Thomson, Sir Joseph John (1856-1940)

was the British physicist who discovered the electron, a fundamental atomic particle. Thomson received the 1906 Nobel Prize in physics for this work. In the middle 1890's, Thomson conducted a series of experiments on cathode rays. These rays are produced in a vacuum tube equipped with a positive terminal (anode) and a negative terminal (cathode). The cathode rays result when high-voltage electrical current is supplied to the cathode. Thomson believed that cathode rays were actually streams of tiny charged particles. He devised experiments to deflect, or bend, the cathode rays from their normal path in the tube. Then he worked out mathematical calculations on the experimental data. Thomson concluded that the rays were indeed composed of tiny charged particles, which he named "corpuscles." Later, the name "electron" was adopted. Thomson demonstrated that the particles are negatively charged, can generate heat, and have very little mass-about 1,000 times less mass than a hydrogen ion (proton), in fact. Furthermore Thomson showed through repeated experiments that electrons are present in many chemical elements. And he theorized (correctly) that electrons are a fundamental part of all matter
Thomson's discovery revolutionized scientific understanding of the atom. Before Thomson, most scientists had believed that the atom was indivisible, the smallest particle of matter that could exist His work proved that the atom could indeed be broken down into smaller particles. For this reason, Thomson can be considered the founder of modern atomic physics. Based on his work with electrons, Thomson proposed an atomic model. He suggested that the atom is a sphere (a round ball) with the electrons embedded throughout its inner volume. According to Thomson's model, the interior of an atom would resemble a watermelon with embedded seeds. Within the following 20 years, however, physicists Ernest Rutherford and Niels Bohr would develop a more workable atomic model.
Thomson discovered the first isotopes of a chemical element, specifically of the element neon. An isotope is a form of a chemical element that has a different atomic weight than the element in its normal form. Later, a pupil of Thomson, Francis Aston, invented the mass spectrograph. This device separates atoms of differing atomic weights in a substance. In 1903 Thomson proposed a discontinuous theory of light. By this, Thomson meant that light rays are composed of separate particles rather than continuous streams. Several years later, Einstein developed the photon theory of light This theory proposes that light is made up of packets of energy called photon£ Thomson had a great impact on the field of atomic physics as a teacher. He was director of the Cavendish Laboratory at Cambridge University for the eventful years in the 1890's and early 1900's. During this period, modern atomic physics came into being.