Rutherford

Ernest Rutherford (1871-1937)

Ernest Rutherford, Baron Rutherford of Nelson, nuclear physicist and Nobel Prize winner, is to be ranked in fame with Sir Isaac Newton and Michael Faraday. Indeed, just as Faraday is called the “father of electricity,” so a similar description might be applied to Rutherford in relation to nuclear energy. He contributed substantially to the understanding of the disintegration and transmutation of the radioactive elements, discovered and named the particles expelled from radium, identified the alpha particle as a helium atom and with its aid evolved the nuclear theory of atomic structure, and used that particle to produce the first artificial disintegration of elements. In the universities of McGill, Manchester, and Cambridge he led and inspired two generations of physicists who–to use his own words–“turned out the facts of Nature,” and in the Cavendish Laboratory his “boys” discovered the neutron and artificial disintegration by accelerated particles. (see also Index: nuclear physics)

.. A scholarship allowed him to enroll in Canterbury College, Christchurch, from where he graduated with a B.A. in 1892 and an M.A. in 1893 with first-class honours in mathematics and physics. Financing himself by part-time teaching, he stayed for a fifth year to do research in physics, studying the properties of iron in high-frequency alternating magnetic fields. He found that he could detect the electromagnetic waves–wireless waves–newly discovered by the German physicist Heinrich Hertz, even after they had passed through brick walls.

On his arrival in Cambridge in 1895, Rutherford began to work under J.J. Thomson, professor of experimental physics at the university’s Cavendish Laboratory.

Rutherford made a great impression on colleagues in the Cavendish Laboratory, and Thomson held him in high esteem. He also aroused jealousies in the more conservative members of the Cavendish fraternity, as is clear from his letters to Mary. In December 1895, when Röntgen discovered X rays, Thomson asked Rutherford to join him in a study of the effects of passing a beam of X rays through a gas. They discovered that the X rays produced large quantities of electrically charged particles, or carriers of positive and negative electricity, and that these carriers, or ionized atoms, recombined to form neutral molecules. Working on his own, Rutherford then devised a technique for measuring the velocity and rate of recombination of these positive and negative ions. The published papers on this subject remain classics to the present day.

In 1896 the French physicist Henri Becquerel discovered that uranium emitted rays that could fog a photographic plate as did X rays. Rutherford soon showed that they also ionized air but that they were different from X rays, consisting of two distinct types of radiation. He named them alpha rays, highly powerful in producing ionization but easily absorbed, and beta rays, which produced less radiation but had more penetrating ability. He thought they must be extremely minute particles of matter.

Toward the end of the 19th century many scientists thought that no new advances in physics remained to be made. Yet within three years Rutherford succeeded in marking out an entirely new branch of physics called radioactivity. He soon discovered that thorium or its compounds disintegrated into a gas that in turn disintegrated into an unknown “active deposit,” likewise radioactive. Rutherford and a young chemist, Frederick Soddy, then investigated three groups of radioactive elements–radium, thorium, and actinium. They concluded in 1902 that radioactivity was a process in which atoms of one element spontaneously disintegrated into atoms of an entirely different element, which also remained radioactive.

Rutherford’s outstanding work won him recognition by the Royal Society, which elected him a fellow in 1903 and awarded him the Rumford medal in 1904. In his book Radio-activity he summarized in 1904 the results of research in that subject. The evidence he marshaled for radioactivity was that it is unaffected by external conditions, such as temperature and chemical change; that more heat is produced than in an ordinary chemical reaction; that new types of matter are produced at a rate in equilibrium with the rate of decay; and that the new products possess distinct chemical properties.

With the ingenious apparatus that he and his research assistant, Hans Geiger, had invented, they counted the particles as they were emitted one by one from a known amount of radium; and they also measured the total charge collected from which the charge on each particle could be detected. Combining this result with the rate of production of helium from radium, determined by Rutherford and the American chemist Bertram Borden Boltwood, Rutherford was able to deduce Avogadro’s number (the constant number of molecules in the molecular weight in grams of any substance) in the most direct manner conceivable. With his student Thomas D. Royds he proved in 1908 that the alpha particle really is a helium atom.

In 1911 Rutherford made his greatest contribution to science with his nuclear theory of the atom. He had observed in Montreal that fast-moving alpha particles on passing through thin plates of mica produced diffuse images on photographic plates, whereas a sharp image was produced when there was no obstruction to the passage of the rays. He considered that the particles must be deflected through small angles as they passed close to atoms of the mica, but calculation showed that an electric field of 100,000,000 volts per centimetre was necessary to deflect such particles traveling at 20,000 kilometres per second, a most astonishing conclusion. This phenomenon of scattering was found in the counting experiments with Geiger; Rutherford suggested to Geiger and another student, Ernest Marsden, that it would be of interest to examine whether any particles were scattered backward–i.e., deflected through an angle of more than 90 degrees. To their astonishment, a few particles in every 10,000 were indeed so scattered, emerging from the same side of a gold foil as that on which they had entered. After a number of calculations, Rutherford came to the conclusion that the requisite intense electric field to cause such a large deflection could occur only if all the positive charge in the atom, and therefore almost all the mass, were concentrated on a very small central nucleus some 10,000 times smaller in diameter than that of the entire atom. The positive charge on the nucleus would therefore be balanced by an equal charge on all the electrons distributed somehow around the nucleus.

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“Ernest Rutherford”
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[Accessed 10 May 1998](T.E.A./Ed.)

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