Discovery of the Electron

10 Key excerpts on "Discovery of the Electron"

• 

  eBook - PDF

  Half-Hours with Great Scientists

  The Story of Physics

  • Charles G. Fraser(Author)
  • 2019(Publication Date)
  • University of Toronto Press

    (Publisher)

  CHAPTER TWENTY-FOUR The Electron Theory T HE seventh epoch in the story of electricity is characterized by the electron hypothesis and its applications. The advent of this epoch was brought about by the work of so many able investigators that it is not easy to single out one man and accord him the laurels. If there is one name, however, that stands above all others, it is prob-ably that of the gifted physicist and mathematician, Sir J. J. Thomson, who became Maxwell's successor as Director of the Cavendish Labor-atory at Cambridge. A paper of his, published in 1896, proposed the assumption that atoms are composed of smaller particles—now called electrons, protons, neutrons, etc. Only a man with courage, much prestige, and irrefutable experimental data could have dared to stand up and tell the scientists of that day that what they called atoms (literally, not divisible) should really be called toms. Many of those scientists were such fanatical subscribers to the atomic theory that they resented an attack upon it as violently as they would have an attempt to undermine their religious faith—just like the peripatetics in Galileo's day. Science will have made an important advance when the fanatical support of any hypothesis is frowned out of court. Professor Jean Perrin of Paris, a friend of Pierre and Marie Curie, had shown, by using a pierced anode, that cathode rays carry negative charges. By modifying Perrin's apparatus Thomson succeeded in making an ingenious series of measurements and calculations, and his conclusions from these brought the electron theory into the world— and a new and marvellous epoch into not only electricity but also some other sciences: The apparatus used is represented in fig. [333]. The rays from the cathode C pass through a slit in the anode A, which is a metal plug . . . connected with the earth; after passing through a second slit B, .

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Science In The Making

  1850-1900

  • E. A. Davis(Author)
  • 1997(Publication Date)
  • CRC Press

    (Publisher)

  Part Six J. J. Thomson and the Discovery of the Electron Part Six J. J. Thomson and the Discovery of the Electron J. J. Thomson (1856-1940) Joseph John Thomson, frequently referred to as 'J ]', was born near Manchester to a family of modest means. His father was a bookseller and publisher who died two years after sending his son to Owen's College at the age of 14. There Thomson had his first glimpse of science via the lectures of Balfour Stewart and Osborne Reynolds, the professors of physics and engineering at the College. He had the opportunity to undertake laboratory experiments and at the age of 20 published his first paper (Philosophical Magazine 1877 3 389) 'Experiments on contact electricity between non-conductors'. Thomson won a scholarship to Trinity College, Cambridge in 1876. He was to remain a member of that College all his life, eventually becoming its Master. He graduated as Second Wrangler in the Mathematical Tripos with a thorough grounding in mathematical physics. Immediately after taking his degree, Thomson began work in the Cavendish Laboratory and soon found himself working on the precision determination of electrical standards under Lord Rayleigh who had succeeded Maxwell as the Cavendish Professor of Experimental Physics in 1879. Five years later Thomson himself was elected to this prestigious position. Thomson published extensively in the Philosophical Magazine both before and after his appointment as Cavendish Professor in 1884, with over sixty papers authored or coauthored by him appearing in its pages. In 1892 he wrote to Richard Threlfall (who worked as Thomson's assistant in the laboratory and later became Professor of Physics at the University of Sydney) I think myself that the Philosophical Magazine is a better means of publication than even the Royal Society as the circulation is larger and the delay very much less.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Understanding The Universe: From Quarks To The Cosmos

  From Quarks to the Cosmos

  • Donald Lincoln(Author)
  • 2004(Publication Date)
  • World Scientific

    (Publisher)

