H.F. Newall.
One-time assistant to the young Professor Thomson.
In the latter half of the 19th century, one topic that physicists became deeply interested in was the passage of electricity through gases. It had been known for centuries that gases, although ordinarily electrically insulating, could 'breakdown' under the influence of a sufficiently high voltage. Such breakdowns occur quite normally in everyday life, ranging from the small sparks between a metal door knob and the hand that anyone who walks across a carpeted floor wearing rubber shoes often experiences, to lightning, a large-scale but common phenomenon during thunderstorms. Following the invention of the diffusion pump by Heinrich Geissler (1814-1879) it became possible for the pressure of a gas within a glass tube to be reduced to 1/10,000 of an atmosphere; then, when he sealed wires into the ends of an evacuated Geissler tube, Julius Plücker (1801-1868) found, to his great surprise, that an electrical current flowed through the tube. As a result, research into electrical discharge through gases became of major interest. Among those at the forefront of research during that time were J.W. Hittorf (1824-1914), Sir William Crookes (1832-1919), Eugen Goldstein (1850-1930), Philipp Lenard (1862-1947) and Jean-Baptiste Perrin (1870-1942); each in their own way contributed to the qualitative aspects of gas discharges [1]. For example, Crookes, whose beliefs in spiritualism and the supernatural partially obscured his scientific contributions, discovered that the passage of electricity through gases takes place in a much more controlled way if the pressure of the gas in the tube was reduced to a small fraction of an atmosphere. Soon, in many laboratories, Crookes' tubes were glowing with a quiet light (or discharge) whose color depended on the type of gas. (Today, they are glowing on the streets of cities advertising motels, clubs and a thousand and one other things!) Much effort was expended in studying these discharges - as a function of the pressure, electric and magnetic fields, etc. - and there had been many important advances before the discovery of x-rays. It was known, for instance, that in the discharge some sort of beam of radiation was emitted from the cathode (negative electrode) that had some interesting properties; one of the more significant was the green colored fluorescence that appeared where the beam of radiation fell on the glass. In 1869 Hittorf showed that the radiation traveled in straight lines because placing obstacles in the path cast 'shadows' in the fluorescence. Several years later Goldstein confirmed Hittorf's observations and introduced the term cathode rays to identify the radiation. Goldstein, along with many of his colleagues, believed the radiation to be wave-like. Strong evidence for this belief was found by Lenard, who had discovered that the cathode rays could pass quite easily through various thin screens placed in their way without making holes in them.
argued Lenard. Today, Lenard's argument sounds rather weak but at the time it was a strongly held belief, particularly among German scientists. A contrary view was put forward in England by Crookes, who showed that the radiation could be deflected by a magnet placed near the tube as would be the case if there were a flow of negatively charged particles flying from the cathode. At about the same time in France, Perrin found that a metal plate placed in the way of the beam became negatively charged [2]. So the view in Britain and France was that cathode rays were, in fact, particles, flying through the rarefied gas similar to the way that Faraday's ions moved through liquid electrolytes during electrolysis [3]. A spirited controversy arose between the two schools of thought that occasionally led to conflict over the originality of discoveries. However, the dispute seemingly came to an end when the British physicist J.J. Thomson on the basis of several, carefully conducted experiments, proved conclusively that cathode rays are streams of particles originating near the cathode. He further proposed that these mysterious particles are much smaller than atoms, whose existence had been firmly established through chemical evidence and the kinetic theory of gases. Thomson boldly suggested that these particles were small and that they make up all of the matter in atoms. This idea was highly controversial and startling since the general view was that atoms were indivisible, the fundamental building blocks of all matter. Of course, we now know that Thomson's speculations are generally true; cathode rays are electrons [4], small negatively charged particles that are indeed fundamental parts of every atom but they are not the only constituents of the atom as Thomson had hypothesized.
Joseph John Thomson was born on December 18, 1856 near Manchester, England. In his autobiography, Thomson recalled
His contemporaries were Hertz, Röntgen and Becquerel and they all, together with Thomson, won Nobel prizes, but more importantly, heralded the modern age of physics, which completely refashioned the understanding of physical phenomena.
In 1882 Thomson was appointed a lecturer in mathematics at Trinity and in 1883 he became a university lecturer. In January 1890 he married Rose Paget, one of the first generation of women researchers at the Cavendish Laboratory at Cambridge. They had two children, George Paget (G.P.) Thomson, who became a very prominent physicist himself (as we will see) and won the Nobel Prize, and Joan Paget Thomson. In 1894 he succeeded Lord Rayleigh (1842-1919) as Cavendish professor of experimental physics; for some time prior he had been investigating the properties of electrical discharges in gases, but it was not until 1895 that he was on the right track and in 1897 he discovered the electron.
He comments that:
He found that the ratio m/e is independent of the nature of the gas and the metal used for the cathode, suggesting to him that the particles were constituents of the atoms of all substances. Also, the ratio is at least 1000 times smaller than the value for the hydrogen ion [5] in electrolysis. Thomson remarked that:
By citing Lenard, who had shown that the range of cathode rays in air was much greater than that of molecules, Thomson argued therefore that m was small; the particles must be much smaller than ordinary molecules since they travel so much further than molecules before colliding with air molecules.
