(1856-1940) British Experimentalist (Classical Electromagnetism), Atomic Physicist
J. J. Thomson was a brilliant experimentalist whose exploration of the nature of cathode rays led him to the discovery of the electron. He was awarded the 1906 Nobel Prize in physics for his research on the conduction of electricity through gases.
Born in Manchester, England, on December 18, 1856, Thomson became a physicist by default. Intending to become an engineer, he enrolled at Owens College (later to become Manchester University) at the age of 14. But when his father, a seller of antique books, died two years later, he found himself without the means to finance an engineering apprenticeship. Instead, he managed to win a scholarship to study mathematics, physics, and chemistry and, in 1876, entered Trinity College, Cambridge.
After graduating in mathematics in 1880, he worked at the Cavendish Laboratory under lord rayleigh (john william strutt), whom he succeeded as Cavendish Professor of Experimental Physics in 1884. Over the next 35 years he would develop the Cavendish Laboratory into the world’s leading center for subatomic physics. It was at the Cavendish that Thomson, applying his genius for designing apparatus turning to the mysteries of atomic structure, made his breakthrough discoveries.
His Treatise on the Motion of Vortex Rings, which won him the Adams Prize in 1884, approached the subject from the point of view, espoused by many of Thomson’s peers, that atoms exist in vortex rings within a hypothetical “ether,” which was thought to permeate all of space. From there he turned his attention to the current debate over the nature of cathode rays, that is, the phenomenon that produced a fluorescent glow when most of the air was pumped out of a glass tube with wires embedded at each end and a high voltage was sent across it. Physicists asked whether cathode rays were charged particles or some undefined wavelike process in the ether. When Thomson performed his first experiment, prior evidence seemed to favor waves. The German physicist heinrich rudolf hertz had apparently shown that cathode rays were not deflected by an electric field; this indicated that they did not possess electric charge and, therefore, did not have a particle nature. In 1897, Thomson invalidated Hertz’s results, showing that they were caused by his use of an insufficiently evacuated cathode ray tube. When Thomson used a new technique to try to bend cathode rays with an electric field, extracting nearly all the gas from a tube, he succeeded where Hertz had failed.
He then sought to determine the basic properties of the cathode ray particles. Although he lacked the means to measure the mass or electric charge of such particles, he could measure how much the rays were bent by a magnetic field and how much energy they carried. From these data he could calculate the ratio of the electric charge of a particle to its mass. He collected data by using a variety of tubes as well as different gases. Whatever gas he used, he found that the ratio of the charge divided by the mass of the cathode ray particles was constant and had a value nearly 1000 times larger than that of a charged hydrogen atom. These results led Thomson to a logical fork in the road: either the cathode rays carried an enormous charge, as compared with a charged atom, or else they were extraordinarily light relative to their charge. The question was settled by philipp von lenard in experiments on how cathode rays penetrate gases. He showed that if cathode rays were particles they had to have a mass much smaller than the mass of any atom. Subsequent experiments by robert andrews millikan measured the charge directly and confirmed Lenard’s conclusions.
Joseph John (J. J.) Thomson’s exploration of the nature of cathode rays led him to the discovery of the electron.
At a historic meeting of the Royal Institution in 1897 Thomson announced his results. Having shown that cathode rays were fundamental, negatively charged particles with a mass much less than that of the lightest atom known, he went on to suggest that these material particles were the building blocks of the atom, the basic unit of all matter in the universe that physicists had long been seeking. For Thomson, however, this discovery only led to another, deeper mystery:
I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter… [but] what are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?
What Thomson called “corpuscles” or “particles” would later be given the name electrons, and physicists would devise numerous theories to explain how they combined to form atoms. Thomson himself suggested a model for the atom called the “plum pudding” or “raisin cake model,” in which thousands of tiny, negatively charged corpuscles move inside a cloud of positive charge. Later on, using alpha particles (a different kind of particle beam), Thomson’s former student ernest rutherford disproved Thomson’s model, replacing it with his solar system model of the atom: a massive, positively charged center circled by only a few electrons.
After investigating the nature and properties of electrons for several more years, Thomson began researching “canal rays,” streams of positively charged ions (i.e., atoms that have lost one of their negatively charged electrons), which he named positive rays. In the process he learned how to use positive rays for separating different kinds of atoms and molecules. His technique led to the development of the mass spectroscope, an instrument that measures the charge-to-mass ratio. Using magnetic and electric fields to deflect these rays, Thomson found, in 1912, that ions of neon gas were deflected by different amounts, indicating that they consist of a mixture of ions with different charge-to-mass ratios. He confirmed the existence of isotopes, earlier proposed by the British chemist Frederick Soddy, when in that same year he identified the isotope neon 22. Later many more isotopes would be discovered.
In 1919, Thomson resigned his post at the Cavendish Laboratory, after being elected Master of Trinity College. He would remain at Trinity until his death on August 30, 1940, at the age of 84. As an educator he devoted much attention to the problems of teaching science at secondary and university levels. Many universities used his prolific writings on electricity, magnetism, and other topics. His profound personal impact on his students is reflected in the fact that seven of them—including his own son, George—went on to win the Nobel Prize in physics.
Most importantly, however, through his discovery of the first known elementary particle, the electron, Thomson initiated a period of exploration that would lead 20th-century physicists to uncover a new universe of the infinitesi-mally small.
