J.J. Thomson

J.J. Thomson the British physicist (1856-1940) who discovered the electron in 1897 and thus demonstrated that the atom is divisible, overturning the long-held belief that atoms were indivisible units. He received the 1906 Nobel Prize in Physics and trained a remarkable generation of physicists, several of whom themselves won Nobel Prizes.

Early Life and Education

Joseph John Thomson was born on 18 December 1856 near Manchester, the son of a bookseller. He studied engineering at Owens College before moving to Trinity College, Cambridge, where he excelled in mathematics and was elected a fellow in 1880. In 1884, at the unusually young age of twenty-eight, he was appointed Cavendish Professor of Experimental Physics, succeeding Lord Rayleigh and following in the chair once held by Maxwell.

At the Cavendish Laboratory Thomson built a school of experimental physics that became the most productive in the world. He was an inspiring teacher with a genius for attracting able students and for setting them problems at the frontier of the subject. His own early research treated the discharge of electricity through gases, a field full of puzzles that would lead him to his great discovery. He also wrote one of the first comprehensive textbooks on the conduction of electricity through gases, a work that guided the next generation of researchers.

Discovery of the Electron

In the 1890s physicists studied the mysterious rays produced in evacuated glass tubes when a high voltage was applied, known as cathode rays. Some held that they were a form of wave in the ether, others that they were streams of particles. Thomson settled the question by measuring how the rays were deflected by electric and magnetic fields. His careful apparatus allowed him to apply the fields separately and in combination, and to determine both the velocity of the rays and the ratio of charge to mass of their constituents.

By comparing the deflection with the known fields, Thomson determined the ratio of the electric charge of the particles to their mass. The ratio was about a thousand times larger than that of a hydrogen ion, the lightest known charged particle. He concluded that the carriers of the cathode rays were negatively charged particles far smaller than any atom, common to all substances, and he called them corpuscles; they are now known as electrons. Their universality suggested that they were a fundamental constituent of all matter, whatever its source.

Isotopes and the Atomic Model

Thomson went on to study the conduction of electricity through gases and, using a device he called the parabola method, showed that atoms of the same element could have different masses. This work, extended by his assistant Francis Aston, established the existence of isotopes and laid the groundwork for the mass spectrograph. Thomson was one of the first to suggest that the chemical elements might be built from a small number of primordial constituents. His analysis of positive rays, the streams of ionised atoms produced in discharge tubes, opened a new window on the composition of matter.

To account for the atom's overall neutrality, Thomson proposed a model in which negatively charged electrons were embedded in a sphere of positive charge, popularly likened to plums in a pudding. The model was later displaced by Ernest Rutherford's nuclear atom, but it represented an important first attempt to picture the internal structure of the atom and to explain the phenomena of radioactivity.

Teaching and the Cavendish School

Thomson's influence as a teacher was almost as consequential as his research. Seven of his research students and assistants, including Rutherford, Aston, Charles Wilson and Lawrence Bragg, went on to win Nobel Prizes, and the Cavendish Laboratory under his direction became a byword for excellence in experimental physics. He encouraged his students to think independently and to pursue their own lines of inquiry, and he was generous in crediting their contributions.

He served as president of the Royal Society and, after resigning the Cavendish chair in 1918, as master of Trinity College, a post he held until his death. He was knighted in 1908 and received the Order of Merit in 1912. He continued to write and lecture on physics and on the philosophy of science in his later years, and he presided over the Cavendish during a period in which it produced an extraordinary succession of fundamental discoveries.

Influence and Legacy

The discovery of the electron was one of the decisive events that opened the modern era of physics. It showed that the atom has a structure and a history, and it led directly to the understanding of electricity in metals, of chemical bonding and of the processes of ionisation. The concept of the electron underlies all of chemistry and much of technology, from the vacuum tube to the transistor and the whole of electronics.

Thomson's recognition that particles could exist that were smaller than atoms also helped to shift physics away from the assumption that matter is ultimately simple and indivisible. His laboratory became the training ground for the physicists who built atomic and nuclear physics in the twentieth century, and his own career embodied the union of precise experiment with bold theoretical imagination. He died at Cambridge on 30 August 1940, honoured as the man who had first shown that the atom could be divided. His son George Paget Thomson later won a Nobel Prize for demonstrating the wave nature of the electron.

See also

• Electron
• Cathode rays
• Isotope
• Cavendish Laboratory