  If, in the very intense electric field in the neighbourhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the elec-tric field, they would behave exactly like the cathode rays. Thus for many historians of science, this lecture heralded the age of modern particle physics. Thomson called his discovery “corpuscles,” although in this he was quickly out of step with his colleagues. While most were very skeptical of his assertions (a distinguished member of his audience 46 u n d e r s t a n d i n g t h e u n i v e r s e later told Thomson that he thought that Thomson was pulling their leg), evidence quickly grew. Physicists began to call the new particles “electrons,” a term coined by G. Johnstone Stoney in 1891, in an entirely different context. Stoney used the term electron to describe the smallest unit of charge found in an experiment that passed current through chemicals. Thomson did not use the term “electron” for more than 20 years. While Thomson had measured the mass to charge ratio, he could say little about either independently. Using entirely different tech-niques, two years later Thomson showed that the charge of one of his “corpuscles” was about the same as that carried by a hydrogen ion. Consequently, one was led to the inescapable conclusion that the mass of an electron was very small (modern measurements give it a mass of 1 / 1886 that of a hydrogen atom). The electron became a very light component of all atoms, each carrying the same charge as an ion. For his brilliant understanding of the data, as well as a few delicate experiments of his own, Thomson was awarded the Nobel Prize in Physics in 1906, for “researches into the discharge of electricity in gases.” Knighthood followed in 1908 and the Order of Merit in 1912.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Restless Universe

  • Max Born(Author)
  • 2013(Publication Date)
  • Dover Publications

    (Publisher)

  CHAPTER IV The Electronic Structure of the Atom

  1. The Discovery of the Positive Charges in the Atom.

  N OW we know enough about the electron to be able to study the structure of the atom accurately.

  The first question is one which we have already asked more than once, but have not yet answered: where is the positive electricity in the atom? For, as the whole atom is electrically neutral and it has negative electrons in its outer regions, there must be a positive charge somewhere inside.

  We must therefore penetrate into the interior of the atom. For this purpose we shall find fast particles useful, as they have great momentum. The first systematic investigations of this kind were made by Lenard, using fast cathode rays. As we have already mentioned (p. 94 ), these will pass through thin layers of metal. In solid bodies the atoms are tightly packed together. The outer shells of electrons of one atom are almost touching those of the next. If an electron flies through with only a small deviation of direction it must have passed through the interior of the atom almost undisturbed (65). This interior must therefore be comparatively empty, in spite of its compact outer layer.

  Occasionally, an electron is strongly deviated; we naturally suppose that it has struck some solid obstacle. Lenard was able to explain the bulk of his observations by assuming that in the interior of the atom there are a number of heavy positively charged particles, which he called “dynamids”.

  We shall not, however, use these experiments to explain how such conclusions can be drawn, but those of Rutherford instead. Rutherford used heavier projectiles, the α-particles from radioactive substances, which we have already mentioned (p. 102

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Maxwell's Enduring Legacy

  A Scientific History of the Cavendish Laboratory

  • Malcolm Longair(Author)
  • 2016(Publication Date)
  • Cambridge University Press

    (Publisher)

  Intriguingly, the ferment of continental physics and theoretical physics had relatively little impact upon the experimental programme of the Laboratory, which continued the tradition of carrying out ingenious experiments and model-building, which informed the deliberations of the theorists. 7.1 The problems of building models of atoms Thomson summarised the researches which led to his Discovery of the Electron and its universality in his influential book Conduction of Electricity through Gases (Thomson, 1903a). As he wrote in the introduction: The study of the electrical properties of gases seems to offer the most promising field for investigating the Nature of Electricity and the Constitution of Matter, for thanks to the Kinetic Theory of Gases our conceptions of the processes other than electrical which occur in gases are much more vivid and definite than they are for liquids or solids; in consequence of this the subject has advanced very rapidly and I think it may now fairly be claimed that our knowledge of and insight into the processes going on when electricity passes through a gas is greater than it is in the case either of solids or liquids. The pos- session of a charge by the ions increases so much the ease with which they can be traced and their properties studied that, as the reader will see, we know far more about the ion than we do about the uncharged molecule. 139 140 The Thomson era, 1900–1919: atomic structure With the discovery and study of Cathode rays, Röntgen rays and Radio-activity a new era has begun in Physics, in which the electrical properties of gases have played and will play a most important part; the bearing of these discoveries on the problems of the Con- stitution of Matter and the Nature of Electricity is in most intimate connection with the view we take of the processes which go on when electricity passes through a gas.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Britannica Guide to the Atom

  • Britannica Educational Publishing, Erik Gregersen(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  From the magnitude of the electrical and magnetic deflections, Thomson could calculate the ratio of mass to charge for the electrons. This ratio was known for atoms from electrochemical studies. Measuring and comparing it with the number for an atom, he discovered that the mass of the electron was minuscule, merely 1/1,836 that of a hydrogen ion. When scientists realized that an electron was virtually 1,000 times lighter than the smallest atom, they understood how cathode rays could penetrate metal sheets and how electric current could flow through copper wires. In deriving the mass-to-charge ratio, Thomson had calculated the electron’s velocity. It was 1/10 the speed of light, thus amounting to roughly 30,000 km (18,000 miles) per second. Thomson emphasized that

  we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state; a state in which all matter, that is, matter derived from different sources such as hydrogen, oxygen, etc., is of one and the same kind; this matter being the substance from which all the chemical elements are built up

  .

