Robert Millikan (1868-1953) won the 1923 Nobel Prize in physics for his work on the elementary electric charge and on the photoelectric effect. He also did much work on cosmic rays, which he named. He is seen here (right) in his basement with his assistant and his self-recording electroscope. Under Millikan’s leadership the California Institute of Technology quickly developed into one of the foremost scientific centers in the world. (The Bettmann Archive)
Millikan also found that a charge always appears to be in exact integer multiples of plus or minus e; in other words, the charge is quantized. Other elementary particles discovered later were also found to have a charge of plus or minus e. For example, the positron, discovered in 1932 by Carl David Anderson of the California Institute of Technology, is exactly the same as the electron, except that it has a charge of +e.
Atomic Structure
Bulk matter is normally neutral. The tendency is for every positive proton in an atom to be electrically balanced against a negative electron, and the sum is as close to zero as anyone has been able to measure. In 1911, Ernest Rutherford proposed the nuclear atom. He suggested that electrons orbit a positively charged nucleus less than 1/100,000,000,000,000 meters in diameter, just as planets orbit the Sun. Rutherford also suggested that the nucleus is composed of protons, each having a charge +e.
This view of matter, still considered correct in many ways, established the electrical force as that which holds an atom together. After Rutherford presented his atom, the Danish physicist Niels Bohr proposed that the electrons have only certain orbits about the nucleus, that other orbits are impossible.
Quantum Theory
Early in the 20th century the quantum theory was developed. According to this theory, the electron is a smeared cloud of mass and charge. In some situations the electron cloud might be so small that the particle appears to be much like the tiny, charged marble of earlier views. In other situations, such as when the electron is in an atomic orbit, the cloud is many times larger.
In 1963, Murray Gell-Mann and George Zweig of the California Institute of Technology proposed a theory according to which the electronic charge e might not be the fundamental charge after all. In their theory, heavy particles such as protons and neutrons consist of various combinations of particles called quarks. One quark is supposed to have charge (-1/3)e and another (-2/3)e. This theory has prompted a major search for quarks.
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