consequences, for experience has shown that a unit in daily use by engineers soon becomes a familiar quantity, and as such is preferred for quantitative work even by the pure physicist. The statistical examination of tho question by G. A. Campbell1 shows this very clearly: the practical units, volt, ohm, ampere, coulomb, etc., are used by the pure physicist far more than the corresponding C.G.S.M. or C.G.S.K. units. Since the M.K.S. system also possesses advantages for mathematical work, it is at least possible that it may ultimately become universally used.

MY chief reason for desiring to reply to Prof. Howe's letter on this subject in NATURE of March 13 is that the last paragraph of the letter gives the impression, inadvertently I presume, that the implications of the adoption of Ampere's theory of magnetism had been considered fully by the International Committee when they made the recommendation that B and H were to be considered respectively in the roles of effect and cause, and their ratio therefore as an entity having physical dimensions. This is not the case.

THE suggestion by Dr. Drysdale, in NATURE of August 13, that by resorting to the experiments and equations of Ampere relating to the forces between current-carrying conductors, magnetism may be eliminated from fundamental physical concepts, reminds me that in the course of conversation with the late Sir Horace Lamb some years ago, he remarked that it would greatly simplify mathematical treatment if we could dispense with the duality of electricity and magnetism and concentrate on one of them, as Ampere appeared to have done. The hint was the more impressive because Sir Horace, in his “Hydrodynamics”, had set forth the vortex theory so convincingly.

THE following version of Ampere's Rule is one which I communicated some time ago to a few friends, but it did not appear to me to be expressed in language sufficiently grave to justify its publication. Still, as the Rule is a simple and useful one, your readers, in general, may be disposed to overlook its levity.

I DOUBT very much whether Sir James Henderson1 is justified in assuming that the members of the International Committee—and presumably also those of the various congresses and committees which have from time to time reaffirmed the decision that the permeability of space should be regarded as an entity having physical dimensions (the so-called Oslo convention)—were not as fully aware as he himself of ” the implications of the adoption of Ampere's theory of magnetism”. That their interpretation of these implications differs from his I do not doubt. When I read that ” Prof. Howe would have us jettisonthat wonderful machine which has produced the electromagnetic theory of light, to which is due the discovery of the quantum theory and all the other developments of modern theoretical physics”, I can only smile and say that it sounds magnificently foolish, but bears little, if any, relation to the facts of the case. In his paper he showed that it was not only possible, but also very tempting, to assume that permeability is merely a numeric, and that in space the magnetic induction B and the magnetizing force H are one and the same thing. This is, however, a very old assumption; Gauss and Weber made it and on it built the electromagnetic system of units, but they never deceived themselves into thinking that it was anything more than a conventional assumption. Sir James is mistaken in thinking that I object to the mathematical nature of his arguments; on the contrary, my objection is to the assumptions which he introduces. The question is: What are the dimensional relations between H and B in space? I confess in all humility that I do not know, and I put B/H = ;0 a constant of space analogous to the gravitational or dielectric constants of space. Sir James Henderson, however, says, ” According to Ampere's theory B and H are physically identical” and apparently expects the scientific world, which has debated the matter for at least sixty years, to take his word for it.

ACCORDING to Ampere's theorem, the magnetic field due to an electric current flowing in any circuit is equivalent at external points to that due to a simple magnetic shell the bounding edge of which coincides with the conductor, and the strength of which is equal to the strength of the current.

IT was originally supposed that separating contacts in low-voltage, low-inductance circuits interrupted the current without arcing provided the supply voltage was lower than the ionization potential of the contact metal, and that the circuit inductance was not greater than about a microhenry. However, inconsistencies in the values of the measured ‘fine transfer’ obtained under these so-called arc-free conditions led to the supposition that arcs might be present1, and this was confirmed by Lander2, who published an oscillographic record of an arc obtained between separating gold contacts in a 6-volt, 1 microhenry circuit carrying a current of 1 ampere.

THE Department of Natural Philosophy in this University has recently acquired, through a fund provided by the liberality of the Bellahouston Trustees of Glasgow, a fine echelon grating of twenty-six plates made by Hilger, London. The instrument has excellent definition, and its great power enables it to show Zeeman effects with moderate magnetic fields. In conjunction with a new electromagnet which has been constructed for the Department by Messrs. Mavor and Coulson from the same fund, we have been able to make some preliminary observations which may be of interest to readers of NATURE. The magnet and spectroscope were shown at the recent British Association meeting, when some account of the power of the set of instruments was given. The magnet when excited by a current of only five or six amperes gave a field of about 50,000 C.G.S., and was found capable of giving still higher intensities.

THE maintenance of oscillations of a sonometer wire by the passage of an alternating current through it has been studied in detail by Krishnaiyer and others (Phil. Mag., 1922, etc.). If, however, direct current of the value of about an ampere be passed through the wire, it is found that the frequency of the oscillations, for a fixed position of the two sonometer bridges, is slightly lower than what it is when no current is passing. This effect is best observed by tuning the sonometer with an electrically excited fork placed on the sonometer board. When the wire is tuned to this frequency it begins to vibrate with a large amplitude. These vibrations are observed with a low power microscope. If the direct current be now passed through the wire, the amplitude of the vibrations is immediately reduced, and can be restored again to its original strength by shortening a little the length of the wire between the bridges.