Millikan's Experiment

10 Key excerpts on "Millikan's Experiment"

• 

  eBook - PDF

  Arthur Haas: Introduction to Theoretical Physics. Volume 2

  • Arthur Haas, T. Verschoyle(Authors)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  Not only theoretical con-siderations, however, have given rise to this fundamental assumption of modern atomic theory, for a direct empirical confirmation is to be found in some experiments of Millikan, which have, moreover, rendered possible a direct and extremely accurate determination of the elementary quantum of electricity. Millikan's Experiments consisted in the observation with a microscope of minute, individual particles of matter carrying feeble electric charges, and moving under the two-fold influence of gravity and of a field directed vertically upwards. 3 B 2 4 THEORETICAL PHYSICS This method of so-called individual observation 1 was devised in 1909 by Ehrenhaft and a little later also by Millikan, and was further developed and perfected in the following years. 2 Millikan worked with oil droplets with a diameter of aboutlO-4 cm. The advantage of oil droplets, as compared with particles of other substances, is that they do not change in size owing to evaporation. By means of a spray-diffuser a fine mist of oil is blown into a chamber, and through a small hole in the bottom of the latter a droplet is from time to time admitted into a condenser placed beneath the chamber, the condenser consisting of two parallel horizontal plates. In consequence of the friction due to the spraying, these droplets are almost always electrically charged. The plates of the condenser can be connected through a commutator to a battery of accumulators giving some 10,000 volts. The particle in the interior of the condenser is illuminated by a strong source of light placed at one side, and it is at the same time observed by a microscope through a window in the condenser ; it then appears as a bright point on a black background, although its true shape cannot be recognized. When the electric field is cut out, a particle of this kind slowly falls with a constant velocity, in consequence of its own weight and the strong frictional resistance.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Atomic Theory

  An Elementary Exposition

  • Arthur Haas, T. Verschoyle(Authors)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  Not only theoretical con-siderations, however, have given rise to this fundamental assumption of modern atomic theory, for a direct empirical confirmation is to be found in some experiments of Millilcan, which have, moreover, rendered possible a direct and extremely accurate determination of the elementary quantum of electricity. Millikan's Experiments consisted in the observation with a microscope of minute, individual particles of matter carrying feeble electric charges, and moving under the two-fold influence of gravity and of a field directed vertically upwards. A.T. 1 B 2 ATOMIC THEORY This method of 60-called individual observation 1 was devised in 1909 by Ehrenhaft and a little later also by Millikan, and was further developed and perfected in the following years. 2 Millikan worked with oil droplets with a diameter of aboutl0-4 cm. The advantage of oil droplets, as compared with particles of other substances, is that they do not change in size owing to evaporation. By means of a spray-diffuser a fine mist of oil is blown into a chamber, and through a small hole in the bottom of the latter a droplet is from time to time admitted into a condenser placed beneath the chamber, the condenser consisting of two parallel horizontal plates. In consequence of the friction due to the spraying, these droplets are almost always electrically charged. The plates of the condenser can be connected through a commutator to a battery of accumulators giving some 10,000 volts. The particle in the interior of the condenser is illuminated by a strong source of light placed at one side, and it is at the same time observed by a microscope through a window in the condenser ; it then appears as a bright point on a black background, although its true shape cannot be recognized. When the electric field is cut out, a particle of this kind slowly falls with a constant velocity, in consequence of its own weight and the strong frictional resistance.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Radioactivity

  History, Science, Vital Uses and Ominous Peril

  • Michael F. L'Annunziata(Author)
  • 2022(Publication Date)
  • Elsevier

    (Publisher)

  He subsequently went to work earnestly to disprove Einstein's concept of the photon as an elementary particle in the photoelectric effect, and after 10 years of effort by 1921 (Millikan, 1914 ; 1916a,b, 1921) all of his research in this direction proved to be a great contribution to physics, because it gave more credence to Einstein's light quantum or photon. Millikan was thus awarded the Nobel Prize in Physics in 1923 for his dual contribution on his measurement of the elementary charge of electricity and the photoelectric effect. The significance of Millikan's work on the charge of the electron was highlighted by Prof. A. Gullstrand, Chairman of the Nobel Committee (Gullstrand, 1923) when he stated: Millikan's aim was to prove that electricity really has the atomic structure, which, on the base of theoretical evidence, it was supposed to have. To prove this, it was necessary to ascertain, not only that electricity, from whatever source it may come, always appears as a unit of charge or as an exact multiple of units, but also that the unit is not a statistical mean, as, for instance, has of late been shown to be the case with atomic weights. In other words, it was necessary to measure the charge of a single ion with such a degree of accuracy as would enable him to ascertain that this charge is always the same, and it was necessary to furnish the same proofs in the case of free electrons. By a brilliant method of investigation and by extraordinary exact experimental technique Millikan reached his goal. To prove the nature and charge of the electron, Millikan and Fletcher devised their famous oil-drop experiment. They devised the apparatus, illustrated in Fig. 2.25, which contains two horizontal metal plates separated by a short distance and joined only by an electric field of about 6000 V/cm

