J.J. Thomson and the Discovery of the Electron
Most people regard the identification of cathode rays as electrons as J.J. Thomson’s greatest achievement. This discovery opened the field of subatomic physics to experimental investigation and moved science much closer to understanding the inner workings of the atom. But his influence was far wider as it marked the transition from nineteenth to twentieth century physics. He transformed the Cavendish Laboratory into one of he world’s premier research schools of his day. Through his students, several of which would go on to win Nobel Prizes, he would guide the advancement of British physics into the twentieth century.
Joseph John Thomson, or J.J. as he was called, was born in Manchester, England, on December 18, 1856. His father was a third-generation bookseller and wanted his bright young son to be an engineer. While waiting for an engineering apprenticeship to open, the senior Thomson sent J.J. to Owens College at age 14 to study and wait for the apprenticeship. Thomson later recalled, “It was intended that I should be an engineer…It was arranged that I should be apprenticed to Sharp-Stewart & Co., who had a great reputation as makers of locomotives, but they told my father that they had a long waiting list, and it would be some time before I could begin work.” In 1873, two years into his education at Owens, Thomson’s father died, leaving the family in financial distress. J.J.’s younger brother, Fredrick, left school and got a job to help support the family. Since the family could no longer afford the cost of an engineering apprenticeship for young Thomson, he was forced to make his way with scholarships in the two areas in which he excelled: math and physics. At Owens, he published his first scientific paper, “On Contact Electricity of Insulators," an experimental work elucidating a detail of James Clerk Maxwell’s electromagnetic theory.
Cambridge University and the Cavendish Laboratory
Wanting to continue his education in math and science, Thomson won a scholarship to Trinity College, part of Cambridge University, and began there in 1876. He would remain at Trinity in some capacity for the remainder of his life. Thomson graduated second in his class in mathematics in 1880 and was awarded a fellowship to stay at Trinity for graduate work. During this time, he worked in several areas of mathematical physics, concentrating on expanding the work of James Clerk Maxwell in electromagnetics. Thomson’s fellowship thesis was never published; however, he did publish two long papers in the Philosophical Transaction of the Royal Society, and in a book, published in 1888 and titled, Applications of Dynamics to Physics and Chemistry. In 1882, he was elected to an Assistant Lectureship in mathematics. This required a lot of his time in teaching classes, a task he always said he enjoyed. Even with his heavy teaching load, he didn’t ignore his research and started spending some time in the laboratories working with the equipment.
At Cambridge University, the theoretical aspects of science had always been emphasized rather than the practical laboratory work. As a result, the laboratories at Cambridge were behind the other universities in Britain. This all changed in 1870, when the Chancellor of the University, William Cavendish, 7th Duke of Devonshire, provided the money out of his own pocket to build a world class scientific research facility. William Devonshire was the descendant of Henry Cavendish, the eccentric scientist who had been a pioneer of electrical experiments, discovered the composition of water, and measured the gravitational constant. James Maxwell was hired as the first head of the Cavendish Laboratory and set up a facility that would grow to be second to none in the physical sciences in Britain. Upon the untimely death of Maxwell in 1879, Lord Rayleigh was appointed as Maxwell’s successor and became the Cavendish Professor. Rayleigh was in charge of the laboratory during Thomson’s early days at the university.
Cavendish Professor of Experimental Physics
In the fall of 1884, Lord Rayleigh announced that he was resigning the Cavendish Professorship of Experimental Physics, and the university made attempts to lure Lord Kelvin (William Thomson, 1st Baron Kelvin) away from the University of Glasgow. Lord Kelvin was well established and refused the position, thus it was opened up for competition among five men, Thomson being one of them. Much to Thomson’s surprise and that of many others at the laboratory, he was elected to the position. “I felt," he wrote, “like a fisherman who with light tackle had casually cast a line in an unlikely spot and hooked a fish much too heavy for him to land.” His election to the Cavendish Professorship and this leadership of the laboratory was a pivotal point in his life, as almost overnight he was now the leader of British science. Thomson was young at age 28 to be in charge of the laboratory, especially since his experimental work had been light. Luckily, the personnel of the laboratory remained in their positions with the change in leadership, and all went about their normal business while the new professor found his way and set about to build a research laboratory.