  Thus, the electron was the first subatomic particle identified, the smallest and the fastest bit of matter known at the time.

  MILLIKAN OIL -DROP EXPERIMENT

  In 1909 the American physicist Robert Andrews Millikan greatly improved a method employed by Thomson for directly measuring the electron charge. He began by measuring the course of charged water droplets in an electrical field. The results suggested that the charge on the droplets is a multiple of the elementary electric charge, but the experiment was not accurate enough to be convincing.

  In Millikan’s subsequent experiment, he produced the first direct and compelling measurement of the electric charge of a single electron by measuring the minute electric charge that is present on many of the droplets in an oil mist. The force on any electric charge in an electric field is equal to the product of the charge and the electric field. Millikan was able to measure both the amount of electric force and magnitude of electric field on the tiny charge of an isolated oil droplet and from the data determine the magnitude of the charge itself.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Revolutions in Twentieth-Century Physics

  • David J. Griffiths(Author)
  • 2012(Publication Date)
  • Cambridge University Press

    (Publisher)

  As the name suggests, cathode rays were presumed to be some sort of invisible radiation, like ultraviolet light or X-rays. But in 1897 J. J. Thomson showed that these “rays” could be deflected by electric and magnetic fields, indicating that they carry electric charge. He found that the charge was in fact negative, and he was able to determine the charge-to-mass ratio, e/m. Although he could not measure e or m separately, it was clear that a cathode “ray” was actually a stream of charged particles; they came to be called electrons. 1 The charge of the electron was measured by Robert Millikan, in 1909. It so happens that the tiny droplets of oil produced by an “atomizer” (perfume sprayer) often pick up (or lose) a few electrons. By watching them move up or down in an electric field, Millikan was able to determine the charges of the drops. The smallest charge he got was e D 1.60  10 19 C, (4.1) but some drops had a charge of 2e, or 3e, or e, etc. He concluded that the charge of the electron itself was e, and that the droplets had lost (or gained) one, two, three, . . . of them. Combining Millikan’s e with Thomson’s value for 1 The name itself was introduced by George Stoney, in 1894, to denote the hypothetical smallest unit of electric charge. 100 Elementary particles e/m yields the mass of the electron, m e D 9.11  10 31 kg. (4.2) Thomson guessed (correctly) that electrons are basic constituents of atoms. However, since atoms are electrically neutral, and much heavier than electrons, there had to be some other stuff, with positive charge and most of the mass. He pictured the atom as a sort of “plum pudding,” with electrons embedded (like the plums) in a heavy positive dough. But Thomson’s plum pudding model was decisively repudiated by Rutherford’s scattering experiments. Rutherford and his assistants fired alpha particles (more on these in a moment) into a sheet of gold foil.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to Chemistry

  • Amos Turk(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  10 • ELECTRONIC STRUCTURES OF ATOMS tube and of the materials of which the tube is composed. Also, elec-trons from other sources, such as a hotpathode filament or photoelec-trons, yield the same ratio. This, with the observation that the mass of the electron is about 2 10 3 times smaller than the mass of the lightest atom known, implies that the electron is a universal con-stituent of matter. Originally referred to as rays because they were believed to be light waves, cathode rays would be more accurately named electron beams. Electromagnetic radiation (light), in contrast, is not deflected in electric or magnetic fields. Wilhelm Rdntgen, while performing experiments on the fluorescence produced by cathode rays, discovered in 1895 a radiation that pene-trates glass and metal plates opaque to light and cathode rays. This radiation, named the χ is produced when high-speed electrons strike an object. X rays darken a photographic plate, exhibit the phe-nomena of diffraction, refraction, and reflection, but are not deflected in electric or magnetic fields. They are therefore electromagnetic waves of the same nature as light, radio waves, and ultraviolet radia-tion. Their wavelengths are of the order of 1 A, compared to 4-7 10 3 A for visible light. In addition, they cause luminescence and ionize gases. (The conversion of neutral particles into ions is called 2.3 • THE POSITIVE IONS (POSITIVE RAYS) In the discharge tube, neutral particles—atoms or molecules—are ion-ized into electrons and positive ions. These charged particles acquire kinetic energy under the influence of the electric field. Many of the positive ions collide with the cathode. Evidently, if a perforated cath-ode is used, some positive ions will emerge behind it. Eugen Goldstein performed such an experiment in 1886 and detected what he called the positive rays.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Great Experiments in Physics

