A fine mist of oil was sprayed into a chamber above the plates. Millikan set up a pair of parallel conducting plates horizontally, one above the other, with a large electric field between them that could be adjusted. Millikan’s student Harvey Fletcher played an important role in implementing this improvement. In Millikan’s apparatus, the water drops would have quickly evaporated, whereas individual oil drops could be studied for a long time. One of the great improvements was the use of oil drops instead of the cloud of water drops that Thomson used. In 1906, Millikan began experiments at the University of Chicago to attempt to measure individual electron charges, and with much greater accuracy than Thomson and co-workers had been able to achieve. This was consistent with his hypothesis that the charge on the electron was the same as that found in hydrogen. Measuring the charge on clouds of water droplets in a cloud chamber, he and his collaborators were able to determine that the charge on the electron, or at least the average charge on the electrons in a cloud, was roughly 10 -19 Coulombs (the Coulomb is the unit of charge in the metric system). Thomson believed this was because the charge was the same, but the mass was some 1700 times smaller. By bending electrons in electric and magnetic fields, investigators could tell that they were negatively charged, and that the ratio of charge to mass, e/m, was the same for all electrons, and about 1700 times larger than that for the ionized hydrogen atom.
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Thomson of Cambridge University, and now called electrons. He and most physicists at the time believed that charge, like mass, was an infinitely divisible quantity.īut in 1897, it was realized that cathode rays were in fact tiny charged particles, dubbed “corpuscles” by their discoverer, J. Faraday’s laws of electrolysis, which were discovered around 1840, provided strong evidence of the quantization of charge, but Faraday never supported the idea. The prevailing theory in the late 19 th century of how charge was produced, held that charge was a type of “strain on the ether,” something that could grow or shrink without restrictions. Millikan received the Nobel Prize in 1923 in recognition of two major achievements: measuring the charge of the electron in his famous oil-drop experiment (see “ This Month in Physics History,” APS News, August/September 2006), and verifying Einstein's prediction of the relationship between light frequency and electron energy in the photoelectric effect, a phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light. Pierre Curie carried a vial of radium in his coat pocket to demonstrate its greenish glow, a habit that caused him to become ill from radiation poisoning well before he was run over by a horse-drawn wagon and killed instantly in 1906.On November 30, 2006, as part of its Historic Sites initiative, the American Physical Society presented a plaque to the University of Chicago, to honor Robert A. Starting with several tons of pitchblende, the Curies isolated two new radioactive elements after months of work: polonium, which was named for Marie’s native Poland, and radium, which was named for its intense radioactivity. She found that one particular uranium ore, pitchblende, was substantially more radioactive than most, which suggested that it contained one or more highly radioactive impurities. Marie Curie coined the term radioactivity (from the Latin radius, meaning “ray”) to describe the emission of energy rays by matter. Becquerel’s work was greatly extended by Marie Curie (1867–1934) and her husband, Pierre (1854–1906) all three shared the Nobel Prize in Physics in 1903. The second line of investigation began in 1896, when the French physicist Henri Becquerel (1852–1908) discovered that certain minerals, such as uranium salts, emitted a new form of energy. With this information and Thomson’s mass-to-charge ratio, Millikan determined the mass of an electron: Subsequently, the American scientist Robert Millikan (1868–1953) carried out a series of experiments using electrically charged oil droplets, which allowed him to calculate the charge on a single electron. Another set of electrode plates deflect the ray, with the ray bending towards the positive plate. Schematic of cathode ray tube with deflection. Image used with Permission (CC BY-SA-NC).
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As the cathode rays travel toward the right, they are deflected toward the positive electrode (+), demonstrating that they are negatively charged. \): Deflection of Cathode Rays by an Electric Field.