A Family Man
With Thomson’s new position there was a large bump in salary and now he was one of the most eligible bachelors in Cambridge. It was not long before he met Rose Paget, one of the daughters of a professor at the university. Rose was four years younger than J.J., had little formal education, but was well read and possessed a love of science. They were married on January 2, 1890, and their house soon became the hub of Cambridge University society. Rose was important for the life of the laboratory, as she held teas and dinners for the students and staff, took an interest in their personal lives, and gave hospitality to the fiancées of the young researchers. As the complexion of the laboratory students and researchers became more international, Rose and J.J. were the “glue” that held various factions in place and kept the work moving forward. The couple had a son, George, born in 1892 and a daughter, Joan, born in 1903. George would follow in his father’s footsteps and become a physicist and go on to continue his father’s work into the nature of the electron. The Thomsons would remain married to each other for the remainder of their days.
Science at the Cavendish Laboratory
Now as the head of the Cavendish, he had a duty to experiment with the added luxury of being able to choose his own course of investigation. Thomson was initially interested in pursuing the theories of his predecessor at the Cavendish, James Maxwell. The phenomena of gas discharge had attracted much attention in the early 1880s due to the work of the British scientist William Crookes and the German physicist Eugen Goldstein. Gaseous discharge is the phenomenon seen when a glass vessel (cathode tube) is filled with gas at low pressure and an electric potential is applied across the electrodes. As the electrical potential is increased across the electrons, the tube will begin to glow, or the glass tube will begin to fluoresce. The phenomenon has been known since the seventeenth century, and today it is the same effect we see in fluorescent light bulbs. Thomson wrote of gaseous discharge: “Preeminent for the beauty and variety of the experiments and for the importance of its results on electrical theories.”
The exact nature of the cathode rays was not known, but there were two schools of thought. The English physicists, like Thomson, believed them to be streams of charged particles, primarily because their path curved in the presence of a magnetic field. The German scientists argued that, since the rays caused gas to fluoresce, they were a form of “ether disturbance” similar to ultraviolet light. The problem was that the cathode rays did not seem to be affected by an electric field, as would be expected by a charged particle. Thomson was able to demonstrate the deflection of the cathode rays by an electric field by using highly-evacuated cathode tubes. Thomson published his first paper on discharge in 1886, titled “Some Experiment on the Electrical Discharge in a Uniform Electric Field, with Some Theoretical Considerations about the Passage of Electricity Through Gases.”
Around 1890, Thomson’s research on gaseous discharges took a new direction with the announcement of the results of the German physicist Heinrich Hertz’s experiment demonstrating the existence of electromagnetic waves in 1888. Thomson was beginning to realize that the cathode rays were discrete charges rather than a mechanism for energy dissipation. By 1895, Thomson’s theory of discharge had evolved; he maintained throughout that gaseous discharge was similar to electrolysis, in that both processes required chemical disassociation. He wrote: “…The relations between matter and electricity is indeed one of the most important problems in the whole range of physics…These relations I speak of are between charges of electricity and matter. The idea of charge need not arise, in fact does not arise as long as we deal with the ether alone.” Thomson was starting to develop a clear mental picture of the nature of an electric charge, that it was related to the chemical nature of the atom.
Discovery of the Electron
Thomson continued to investigate the cathode rays, and he calculated the velocity of the rays by balancing the opposing deflection caused by magnet and electric fields in a cathode ray tube. By knowing the velocity of the cathode rays and using a deflection from one of the fields, he was able to determine the ratio of electric charge (e) to the mass (m) of the cathode rays. He continued this line of experimentation and introduced various gases into the cathode tube and found that the ratio of the charge to mass (e/m) didn’t depend on the type of gas in the tube or the type of metal used in the cathode. He also determined that the cathode rays were about a thousand times lighter than the value already obtained for hydrogen ions. In further investigations, he measured the charge of electricity carried by various negative ions and found it to be the same in gaseous discharge as in electrolysis.
From his work with the cathode tube and comparison with results derived from electrolysis, he was able to conclude that cathode rays were negatively charged particles, fundamental to matter, and much smaller than the smallest known atom. He called these particles “corpuscles.” It would be a few years later before the name “electron” would come into common usage.
Thomson first announced his idea that cathode rays were corpuscles at a Friday evening meeting of the Royal Institution in late April 1897. The suggestion put forth by Thomson that the corpuscles were about one thousand times smaller than the size of the then smallest particle known, the hydrogen atom, caused a stir in the scientific community. Also, the idea that all matter was made up of these small corpuscles was a real change in the view of the inner workings of the atom. The notion of the electron, or the smallest unit of negative charge, was not new; however, Thomson’s assumption that the corpuscle was a fundamental building block of the atom was radical indeed. He is credited with the discovery of the electron since he provided experimental evidence of the existence of this very small fundamental particle—of which all matter consists. His work would not go unnoticed by the world, and in 1906 he was awarded the Nobel Prize in physics "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." Two years later, he was knighted.
