“Rays of Positive Electricity.”

In 1886, Goldstein observed that when the cathode in a vacuum tube was pierced with holes, the electrical discharge did not stop at the cathode; behind the cathode, beams of light could be seen streaming through the holes in the way represented in fig. 1. He ascribed these pencils of light to rays passing through the holes into the gas behind the cathode; and from their association with the channels through the cathode he called these rays Kanalstrahlen. The colour of the light behind the cathode depends upon the gas in the tube: with air the light is yellowish, with hydrogen rose colour, with neon the gorgeous neon red, the effects with this gas being exceedingly striking. The rays produce phosphorescence when they strike against the walls of the tube; they also affect a photographic plate. Goldstein could not detect any deflection when a permanent magnet was held near the rays. In 1898, however, W. Wien, by the use of very powerful magnetic fields, deflected these rays and showed that some of them were positively charged; by measuring the electric and magnetic deflections he proved that the masses of the particles in these rays were comparable with the masses of atoms of hydrogen, and thus were more than a thousand times the mass of a particle in the cathode ray. The composition of these positive rays is much more complex than that of the cathode rays, for whereas the particles in the cathode rays are all of the same kind, there are in the positive rays many different kinds of particles. We can, however, by the following method sort these particles out, determine what kind of particles are present, and the velocities with which they are moving. Suppose that a pencil of these rays is moving parallel to the axis of x, striking a plane at right angles to their path at the point O; if before they reach the plane they are acted on by an electric force parallel to the axis of y, the spot where a particle strikes the plane will be deflected parallel to y through a distance y given by the equation y = e/mv2 A, where e, m, v, are respectively the charge, mass, and velocity of the particle, and A a constant depending upon the strength of the electric field and the length of path of the particle, but quite independent of e, m, or v.

The discharge of electricity through gases is a subject that has had a most wonderful growth, — a growth possibly greater than that of any other single division in physics. With the discovery of cathode rays, X-rays, radioactivity, and rays of positive electricity, a new era was begun. The cathode rays were the first of the above to be brought to our attention, however, but little was known of their properties until the researches of the last decade. About fifteen years ago the wonderful X- or Roentgen rays were discovered. A few years later came that almost revolutionizing discovery of radioactivity, — revolutionizing because we are obliged to change our conceptions regarding the molecule and atom. Another of equal importance because of its bearing upon chemical composition, is afforded by J. J. Thomson's recent investigations on rays of positive electricity.

The 2007 Benjamin Franklin Medal in Chemistry Presented to Klaus Biemann, Ph.D., of The Massachusetts Institute of Technology Cambridge, Massachusetts

This investigation was undertaken in order to discover whether the socalled canal rays (Canalstrahlen) discovered by Goldstein, which, at suitable pressures, visibly stream through the apertures in a perforated cathode, backward away from the anode, share with cathode rays the property discovered, as regards the latter, by Sir William Crookes many years ago, of producing a sufficient mechanical pressure to cause small and light mica mill-wheels to rotate. Several tubes were constructed and experimented with, and all showed that canal rays do cause mica mill-wheels to rotate quite rapidly. Fig. 1 is the form of tube with which this was most conclusively demonstrated. A 1 is the anode, the lower end of which is tipped with a glass plate, so as to preclude the transmission of cathode rays vertically downwards in the event of this electrode acquiring at any moment a negative charge due to oscillations in the electric discharge.

Rontgen and his pupils had always held that light waves were identical in nature with electrical waves produced by mechanical means, but there was a gap, on which very little work had been done, between the longest infra-red radiation and the shortest electrical wave that could be mechanically produced. He believed the investigation of this gap to be essential to the proper study of the constitution of the atom. The work already done on X-rays had demonstrated the existence of two separate rings of electrons in the atom, one within the other. These rings were responsible for the K and L types of radiation respectively. The L radiation was so much softer than the K that if a third ring of electrons existed, the radiation from which was proportionately softer than that of the L type, this radiation would fall well within the gap already mentioned. In the first experiment described a special form of discharge tube was employed. The positive rays passed through a tubular perforation in the cathode and impinged obliquely on a metal target. A photographic plate of the Schumann type was situated at the further end of a branch tube in such a position that no solid obstacle interposed between the target and the plate. When the discharge passed between the electrodes the photographic plate was affected. The application of an intense transverse electrostatic field between two metal plates situated between the cathode and the target completely stopped the effect, showing that this was not due to stray radiation reflected from the target, since, while charged particles would be swept to one side, radiation would not be affected by the field. Hence the passage of positive particles from the cathode to the target was essential. On the other hand, a strong transverse electrostatic field in the branch tube had no effect, showing that a radiation was passing between the target and the plate, which was not, therefore, merely affected by positive particles rebounding down the side tube after impact on the target. The properties of this radiation were intermediate between ordinary X-rays and Schumann waves. They were susceptible to reflection by metal surfaces, and their penetrating power was very small. They were completely stopped by the finest collodion film obtainable. It was shown that the quality of the radiation did not depend on the energy of the moving particles which gave rise to it, but on the velocity. Hence equally soft rays should be produced by cathode particles if these were travelling as slowly as the positive rays. A discharge tube was constructed in which the cathode rays, leaving the cathode with the ordinary velocity, could be subjected to a retarding electrostatic field of variable strength before impinging on the target. It this way the velocity of impact could be varied over a large range, and radiations were obtained varying in quality from ordinary hard X-rays to the so-called Schumann waves. It was hoped by the study of these radiations to be able to determine not only the number of rings of electrons within the atom, but the number of electrons in each ring.

