Cathode Rays & the Discovery of the Electron

A Thumbnail History of Electronics

I. Cathode Rays & the Discovery of the Electron
Although many of the pioneers of 19^th Century physics, including Faraday, were convinced on the basis of chemistry and the phenomena observed in electrolysis that electric current consisted of the flow of particles of charge, the nature of these charges was not understood. Even the basic question of whether the charge of the particles was positive or negative remained undetermined. The answers to these questions, and to the basic structure of matter, were resolved by experiments that began with the study of electric discharges in evacuated tubes. Along the way a series of discoveries were made which led to the technological revolution of the 20^th Century.

William Crookes (1832-1919), heir at an early age to a large fortune, carried out his investigations in a private laboratory. His studies of electrical discharges in gases, which followed the development of the cathode ray tube by Pluecker and Hittorf, and his observations of cathode rays and the dark space at the cathode led to the discovery of x-rays and of the electron. Crookes also invented the radiometer, whose eventual explication verified the kinetic theory of gases. Curiously, Crookes was a believer in the occult and in the 1870’s claimed to have verified the authenticity of psychic phenomena. Later he became involved in the Theosophical Movement and there are references to his having exorcised demons. In 1897 Crookes was knighted by Queen Victoria (who is also reputed to have had an interest in the occult) and in 1909 was elected president of the Royal Society.
Karl Ferdinand Braun (1850-1918) was director of the Physical Institute and a professor of Physics at the University of Strasbourg when he demonstrated the first cathode ray tube oscillograph, guiding a narrow stream of electrons to a fluorescent screen and presaging the modern television screen. Although little remembered today, Braun made several important contributions. He discovered that rectification occurs at a crystal/metal junction, leading to the introduction of crystal receivers. In 1899, he introduced (sparkless) inductive coupling to antennas and the first directive beam antenna. He received the Nobel Prize in 1909 along with Guglielmo Marconi. Braun was in New York to testify in a patent suit when the United States entered World War I; he was interned as an enemy alien and died before the war ended.
Wilhelm Conrad Roentgen (1845 -1923) was 44 years old, head of the Physical Institute and recently retired Rector (President) of the University of Wurzburg when, in November, 1895, he discovered that some unknown radiation coming from a Crookes tube could cause crystals to fluoresce, pass through solid objects, and affect photographic plates. Working alone, sometimes sleeping in his laboratory, and maintaining great secrecy, he completed his research and eight weeks later announced his discovery. The scientific and medical implications of his work were immediately recognized and reported world-wide following its publication on New Year’s Day in 1896. Within a few weeks some hospitals began to use x-rays. Roentgen became one of the most renowned scientists in the world. He received many honors, including the first Nobel prize in Physics and an offer (refused) to be raised to the nobility.
J(oseph) J(ohn) Thomson (1856-1940), the son of a Manchester bookseller, entered college at fourteen and at twenty-eight was elected a fellow of the Royal Society and appointed to the Chair of Physics at the Cavendish Laboratory. His great discovery occurred in 1897 during the course of his investigations of cathode rays. Thomson provided convincing evidence that the rays consisted of charged particles; he measured the ratio of charge to mass and was able to estimate that the mass was equal to about 1/1800 of the mass of a hydrogen atom. His discovery of the electron won the Nobel Prize in 1906 and he was knighted two years later. Thomson was described by Rutherford as having "a most radiating smile, … when he is scoring off anyone."
Robert A. Millikan (1868 -1953) began his career as a classics major at Oberlin College, but agreed to teach Physics in order to earn more money. When he was offered a fellowship in Physics at Columbia he accepted, but again only because it was the best offer he could get financially. His academic career at the University of Chicago was at first devoted to teaching and administration and he did not begin to do research seriously until he was almost forty. Then, in 1906 he began to devise a series of improvements to the Thomson experiment that led to the oil-drop apparatus in which the charge of the electron was measured conclusively. His results were published in 1910 and the last resistance to the atomic theory of matter was dispelled. In 1914 he published the results of the research for which he was awarded the Nobel Prize - the direct determination of Plank’s constant using the photoelectric effect - verifying the 1905 Einstein theory of the photoelectric effect and the quantum nature of light.

Return to "Contents"	Next