J. J. Thomson
Sir Joseph John Thomson was a British physicist. He received the 1906 Nobel Prize in Physics "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." In 1897, he showed that cathode rays were composed of previously unknown negatively charged particles, which he calculated must have bodies much smaller than atoms and a very large charge-to-mass ratio. The electron was the first subatomic particle to be discovered.
Thomson is credited with finding the first evidence for isotopes of a stable element in 1912, as part of his exploration into the composition of canal rays. His experiments to determine the nature of positively charged particles, with Francis William Aston, were the first use of mass spectrometry and led to the development of the mass spectrograph.
Thomson was an influential teacher, and seven of his students went on to win Nobel Prizes: Ernest Rutherford, Lawrence Bragg, Charles Barkla, Francis Aston, Charles Thomson Rees Wilson, Owen Richardson and Edward Appleton. His son, George Paget Thomson, shared the 1937 Nobel Prize in Physics with Clinton Davisson "for their experimental discovery of the diffraction of electrons by crystals".
Biography
Joseph John Thomson was born on 18 December 1856 in Cheetham Hill, Manchester. His mother, Emma Swindells, came from a local textile family. His father, Joseph James Thomson, ran an antiquarian bookshop founded by Thomson's great-grandfather. Joseph John had a brother, Frederick Vernon Thomson, who was two years younger than he was. Thomson was a reserved yet devout Anglican.Education
Thomson's early education was in small private schools where he demonstrated outstanding talent and interest in science. In 1870, he was admitted to Owens College in Manchester at the unusually young age of 14, and came under the influence of Balfour Stewart, Professor of Physics, who initiated him into physical research. He began experimenting with contact electrification and soon published his first scientific paper. His parents planned to enroll him as an apprentice engineer to Sharp, Stewart & Co, a locomotive manufacturer, but these plans were cut short when his father died in 1873.In 1876, Thomson moved on to Trinity College, Cambridge. In 1880, he received his B.A. in mathematics. He applied for and became a Fellow of Trinity College the following year. He obtained an M.A. in 1883.
Career
On 22 December 1884, Thomson was appointed Cavendish Professor of Physics at the University of Cambridge. This appointment caused considerable surprise; candidates such as Osborne Reynolds and Richard Glazebrook were older and more experienced in laboratory work, whereas Thomson was known for his work as a mathematician—being recognised as an exceptional talent.Thomson was knighted in 1908 and appointed to the Order of Merit in 1912. At Oxford, he gave the 1914 Romanes Lecture titled The Atomic Theory. In 1918, he became Master of Trinity College, Cambridge, a position he held until his death on 30 August 1940. His ashes rest in Westminster Abbey, near the graves of Isaac Newton and his former student, Ernest Rutherford.
Rutherford succeeded him as Cavendish Professor. Six of Thomson's research assistants and junior colleagues won the Nobel Prize in Physics, and two won the Nobel Prize in Chemistry. Thomson's son, George Paget Thomson, won the 1937 Nobel Prize in Physics for proving the wave-like properties of electrons.
Research
Early work
Thomson's prize-winning master's work, Treatise on the motion of vortex rings, shows his early interest in atomic structure. In it, Thomson mathematically described the motions of Lord Kelvin's vortex theory of the atom.Thomson published a number of papers addressing both mathematical and experimental issues of electromagnetism. He examined the electromagnetic theory of light of James Clerk Maxwell, introduced the concept of electromagnetic mass of a charged particle, and demonstrated that a moving charged body would apparently increase in mass.
Much of his work in mathematical modelling of chemical processes can be thought of as early computational chemistry. In further work, published in book form as Applications of dynamics to physics and chemistry, Thomson addressed the transformation of energy in mathematical and theoretical terms, suggesting that all energy might be kinetic. His next book, Notes on recent researches in electricity and magnetism, built upon Maxwell's Treatise upon electricity and magnetism, and was sometimes referred to as "the third volume of Maxwell." In it, Thomson emphasized physical methods and experimentation and included extensive figures and diagrams of apparatus, including a number for the passage of electricity through gases. His third book, was a readable introduction to a wide variety of subjects, and achieved considerable popularity as a textbook.
A series of four lectures, given by Thomson on a visit to Princeton University in 1896, were subsequently published as Discharge of electricity through gases. He also presented a series of six lectures at Yale University in 1904.
Discovery of the electron
Several scientists, such as William Prout and Norman Lockyer, had suggested that atoms were built up from a more fundamental unit, but they envisioned this unit to be the size of the smallest atom, hydrogen. Thomson in 1897 was the first to suggest that one of the fundamental units of the atom was more than 1,000 times smaller than an atom, suggesting the subatomic particle now known as the electron. Thomson discovered this through his explorations on the properties of cathode rays. Thomson made his suggestion on 30 April 1897 following his discovery that cathode rays could travel much further through air than expected for an atom-sized particle. He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1,000 times lighter than the hydrogen atom, but also that their mass was the same in whichever type of atom they came from. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles "corpuscles", but later scientists preferred the name electron, which had been suggested by George Johnstone Stoney in 1891, prior to Thomson's discovery.In April 1897, Thomson had only early indications that the cathode rays could be deflected electrically. A month after Thomson's announcement of the corpuscle, he found that he could reliably deflect the rays by an electric field if he evacuated the discharge tube to a very low pressure. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he obtained more robust measurements of the mass-to-charge ratio that confirmed his previous estimates. This became the classic means of measuring the charge-to-mass ratio of the electron. Later in 1899 he measured the charge of the electron to be of.
Thomson believed that the corpuscles emerged from the atoms of the trace gas inside his cathode-ray tubes. He thus concluded that atoms were divisible, and that the corpuscles were their building blocks. In 1904, Thomson suggested a model of the atom, hypothesizing that it was a sphere of positive matter within which electrostatic forces determined the positioning of the corpuscles. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge. In this "plum pudding model", the electrons were seen as embedded in the positive charge like raisins in a plum pudding.
Thomson made the discovery around the same time that Walter Kaufmann and Emil Wiechert discovered the correct mass to charge ratio of these cathode rays.
The name electron was adopted for these particles by the scientific community, mainly due to the advocation by George Francis FitzGerald, Joseph Larmor, and Hendrik Lorentz. The term was originally coined by George Johnstone Stoney in 1891 as a tentative name for the basic unit of electrical charge. For some years Thomson resisted using the word "electron" because he didn't like how some physicists talked of a "positive electron" that was supposed to be the elementary unit of positive charge just as the "negative electron" is the elementary unit of negative charge. Thomson preferred to stick with the word "corpuscle" which he strictly defined as negatively charged. He relented by 1914, using the word "electron" in his book The Atomic Theory. In 1920, Rutherford and his fellows agreed to call the nucleus of the hydrogen ion "proton", establishing a distinct name for the smallest known positively-charged particle of matter.
Isotopes and mass spectrometry
In 1912, as part of his exploration into the composition of the streams of positively charged particles then known as canal rays, Thomson and his research assistant, F. W. Aston, channelled a stream of neon ions through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path. They observed two patches of light on the photographic plate, which suggested two different parabolas of deflection, and concluded that neon is composed of atoms of two different atomic masses, that is to say of two isotopes. This was the first evidence for isotopes of a stable element; Frederick Soddy had previously proposed the existence of isotopes to explain the decay of certain radioactive elements.Thomson's separation of neon isotopes by their mass was the first example of mass spectrometry, which was subsequently improved and developed into a general method by F. W. Aston and by A. J. Dempster.