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Flash of the Cathode Rays

  A History of J J Thomson's Electron

  • Per F Dahl(Author)
  • 1997(Publication Date)
  • CRC Press

    (Publisher)

  To make an exact determination of the value of the elementary electrical charge, which is free from all questionable theoretical assumptions and is limited in accuracy only by the accuracy which is attainable in the measurement of the coefficient of viscosity of air. [16-71] Curiously, the paper is devoid of any figure depicting the apparatus (a shortcoming of the earlier Philosophical Magazine paper as though one is provided in a longer follow-up paper under the same title subsequently published in the Physical Review for April1911 [16-72]. We reproduce that figure here. Once again the basic experimental procedure, which differs in principle from the Millikan-Begeman procedure mainly in the method of producing and admitting the charged droplets, merits retelling. Referring to figure 16.1, a mist of oil was blown from the atomizer A into the dust-free chamber C. One or more of the droplets fell through a pinhole P into the space between the plates M N held apart by ebonite posts, and the pinhole was then closed by an electromagnetically operated cover (not shown) in order to shut out air currents. The droplet, once inside the condenser, was illuminated through a small window by 310 The Electronic Charge Revisited, and One More Controversy A c ~s ~ 1 Eorth. Figure 16.1. Millikan's oil drop apparatus of 1910. Oil drops are blown from atomizer A into chamber C, where they fall under gravity through pinhole P into the space between condenser plates MN supported by ebonite posts a. A thin strip of ebonite encloses the space between the plates, pierced by three glass windows. A beam of light enters one window and emerges through the second; the third window is for observing the illuminated drops floating between the plates. B is a battery, and S is a switch for short-circuiting the charged plates. a beam from an arc light, and was observed through another window, appearing as a brilliant star against a black background in the field of view of a cathetometer telescope.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Cavitation and Associated Phenomena

  • Dmitry Biryukov, Denis Gerasimov, Eugeny Yurin, Dmitry A. Biryukov, Denis N. Gerasimov, Eugeny I. Yurin(Authors)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  As usual, poor understanding of the phenomenon does not prevent us from using it in our life. The most famous example is the photocopying Xerox devices; we dwell on a more scientific case— the discovery of the finite electron charge in the Millikan experiments. This experimental setup is now almost a staple of an educational physical laboratory, so we discuss it slightly thoroughly.

  The scheme of such experiments is shown in Figure 8.1.1 . The method, in its rough explanation, is to measure the electric field strength that is required to hold a charged droplet in place: two forces act on the droplet—gravity mg and the electrical force qE; at equilibrium, we may find the charge of the particle from here:

  q =

  m g

  E

  .

  (8.1.1)

  Millikan established that the charge q quantizes—its value is Ne, where e is the elementary charge, N is an integer number that shows how many elementary charges are located on the droplet.

  From a more accurate theoretical background, some corrections to the relation (8.1.1) have been made; these are not crucial for our purposes. Another question is more interesting: from which source do those electrons appear? The experiment goes as follows: we spray a liquid (an oil) into the gap, apply the electric field from the electric power source, regulating E to obtain its value corresponding to equilibrium, until the droplet is suspended in a gas, and then look carefully inside the gap with microscope, trying to estimate the falling speed of the droplet to determine its size (and, consequently, its mass), when the electric power supply is off.

  Usually, the size of the droplet (the droplet that can be suspended, other ones would leave a gap) is less than 1 μm, which corresponds to the mass of about ~ 10−15 kg. If a droplet has only a single elementary charge in it, then the electric field applied to the plates is ~ 105 V/m—this is a rather small electric filed; it corresponds to the voltage of ~ 102 V applied to the gap of ~ 1 mm This field is far below the critical values of electric field strength to provoke an electrical discharge here—of orders of ~ 106-2 V/m. This means that free electrons cannot be formed in the gas,1

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Physics 1922 – 1941

  Including Presentation Speeches and Laureates' Biographies

  • Sam Stuart(Author)
  • 2013(Publication Date)
  • Elsevier

    (Publisher)