IN 1886 Goldstein discovered that when the kathode in a discharge-tube is perforated, rays pass through the openings and produce luminosity in the gas behind the kathode; the colour of the light depends on the gas with which the tube is filled, and coincides with the colour of the velvety glow which occurs immediately in front of the kathode. The appearance of these rays is indicated in Fig. 1, the anode being to the left of the kathode KK. Since the rays appeared through narrow channels in the kathode, Goldstein called them “Kanalstrahlen”; now that we know more about their nature, “positive rays” would. I think, be a more appropriate name. Goldstein showed that a magnetic force which would deflect kathode rays to a very considerable extent was quite without effect on the “Kanalstrahlen.” By using intense magnetic fields, W. Wien showed that these rays could be deflected, and that the deflection was in the opposite direction to that of the kathode rays, indicating that these rays carry a positive charge of electricity. This was confirmed by measuring the electrical charge received by a vessel into which the rays passed through a small hole, and also by observing the direction in which they are deflected by an electric force. By measuring the deflections under magnetic and electric forces, Wien found by the usual methods the value of e/m and the velocity of the rays. He found for the maximum value of e/m the value of 104, which is the same as that for an atom of hydrogen in the electrolysis of solutions. A valuable summary of the properties of these rays is contained in a paper by Ewers (“Jahrbuch der Radioaktivität,” iii., p. 291, 1906).

ALL physicists and chemists, with many who, though less directly, are yet no less deeply interested in the subjects opened up by the study of the phenomena of the discharge tube, will rejoice that Sir J. J. Thomson has found time, amid his many preoccupations, to bring out this second edition of his well-known monograph on rays of positive electricity. The output of scientific work is now so enormous that it is difficult to keep pace with it even in one's own special line of study. It would be practi cally impossible, if it were not for the assistance given by books such as this, ever to come abreast once more of a subject in which one has once fallen behind. In writing this clear and authoritative account of the present state of a subject which he has done so much to develop, Sir J. J. Thomson has performed a real service to science. Rays of Positive Electricity and their Application to Chemical Analyses. By Sir J. J. Thomson. (Monographs on Physics.) Second edition. Pp. x+237+ix pl. (London: Longmans, Green and Co., 1921.) 16s. net.

It appears that the German physicist, Goldstein,1 was the first to publish the idea that the Sun sends out into space electrical rays analogous to cathode rays, and that we may thus explain the mysterious connection between variations in solar activity and corresponding fluctuations in the magnetic and electric phenomena, on the Earth. Some years later the Danish meteorologist, Adam Paulsen, from his auroral observations in Greenland was led to the hypothesis2 that the aurora is due to cathode rays; but instead of assuming that the rays came from the Sun, he believed that they originated in the upper strata of the atmosphere.Next, in the year 1896 Kr. Birkeland made his remarkable experiments with cathode rays in a magnetic field. He first found that a magnetic pole had an effect on a beam of parallel cathode rays analogous to that of a lens upon a beam of light, viz., to make the rays converge towards a point. This phenomenon led him to the idea3 that the aurora borealîs was due to a similar effect of the Earth's magnetic field on cathode rays coming from the Sun. In order to test his hypothesis, Birkeland exposed a small spherical electromagnet to a stream of cathode rays, and found a series of facts showing analogies to the shape and nature of the aurora. The aurora‐belts in particular were very beautifully produced. These remarkable experiments, which gave the first really good support to the corpuscular theory of aurora, were described on pages 39–42 of the paper “Expédition Norvégienne, 1899–1900, pour l'étude des aurores boréales.4 Notwithstanding that these remarkable experiments tend to show that the aurora is a direct effect of the precipitation of cathode rays in the upper air, Birkeland regarded the aurora as being caused rather by secondary cathode rays produced by strong electric currents in the upper atmosphere;5 in his later publication, however, he arrived at the conviction that the aurora is a direct effect of the electric rays from without.6

