Sir Joseph John Thomson
1856-1940
English Physicist
As the director of the Cavendish Laboratory at Cambridge University, J. J. Thomson was instrumental in many important experiments and advances that marked the transition from classical to modern physics at the turn of the twentieth century. He discovered the electron in 1897 and was awarded a Nobel Prize for Physics in 1906. With his student Ernest Rutherford (1871-1937), also a Nobel laureate, Thomson made many fundamental discoveries concerning the properties of ionizing radiation.
Thomson was born in 1856 in the north of England. He studied physics at Cambridge and, at only 28 years old, was greatly surprised to be simultaneously named director of the Cavendish Laboratory at Cambridge and professor of experimental physics. By his own description, the laboratory he took over used "string and sealing wax" as its equipment, presenting him with a considerable challenge. Under his supervision Cavendish became one of the world's preeminent nuclear physics laboratories.
Just three years later, in 1897, Thomson successfully demonstrated that cathode rays are streams of electrons. radiation. Further research in this area allowed Thomson to determine the mass of the electron to be less than one two-thousandth that of the hydrogen atom, until then the lightest bit of matter known to exist. This led to the realization that electrons are subatomic—units of matter smaller than the atom itself. These same experiments also showed that electrons were negatively charged.
Previous research using magnetic fields had shown that cathode rays (electrons) could be deflected by magnetic fields. This is the principle underlying the operation of televisions, computer monitors, and other CRTs (cathode ray tubes). However, Thomson was the first to place cathode rays in an electrical field by bringing oppositely-charged electric plates next to a beam of cathode rays. When he applied electric current to the plates, the beam deflected towards the positively-charged plate, indicating that the rays had a negative charge. Further experiments in both electric and magnetic fields showed the ratio of mass between hydrogen atoms (protons) and the cathode rays (electrons). For this work, Thomson was awarded the Nobel Prize in 1906.
Additional research by Thomson showed that the interaction between electrons and matter could produce x-rays and that, conversely, xrays interacting with matter could produce electrons. Thomson also developed the first modern atomic model, albeit one that did not stand the test of time. In his "plum pudding" model, Thomson envisaged a sphere of positive changes with an equal number of negative charges (electrons) embedded within. This model was later supplanted by a number of others, leading eventually to the current model in which a cloud of electrons forms a shell surrounding a nucleus comprised of both protons and neutrons.
It may be argued that Thomson's mentoring of Rutherford was as important as his own discoveries in physics. Rutherford, who also went on towin the Nobel Prize for chemistry, was a pivotal figure in the formation of contemporary physics. Among his major discoveries, he proposed that radioactivity results from the disintegration of radioactive atoms, facilitated the development of today's model of atomic structure, and conducted other groundbreaking research into the nature of matter.
Thomson married and his son, George, also went into physics. George Thomson (1892-1975) was awarded the Nobel Prize in 1937 for research in electron diffraction by crystals. It is interesting to note that the work conducted by J.J. Thomson was based on the material properties of the electron while his son's work depended on the wave-like properties of electrons. The elder Thomson was knighted in 1908, and died in 1940 at the age of 84. He was buried near Isaac Newton (1642-1727) in Westminster Abbey in London.
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