Plum Pudding Model of the Atom
Since virtually nothing was known of the structure of the atom, Thomson’s discovery opened up the way for a new understanding of the atom and the new field of subatomic physics. Thomson proposed what has become known as the “plum pudding” model of the atom, in which he speculated that the atom consists of a region of positive charge material that had embedded within it a large number of negative electrons—or the plums in the pudding. In a letter to Rutherford in February 1904, Thomson describes his model of the atom, “I have been working hard for some time at the structure of the atom, regarding the atom as built up of a number of corpuscles in equilibrium or steady motion under their mutual repulsions and a central attraction: it is surprising what a lot of interesting results come out. I really have hope of being able to work out a reasonable theory of chemical combination and my other chemical phenomena.” The reign of the plum pudding model of the atom was short-lived, lasting for only a few years as further investigations revealed weaknesses in the model. The death knell came in 1911 when Thomson’s former student, Ernest Rutherford, a tireless investigator of radioactivity and the inner workings of the atom, proposed a nuclear atom, which is the forerunner of our modern atomic model.
Thomson continued as an active researcher and began following up on Eugen Goldstein’s “canal” or positive rays, which were rays in a discharge tube that streamed backward through a hole cut in the cathode. In 1905, little was known of the positive rays except that they were positively charged and had a charge-to-mass-ratio similar to that of a hydrogen ion. Thomson devised an apparatus that deflected the ion streams by magnetic and electric fields in such a way as to cause ions of different ratios of charge-to-mass to strike different areas of a photographic plate. In 1912, he found that ions of neon gas fell into two different spots on the photographic plate, which seemed to imply that the ions were a mixture of two different types, differing in charge, mass, or both. Fredrick Soddy and Ernest Rutherford had already worked with radioactive isotopes, but here, Thomson had the first indication that stable elements can also exist as isotopes. Thomson’s work would be continued by Francis W. Aston, who would develop the mass spectrometer.
Discovery of The Electron: Cathode Ray Tube Experiment
Teacher and Administrator
When the First World War broke out in 1914, Cambridge University and the Cavendish began to lose students and researchers at a rapid rate as young men went off to war to serve their country. By 1915, the Laboratory was completely turned over for use by the military. Soldiers were housed in the building, and the laboratories were used for making gauges and new military equipment. By that summer, the government had set up a Board of Invention and Research to facilitate the work of scientists in the war. Thomson was one of the board members and spent much of his time smoothing the path between the inventors, producers of the new equipment, and the end user, the military. The most successful new technology that came out of the Laboratory was the development of anti-submarine listening devices. After the war, students returned in droves back to the university to pick up where they left off in their education.
Thomson was a good teacher and took the improvement of science education seriously. He worked diligently to improve science education at both the high school and university levels. As an administrator of the Cavendish Laboratory, he gave his demonstrators and researchers much freedom to pursue their own work. During his tenure, he extended the building twice, once with funds from accumulated laboratory fees and the second time with a generous donation from Lord Rayleigh.
Thomson’s work on the Board of Invention and Research and his role as president of the Royal Society brought him attention from the highest level of the government. He had become the face and voice of British science. When the Master of Trinity College, Cambridge, died in 1917, Thomson was appointed his successor. Unable to run both the laboratory and the college, he retired from the laboratory and was succeeded by one of his best students, Ernest Rutherford. The Thomson family moved into the Trinity Master’s Lodge, where official entertaining became a large part of his role as well as administration of the college. In this position, he promoted research to foster economic benefit to both the college and Great Britain. He became an avid fan of the sports teams and enjoyed attending the football, cricket, and rowing competitions. Thomson continued to dabble in science as an honorary professor until a few years before his death.
He published his memoirs in 1936, titled Recollections and Reflections, just before his eightieth birthday. After that his mind and body started to fail. Sir Joseph John Thomson died on August 30, 1940, and his ashes were buried in Westminster Abbey, near the remains of Sir Isaac Newton and Sir Ernest Rutherford.
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Dahl, Per F. A Flash of the Cathode Rays: A History of J.J. Thomson’s Electron. Institute of Physics Publishing. 1997.
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Navarro, Jaume. A History of the Electron: J.J. and G.P. Thomson. Cambridge University Press. 2012.
West, Doug. Ernest Rutherford: A Short Biography The Father of Nuclear Physics. C&D Publications. 2018.
Questions & Answers
What are the experiments done by Sir George J. Stoney?
Stoney was an Irish physicist (1826-1911). He is most famous for introducing the term electron as the "fundamental unit quantity of electricity". Most of his work was theoretical. He published seventy-five scientific papers in a variety of journals and made significant contributions to cosmic physics and to the theory of gases.