Development of atomic scheme » Studies of the properties of atoms » Discovery of electrons
Cathode-ray studies started in 1854 when Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker, improved the vacuum tube. Plücker exposed cathode rays in 1858 by sealing two electrodes inside the tube, evacuating the air, and forcing electric current linking the electrodes. He found a green glow on the wall of his schooner tube and attributed it to rays emanating from the cathode. In 1869, with better vacuums, Plücker’s pupil Johann W. Hittorf saw a shadow cast by an object placed in front of the cathode. The shadow proved that the cathode rays originated from the cathode. The English physicist and chemist William Crookes investigated cathode rays in 1879 and found that they were bent by a attractive field; the direction of deflection suggested that they were negatively charged particles. As the luminescence did not depend on what gas had been in the vacuum or what metal the electrodes were made of, he surmised that the rays were a property of the electric current itself. As a result of Crookes’s work, cathode rays were widely considered, and the tubes came to be called Crookes tubes.

Thomson repeated Hertz’s conduct experiment with a better vacuum in 1897. He directed the cathode rays linking two parallel aluminum plates to the end of a tube where they were observed as luminescence on the schooner. When the top aluminum plate was negative, the rays went down; when the upper plate was positive, the rays went up. The deflection was proportional to the difference in potential linking the plates. With both attractive and electric deflections observed, it was clear that cathode rays were negatively charged particles. Thomson’s discovery established the particulate nature of electricity. Accordingly, he called his particles electrons.

From the magnitude of the electrical and attractive deflections, Thomson could calculate the ratio of mass to charge for the electrons. This ratio was known for atoms from electrochemical studies. Measuring and comparing it with the number for an atom, he exposed that the mass of the electron was very small, merely 1/1,836 that of a hydrogen ion. When scientists realized that an electron was virtually 1,000 times lighter than the smallest atom, they understood how cathode rays could penetrate metal sheets and how electric current could flow through copper wires. In deriving the mass-to-charge ratio, Thomson had calculated the electron’s velocity. It was 1/10 the speed of set alight, thus amounting to roughly 30,000 km (18,000 miles) per second.

Thus, the electron was the first subatomic particle identified, the smallest and the fastest bit of matter known at the time.