Bakerian Lecture :—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 Figure 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 on 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. Wein, 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 a 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

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.

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.

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

AlSn12 clusters were studied in electric and magnetic beam deflection experiments at nozzle temperatures of Tnozzle = 16-100 K. For 16 K, spatial separation of two fractions of clusters in the molecular beam was achieved by deflection with both an electric and a magnetic field gradient. In the electric deflection experiment, about 76% of the clusters are identified as non-polar and the rest as highly-polar, while the magnetic deflection experiment demonstrates that 37% show an atom-like and 63% a Brillouin-like magnetic response. In order to probe the connection between these fractions in electric and magnetic beam deflection, a combination of these two experiments was performed. This clearly demonstrates that the highly-polar clusters show a Brillouin-like magnetic response and only the non-polar clusters can be deflected atom-like in a magnetic field. This observation suggests that two structural isomers are present in the molecular beam, one of which is highly-symmetric, and demonstrates that spatial isomer separation of metal clusters containing heavy elements is feasible. However, vibrational excitation must also be taken into account to explain the observed magnetic response. A stepwise increase of the cluster temperature shows that suppression of the superatomic response is more sensitive to vibrational excitation than the quenching of the permanent electric dipole moment.

MSn12 clusters (M = Al, Ga, In) were studied in electric and magnetic beam deflection experiments at temperatures of 16 K and 30 K. For all three species, the results of the electric beam deflection experiments indicate the presence of two structural isomers of which one is considerably polar. The magnetic beam deflection experiments show atom-like beam splitting (superatomic behavior) with g-factors of 2.6-2.7 for a fraction of the clusters in the molecular beam, indicating significant spin-orbit coupling. On the one hand, we investigate by several experiments combining electric and magnetic deflectors how the superatomic and polar fractions are linked proving the correlation of the Stark and Zeeman effects. On the other hand, the magnetic deflection behavior is examined more thoroughly by performing quantum chemical calculations. By systematic distortion of an artificial icosahedral tin cage towards the global minimum structure, which has a pyritohedral geometry, the shifts in the magnitude of the g-factor are found to be mainly caused by a single dominant electronic excitation. This allows one to develop a semi-quantitative understanding of the magnetic behavior. On the basis of avoided crossings in the rotational Zeeman diagram, simulations of the magnetic beam deflection comprising computed rotational constants, vibrational modes, g-factors and spin-rotation coupling constants are performed which resemble our experimental findings in satisfactory agreement. With this, a better understanding of the magnetic properties of nanoalloy clusters can be achieved. However, the geometric structures of the polar isomers are still unknown.

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.

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

This paper reviews the relationships, both personal and professional, between J. J. Thomson and Arthur Schuster, the first physicist to attempt quantitative measurements on cathode rays in the years 1884–1890. Their relationships were unusually close, since they both attended the same college (Owens in Manchester), worked in the same research field (discharge of electricity through gases), and served many terms together in executive positions in the Royal Society of London. There were occasional misunderstandings and priority disputes between them about cathode rays, but after Thomson received his Nobel Prize in 1906 harmony again prevailed. In 1897, Thomson had available the important experimental results of Hertz, Lenard, Perrin, Wiechert, and Kaufmann on cathode rays, whereas in 1890, Schuster could benefit from none of this work. But the main reason that Thomson, and not Schuster, “discovered the electron” was that he was a better experimental physicist than Schuster.

While not obvious, deflection aberration is a key aberration in Cathode Ray Tube (CRT) design. A new concept in electron beam deflection with electric fields, originally proposed in 1997, is now being tested in the laboratory. Using a beam injected off the axis of symmetry, deflection aberrations are predicted to be 10 fold reduced compared to symmetrical injection. This would be less than magnetic deflection aberrations. If the invention proves to be valid, important improvements are possible in CRT brightness, resolution, energy consumption, and footprint reduction. As one example, reducing deflection aberrations allows larger beam diameters in the deflection plane as well as large deflection angles. This will reduce space charge spread, allowing larger beam currents and/or smaller focused spot size. Improved medical imaging displays could be built. For another example, much of the energy consumed in a magnetically deflected CRT display is associated with deflection. Electric deflection has a significant advantage in energy consumed compared to magnetic deflection. With 400 million CRTs in daily use in the US consuming 0.54 quads, there is a large incentive to reduce power consumption in CRTs particularly so since excess heat produced adds to office air conditioning loads.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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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.

Chemical action induced by cathode rays and canal rays

List of illustrations. Preface. J J Thomson and his contemporaries. Electromagnetic phenomena unravelled. Cathode rays take center stage. The English get going. Meanwhile, back in Berlin. The English keep going. From Paris to the Scottish highlands. From Liverpool to Princeton. The race for e/m. The charge and the mass. Leiden, 1896. The photoelectric effect revisited. The Beta particle. Evanescent rays: A French cottage industry. Positive rays. The electronic charge revisited, and one more controversy. Dawning of the atomic age. Epilogue: The next twenty years. List of abbreviations. Notes. Bibliography. Name index. Subject index.