  Firsthand Accounts from Galileo to Einstein

  • Morris H. Shamos(Author)
  • 2012(Publication Date)
  • Dover Publications

    (Publisher)

  16

  The Electron

  J. J. Thomson 1856 - 1940

     

  NEW DISCOVERIES in science often lead to developments in the most unexpected quarters. We have seen how Roentgen’s discovery of x-rays led Becquerel to the investigations from which he chanced upon his discovery of radioactivity. The discovery of x-rays led as well to increased activity in the field of gas discharges. The atomic theory of matter was, by that time, firmly established on the strength of chemical evidence and the kinetic theory of gases. The electrical nature of matter was also readily apparent. But the actual connection between electrical and atomic properties was not clearly recognized. No doubt those who investigated the passage of electricity through gases believed that their studies might provide the essential link between these phenomena. At least this was the goal that induced J. J. Thomson to undertake research in gas discharges during the last decade of the nineteenth century. There were others who preceded him, notably J. W. Hittorf (1824–1914), Sir William Crookes (1832–1919), Eugen Goldstein (1850–1930), and Jean Baptiste Perrin (1870–1942), but their investigations added chiefly to the qualitative aspects of gas discharge phenomena.

  The technique was generally the same in each case; a glass tube with platinum electrodes was gradually evacuated while a (large) potential difference was applied between the electrodes. The visible discharge was then studied as a function of pressure, electric field, and nature of the gas. There were several significant advances prior to the discovery of x-rays. It was known, for example, that in these discharges a radiation emanated from the cathode or negative electrode which exhibited certain interesting properties. Perhaps the most significant of these was the green fluorescence that was produced in the glass walls wherever the radiation fell upon them. In 1869 Hittorf showed the rectilinear propagation of the radiation by placing obstacles between the cathode and glass walls and showing that “optical” shadows were formed. This was confirmed several years later by Goldstein, who introduced the name cathode rays

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Cambridge Scientific Minds

  • Peter Harman, Simon Mitton(Authors)
  • 2002(Publication Date)
  • Cambridge University Press

    (Publisher)

  thomson and ernest rutherford 157 publishing the first account of their ionising power, and revising his experimental programme to make the most of a great technical advance. Instead of applying a high voltage to start the discharge and ionise the gas it could now be done without any such fierce shock, and systematic measurements were possible. The way forward was clear, but strangely the discovery for which JJ is chiefly famous owed little to X-rays beyond stimulating his thought. It had been known for decades that in a low-pressure gas dis- charge a radiation was emitted from the negative electrode – the cathode rays – which caused the glass of the tube to fluoresce. The rays propagated in straight lines like light, and cast shadows, but unlike light they were deflected by a magnetic field. They would have been taken for a stream of negatively charged particles but that they were apparently undeflected by an electric field. The general view in Germany was that they were waves, in England that they were indeed particles. JJ (or was it Everett?) managed to obtain a low enough pres- sure, as no one had done before, so that when an electric field was applied it was not neutralised by stray charges in the residual gas. Measurements in 1897 of the electric deflection of the cathode rays clinched the argument for negatively charged particles and showed that they were about a thousand times lighter than a hydrogen atom, the lightest known. At the same time Wiechert and Kaufmann in Germany had both independently reached the same conclusion, but JJ’s special importance lay in his demonstration that the same particle was produced by electrodes of different metals, and in a variety of gases; moreover he and his students extended the work in ways that the others did not, and made it a special task to measure the properties of the new particle they called a corpuscle, clearly a basic constituent of all atoms.

  Sign up to read

  Learn more about book