  Eager attempts were now made to obtain a more exact value for the unit of charge, but the results were not much better than before - until Millikan took up the problem. Millikan's aim was to prove that electricity really has the atomic structure, which, on the base of theoretical evidence, it was supposed to have. To prove this it was necessary to ascertain, not only that electricity, from whatever source it may come, always appears as a unit of charge or as an exact mul-tiple of units, but also that the unit is not a statistical mean, as, for instance, has of late been shown to be the case with atomic weights. In other words it was necessary to measure the charge of a single ion with such a degree of accuracy as would enable him to ascertain that this charge is always the same, and it was necessary to furnish the same proofs in the case of free electrons. By a brilliant method of investigation and by extraordinarily exact exper-imental technique Millikan reached his goal. In his fundamental experiments he had two horizontal metal plates, one a short distance above the other, and by means of a switch he could join them with the poles of a source of high-tension current or short-circuit them. The air between the plates was ionized by radium that could be screened off. There was a minute pin-hole in the middle of the top plate, and over it he had arranged a spray of oil droplets with a radius of about one thousandth of a millimeter. Sooner or later such an oil droplet must fall through the pin-hole and enter the space between the plates, where it was illuminated in such a way that Millikan could see it in a telescope like a bright star on a black background. In the eyepiece of this telescope were placed three cross-hairs, and Millikan measured the time which the droplet required to pass between them. In this way he measured the velocity of fall, which for such small droplets is only a fraction of a millimeter a second.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Physical Chemistry

  • David Ball(Author)
  • 2014(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Obviously, this negatively charged particle was only a piece of an atom. The implication was that atoms were not indivisible. Experiments by Robert Millikan between 1908 and 1917 established the approxi-mate magnitude of the charge, which was then used with Thomson’s e / m ratio to determine the mass of the electron. In his famous oil drop experiment, diagrammed in Figure 9.8, Millikan and coworkers introduced tiny oil droplets in between charged plates, subjected them to ionizing radiation (X rays), and varied the voltage over the plates to try to electrostatically levitate the drops. Knowing the density of the oil, the voltage difference between the plates, the radius of the droplets, and correct-ing for air buoyancy, Millikan calculated an approximate charge of 4.77 3 10 2 10 electrostatic units (esu) or about 1.601 3 10 2 19 coulombs (C). From the e / m , Millikan was able to calculate the mass of the electron as about 9.36 3 10 2 31 kg , about 1/1800 of the mass of a hydrogen atom. (The modern accepted value for the mass of an electron is 9.109 3 10 2 31 kg .) Because there are negatively charged particles in atoms, there should also be positively charged particles, so that matter would be electrically neutral. The proton, a positively charged particle, was identi-fied by Ernest Rutherford in 1911. Following Rutherford and Marsden’s classic experiments with metal foil scat-tering in 1908, Rutherford proposed the nuclear model for atoms. In the nuclear model the majority of the mass—consisting of the protons and the later-discovered neutrons—is concentrated in a central region called the nucleus, and the smaller electrons revolve around the nucleus at some relatively great distance. The experi-ment and the resulting model are illustrated in Figure 9.9. Although the nuclear atom fit the dramatic results of the experiment, there was a major problem: According to Maxwell’s electromagnetic theory, such an atom shouldn’t be stable.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  On Fact and Fraud

  Cautionary Tales from the Front Lines of Science

  • David Goodstein(Author)
  • 2010(Publication Date)
  • Princeton University Press

    (Publisher)

  These cloud chamber experiments were the starting point of Millikan’s efforts. Working with one of his University of Chicago graduate students, Louis Begeman, he had the idea of applying a much stronger electric field than any used previously. His reasoning was that this would completely halt the falling cloud, allowing him to make a much more precise measurement of the charge on its droplets. To his surprise, when he trained a laboratory telescope (not to be confused with an astronomical instrument) on the cloud in order to observe the drops, he found that applying a powerful field had not stopped the cloud’s descent. Instead, nearly all of the ionized droplets dispersed upward or downward, leaving in view just those few droplets whose electric force came sufficiently close to balancing the effect of gravity to temporarily immobilize them. Millikan quickly realized that measuring the charge on individual ionized droplets was a method far superior to finding the charges on droplets in a cloud.

