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ionic compound, positrons, elementary particles, Broglie, light waves

In the early 19th century, British scientist Michael Faraday explored the phenomenon of electrolysis. Electrolysis involves passing an electric current through a substance, such as an ionic compound dissolved in a solution of water. The current separates the constituent elements of the compound—the positively charged ions collect at the (negative) cathode and the negatively charged ions collect at the (positive) anode. Faraday discovered that the amount of an element formed increased in proportion to the amount of electricity passed through the substance (see Electrochemistry). This suggested that atoms, although themselves electrically neutral, are made up of smaller particles that carry electric charge.

Toward the end of the 19th century, physicists realized that if they applied a high voltage between two electrodes (a cathode and an anode) in a vacuum tube, the cathode would release a discharge. This discharge was called a cathode ray. In 1897 the British physicist Sir Joseph J. Thomson revealed that these rays were made up of tiny particles almost 2,000 times lighter than an atom of hydrogen. Thomson also showed that electric and magnetic fields could move around the particles, thus proving they were electrically charged. These tiny, light, and electrically charged particles were named electrons, and because of his work Thomson is regarded as the discoverer of the electron.

In the 1900s, physicists began to realize that light waves could act like particles, so they wondered whether electrons could act like waves. In 1905 German-born American physicist Albert Einstein showed that light—a form of radiation—sometimes behaves as though it is made of particles of fixed energy. In 1923 French physicist Louis de Broglie suggested that electrons—particles of fixed energy—should also be able to behave like radiation. In 1927 American physicists Clinton Davisson and Lester Germer showed that a beam of electrons passing through a crystal diffracts, or bends, in the same way that light does. This dual particle-radiation behavior is the basis of the electron microscope.

Also in 1927, British physicist Paul A. M. Dirac theorized that electrons must have the property now known as spin. The electron was the first elementary particle to be attributed with spin, now considered to be a general attribute of all elementary particles. Dirac also predicted that electrons should have antiparticles, elementary particles with exactly the same properties as electrons but carrying a positive electric charge. In 1932 American physicist Carl David Anderson discovered these electron antiparticles, called positrons.

In modern physics experiments, scientists carefully prepare and collide speeding beams of electrons and positrons. When the beams meet, electrons and positrons destroy each other, producing bursts of energy. The energy released in these collisions can make many new kinds of elementary particles. Such electron-positron colliders are among the main tools of today's particle physics research.

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