Figure 1. J J Thomson giving lecture demonstration of Braun e/m tube in the 1890s. (Photograph courtesy of the Cavendish Laboratory, Cambridge.) In spite of some competitors, Joseph John Thomson, second successor to James Clerk Maxwell in the Cavendish Chair at Cambridge, is generally accepted to be the discoverer of the electron; and although subatomic electrically charged particles were floating throughout the entire history of 19th Century physics, the 30th April 1897, when Thomson gave his lecture on `Cathode rays' at the Royal Institution of Great Britain in London, is being celebrated as the particle's official birthday. The reason is that, after a detailed overview with some demonstrations of facts in cathode-ray physics, Thomson expounded an interesting hypothesis, namely that `atoms of the elements are aggregations of very small particles, all similar to each other; we shall call such particles corpuscles, so that the atoms of ordinary elements are made of corpuscles and holes, the holes being predominant', and that the cathode rays must be identified with these corpuscles [1]. Clearly, this hypothesis combined elements of former knowledge with the speaker's new convictions, notably that all corpuscles had the same mass. He reported on his own method for measuring the ratio of mass m to charge e by magnetic deflection; the value obtained, 1.6 × 10-7, `was small compared with the value 10-4 for the ratio of the mass of an atom of hydrogen to the charge carried by it', and hence it favoured together with other results `the hypothesis that the carriers of the charges are smaller than the atoms of hydrogen'. [2] Thomson concluded finally: `It is interesting to notice that the value of e/m, which we have found from the cathode rays, is of the same order as the value 10-7 deduced by Zeeman from his experiments on the effect of a magnetic field on the period of sodium light.' [2] Thomson's pioneering work of 1897, together with the Dutch investigations of the Zeeman effect and the German studies by Emil Wiechert and Willy Wien (also on cathode rays) established the electron as a constituent of matter. Thus began the electron's great role in science. This issue of European Journal of Physics celebrates the centenary of this occasion, certainly one of the most influential events in the history of physics, with five contributions provided by physicists and historians of science. The first two articles, those of N Robotti and A Kox, describe the discovery of the electron, a veritable double-birth of what soon turned out to be the same child. A condensed (and very selected) chronology of the successful deeds of the electron in the physics of almost the past century by H Rechenberg is followed by a more lengthy narrative of decisive episodes between 1900 and 1930 through which the electron revolutionized the foundations of modern chemistry, presented by T Arabatzis and K Gavroglu. The last paper, by R Penrose, provides a glimpse of the mathematical elegance in the now standard description of the electron by Paul Dirac's famous relativistic equation. Much more can and will be written in many papers devoted this year to the electron's anniversary. It will be argued that: firstly, the electron is (still) a fundamental constituent of all matter; secondly, it is a driving force in many different natural phenomena (or fields of physics); and, thirdly, it exhibits unique properties (such as spin or the particle - antiparticle property) which provide the key to solving the profoundest riddles of Nature and its evolution. That these properties are far from being easily `visualizable' can be derived from the paper of Arabatzis and Gavroglu: at the beginning of this century, chemists endowed the electron with so many abstruse properties that physicists were ultimately happy to have available `non-visualizable' quantum mechanics, allowing the non-classical features of the electron and its behaviour to be described so perfectly. Let us finally recall what Arthur Eddington states in 1926 in the introduction to his The Internal Constitution of Stars: he did not know who was the hero of his story, the big star or the small electron. Today we do know the answer: the electron is the star, both in micro and macro physics.

Countless lives have been redeemed by the accidental revelation of the wonder that is X rays. On November 8, 1895, in the same week that the Lumiere brothers introduced cinematography in France, Wilhelm Conrad Rontgen stumbled across X rays while experimenting with cathode rays at the University of Wurzburg in Bavaria—and was rightly awarded the Nobel prize in 1901 for the achievement. During the course of his analysis, Rontgen had observed a bright fluorescence, which he first thought might be due to the cathode (beta) rays, but later realized that the effect was produced by something as yet beyond his ken and coined the term “X rays” to describe his discovery. He established the potency of X rays after positioning a photographic plate in front of them; the rays left an impression. But his real genius came when he shrewdly recognized the medical importance of the beams once his wife Bertha Rontgen had tried placing her hand between rays and plate, depositing an X-ray image of the bones in her hand on the plate’s emulsion.