Since the electron had to be fitted into the scheme of things, wide avenues for further research and reexamination were immediately opened up; in atomic physics, electricity and magnetism. In all of these areas Thomson played an active and major role. He pioneered the field of mass spectroscopy and in 1913 discovered isotopes; he says:
He calculated also the scattering of x-rays by the electrons bound to atoms from which it appeared that the number of electrons in heavy atoms was roughly one-half of its atomic mass.
Thomson was President of the Royal Society from 1915 to 1920 and during the war years was heavily involved in defense activities for various government agencies. In 1918 he became master of Trinity College but the Cavendish Laboratory continued to be his primary interest throughout his life. He had been a major force in making the Cavendish one of greatest research laboratories in the world and he returned there as often as he could manage. He was appointed professor of Natural Philosophy at the Royal Institution in 1905, received the Nobel Prize in 1906 and was knighted two years later. He resigned his Chairs at the Cavendish in 1919 and at the Royal Institution in 1920 but continued as master of Trinity until his death on August 30, 1940.
Following Thomson's isolation of the electron there were many attempts to determine its properties. He had measured the mass to charge ratio; the next step was to measure their mass or charge separately. However, at the time of Thomson's discovery it had not been established that there was indeed a unique, fundamental charge on the electron. It was the studies of Robert Millikan, beginning in 1907, that produced the next step in establishing the electron as a fundamental particle; not only did he identify that there was a fundamental electrical charge but he measured it accurately also. To explain the connection between matter and electricity, some scientists during the late 19th century argued that there had to be a fundamental unit of electricity. Prior to Thomson's work some estimates had been made of the magnitude of the basic unit of electrical charge. The Irish physicist G.J. Stoney proposed a value in 1881, which was roughly one-tenth of today's value. Better estimates were made by E.O. Meyer and J.S.E. Townsend; the latter was working in Thomson's laboratory but his method, involving the rate of fall of charged clouds of water vapor, was not capable of high precision. In 1903 Thomson and H.A. Wilson carried out similar experiments but with little improvement due principally to the difficulty in reproducing conditions in successive clouds of vapor. So, when Millikan became interested in this problem the value of the fundamental unit of charge was known only within a fairly wide limits. In 1907 he devised a method of studying the motion of a single droplet of water vapor under the action of a vertical electrical and gravitational field. However, because of evaporation of the droplet, even this method achieved only modest accuracy; it wasn't until he used oil-droplets that he was able to carry out reliable and reproducible measurements of the fundamental unit of charge. Millikan's method, which is now known as the oil-drop experiment, established beyond doubt the discreteness of the charge.
With a fully established reputation he turned to other studies and in 1916 he confirmed experimentally Einstein's photoelectric equation thus providing convincing proof of the concept of photons and determining directly the value of Planck's constant.
During World War I he worked for the government in various positions and in 1921 he left Chicago to become director of the newly established Norman Bridge Laboratory of Physics at Pasadena. He also became Chairman of the Executive Council of the California Institute of Technology, a post he held until he retired in 1945. In 1923 Millikan was awarded the Nobel Prize for his studies of the elementary electric charge and the photoelectric effect. He remained active in research until his retirement, particularly in the area of cosmic rays. He died in San Marino, California in 1953.
In the oil-drop experiment, Millikan allowed a single drops of oil that had become charged during their atomization to fall a known distance in air (about 1 cm) and measured the duration of the fall. He then turned on a vertical electric field and measured the time it took for the droplets to travel the same distance upward. These two times allowed him to determine both the mass of the drop and the total charge. During the observations he noted that the charge on the drop sometimes changed spontaneously, by gaining or losing charge from the air. He also induced these changes by using an x-ray or radioactive sources. By measuring the different times it took for the droplets to rise he could calculate the change in charge on the droplets. For example, in the case of one drop, which he observed for a period of four and a-half hours, he found that both the total charge on the drop and the changes in charge were small integral multiples (between 4 and 17) of e, a fundamental unit of charge. After allowing for a small correction that was later found to be necessary, Millikan's value of e was about 0.6% less than today's accepted value.
Millikan associated his measured e both with the charge on Thomson's 'corpuscles' and the hydrogen ion in electrolysis. His value of e with the 'best' m/e measurements from cathode ray experiments therefore provided a value of the mass of the electron, which he found was 1845 times greater than that of the hydrogen atom - very close to today's accepted value of 1837.15.
FOOTNOTES
[1] These scientists used a common technique to study the discharges; a glass tube that was gradually evacuated, with two electrodes - usually of platinum - sealed at the ends of the tube to which was applied a large potential difference (voltage).
[2] Comptes Rendus 121, 1130 (1895).
[3] The difference was that in the case of electrolysis the ions had to make their way through densely packed molecules and would always end up at the anode; the cathode rays in the rarefied gas literally flew in straight lines and hit whatever was in the way.
[4] The word 'electron' was coined in 1891 by G.J. Stoney and had been used to denote the unit of charge found in experiments like those carried out by Michael Faraday (1791-1867), in which an electrical current is passed through an electrolyte (chemicals in solution).
[5] The hydrogen ion is the proton, ehich was not discovered until 1919. Its mass is approximately 1840 times the electron's mass.
[6] There was no strong tradition in the physical sciences at American Universities; Johns Hopkins University, where the tradition first began, had only been established and the American Physical Society had yet to be founded.
REFERENCES
Books
M. Shamos Great Experiments in Physics (Dover Publications, New York - 1987).
A. Franklin Physics Today, 50, 26 (October 1997).
Web sites