  It may have been during this period that Millikan’s wife, Greta, attending a social event while Millikan spent one of his many long evenings in the lab, was asked where Robert was. “Oh,” she answered, “He’s probably gone to watch an ion.” “Well,” one of the faculty wives was later overheard to say, “I know we don’t pay our assistant professors very much, but I didn’t think they had to wash and iron!”3

  Unfortunately, with water as the medium, the single-droplet method had a serious flaw: The water evaporated too rapidly to allow accurate measurements. Millikan, Begeman, and a new graduate student named Harvey Fletcher decided to try doing the experiment with some substance that evaporated less rapidly, and Millikan assigned to Fletcher the job of devising a way to use mercury or glycerin or oil. Fletcher chose oil and almost immediately got a crude apparatus working, using tiny droplets of watch oil that could be sprayed out of a perfume atomizer he had bought in a drugstore. When he focused his telescope on the suspended oil droplets, he could see them dancing around as they collided with unseen air molecules, in a textbook demonstration of Brownian motion. This itself was a phenomenon of considerable scientific interest.

  When Fletcher finally persuaded the busy Millikan to look through his telescope at the dancing, suspended droplets of oil, Millikan immediately abandoned all his efforts to pull off the experiment with water and turned his attention to refining the oil-drop method.

  It took a couple of years, but by around 1910 Fletcher and Millikan had produced two results. The first was an accurate measurement (4.774 × 10-10 electrostatic units) of the unit electric charge (called e ), which they determined from observing how rapidly or how slowly the oil drops rose or fell in gravitational and electric fields. The second result was the determination of Ne

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Let There Be Light: The Story Of Light From Atoms To Galaxies (2nd Edition)

  The Story of Light from Atoms to Galaxies

  • Alex Montwill, Ann Breslin(Authors)
  • 2013(Publication Date)
  • ICP

    (Publisher)

  Thomson showed that cathode rays were not waves in the ether but con-sisted of tiny negatively charged particles (which he called corpus-cles ) of mass more than a thousand times smaller than the atom of hydrogen. This marked the beginning of Millikan’s interest in the electron. On 10 April 1902, Millikan married Greta Blanchard, a recent graduate of the University of Chicago. The ceremony took place in the evening — in his autobiography he confesses that he spent time until late that day reading the proofs of his book on mechanics in the office of the publishers and almost missed his own wedding! All ended well, however, and the young couple spent the next seven months touring Europe on their honeymoon. They were to have three sons, Clark, Glen and Max. Millikan’s first experiments to determine the charge of the electron were made in 1906, when he built a 10,000 V battery designed to create an electric field strong enough to balance the force of gravity and hold charged water droplets in a vapour cloud suspended in the air. He soon improved the method by using oil drops and watching individual drops through a low power microscope. If the number of electrons carried by the droplet happened to change as an electric charge fell off, or an extra charge was added, the oil drop would suddenly begin to move. By putting each drop through a series of up and down trips by varying the strength of the electric field and timing each trip accurately, Millikan discovered that the speeds of a given drop always changed by a series of steps which were a multiple of a certain unit. This unit change took place, he concluded, when the drop lost or gained a single electron as a passenger. ( Continued ) Atoms of Light Acting as Particles 401 To get the data on one droplet usually took hours, and Mrs Millikan often had to change social arrangements at the last minute, because her husband had been detained ‘watching a drop’.

  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)

  They concentrated on the corpuscle’s electric charge and concluded that it was close to, and probably the same as, the charge carried by an ion in a salt solution; this charge had earlier been chris- tened electron by Johnstone Stoney, and eventually the name was transferred to the corpuscle. The method used by JJ’s students involved weighing and counting charged water drops; it was difficult and not 158 brian pippard very accurate, but in 1909 Millikan thought of using oil instead of water and achieved impressive results in Chicago with drops that lasted for hours without evaporating. For a long time his value of the electronic charge figured in tables of atomic constants. Once JJ was assured that all atoms contained negative electrons he began to devise detailed models and quickly ran into difficulties. Since atoms are electrically neutral there must be positive charges as well as electrons, but they never appear in anything like the same form as electrons; the positive particles that are observed carry all the mass of an atom and are indivisible. If they are single particles and their mass is electromagnetic they must be even smaller than electrons, for the electromagnetic mass is inversely proportional to the radius. In such a case the atom would be a collection of tiny particles held together by electrostatic attraction. But it had long been known that such a collec- tion cannot be stable if all parts are at rest, while if they are in motion they will radiate energy and fall together. Although JJ’s lectures, at Princeton University in 1903 and at the Royal Institution in 1906, are a little vague about details (characteristic of one who attached less weight to ascertained truth than to the stimulating power of bold ideas) he seems strongly attracted to the model Kelvin had recently published under the title Aepinus atomized and which became known as the currant-bun model.

  Sign up to read

  Learn more about book