LONDON.Royal Society, April 25.—Sir Archibald Geikie, K.C.B., president, in the chair. —J. S. Townsend: The diffusion and mobility of ions in a magnetic field. The mobility and diffusion of ions in a magnetic field is investigated on the same principles as those employed in the ordinary kinetic theory by considering the motion of an ion along its free paths between collisions with molecules. If U and K be the mobility and coefficient of diffusion when the magnetic force is zero, U, and K7, the corresponding quantities in directions at right angles to a magnetic force H, then where w = He/ra and T the mean interval between collisions. The magnetic deflection 0 of a stream of ions moving with a constant velocity in an electric field is also investigated, and a method is indicated of determining the velocity U due to an electric force X. When 9 is small, tan" = HU/X, and when 9 is large, tanflX/ = HUfc. —J. J. Manley: The observed variations in the temperature coefficients of a precision balance. In this paper is given an account of experiments which supplement and extend an earlier research (Phil. Trans., A, ccx., p. 387) dealing with changes which may be observed in the resting points of precision balances. Attention is directed to the following: - (a) the possibility of the change from a positive to a negative value for the temperature coefficient of a balance; (b) the critical temperature range of a balance; (c) the various causes tending _to give rise to a temperature coefficient; (d) the necessity for the " ageing " of a beam either naturally or artificially. In addition to the above, certain minute and temporary lateral displacements of the whole beam are investigated. A method for measuring these movements is given, and their origin disclosed. - Dr. Guy Barlow: The torque produced by a beam of light in oblique refraction through a glass plate. In accordance with the principle that light carries with it a stream of momentum, the passage of a beam of light through a refracting plate should give rise to a torque on the plate, it being supposed that the reaction is on the matter through which the beam is passing. In 1905 Prof. Poynting and the author made experiments which confirmed this result, but as disturbances, due to gas action, were not eliminated, more exact measurements appeared desirable. In the present experiment the original double-prism arrangement was abandoned in favour of a single cube. A glass cube, of i cm. edge, was suspended axially by a fine quartz fibre. A strong beam of light was sent obliquely through the cube, the angle of incidence having been so adjusted that the beam entered through one half of one face, and emerged through the half-face diagonally opposite. The torque was determined from the observed angular deflection of the cube. Observations were made in hydrogen and air with pressures ranging from o-i to 76 cm. Hg. The disturbance due to radiometer action was found to be inversely proportional to the gas pressure, and could be eliminated. After allowing for the reflected beams, the observed torque (of the order 2xio˜6 dyne cm.) was within 2 per cent, of that calculated from the energy of the beam.-Dr. T. C. Porter: Contributions to the study of nicker. Paper III. This paper is a continuation of two former papers: Proc. Roy. Soc., vol. Ixiii., p. 347, and vol. Ixx., p. 313. If n be the number of revolutions per second for a disc with white sector "TO" and the rest black, just to appear flickerless under illumination " I," then n= -27-83 + (8-57 + 2-79 log I) log iv (360-tt'); this holds when I is greater than 3-98. If I be less than 3-98, then n= - 38'6 + (i2-4 + o-77 log I) log w (360 -w).

Chemical action induced by cathode rays and canal rays

The first page of this article is displayed as the abstract.

When, in 1899, the close friend and colleague of Marie and Pierre Curie, André Louis Debierne, announced the discovery of actinium, he could not imagine that this element would become the first of a group of key elements for understanding atomic structure. A theoretical basis, an initially empirical periodic table, for the systematic study of the atom had been laid down many years earlier by Dmitri Ivanovich Mendeleev in 1869. It turned out to be a tool that could be used to predict what Mendeleev discerned as “missing elements,” elegantly confirmed by the discoveries of gallium in 1875, scandium in 1879, and finally germanium in 1886. Over the course of the following 60 years, a series of discoveries were made that began to reveal the modern picture of the structure of the atom. In chronological order, these were cathode rays, emission spectra, canal rays (protons), X‐rays, radioactivity, the electron, α, β, and γ rays, Planck's law, the photoelectric effect, the atomic nucleus, isotopes, Bohr model of atomic structure, atomic number, and the neutron. It gradually became clear that the number of nuclear protons equaled the nuclear charge and conferred on each atom its unique identity. This allowed scientists to determine how many elements existed in nature, namely 92. It also allowed them to devise experiments to push the envelope beyond 92—to actually create new elements by bombarding and combining the existing atomic nuclei, thus expanding the original periodic table to 118 elements. The impact of these discoveries has changed the course of history. The story of Debierne's discovery, actinium, and the 29 elements that follow it are the subject of this article and this volume.

As is well known, when the lines of force of a magnetic field cut the path of the cathode rays in a vacuum tube, the rays are deflected in one direction or another, according to the polarity of the lines of force. If, on the other hand, the relative positions of the vacuum tube and the magnet are such that the lines of force and the cathode rays are parallel, the rays are not sensibly deflected.