Thomson Cathode Rays

Remembering the Work of the Man Who Discovered The Electron & Changed The Course of History.

British physicist Sir Joseph John Thomson was born exactly 164 years ago, in 1856, and is today credited with discovering the first subatomic particle: the electron, which he under-covered in 1897. Other notable discoveries include the discovery of isotopes as well as the first experimental use of mass spectrometry. Also, Thomson was awarded the Nobel Prize in physics in 1906 for his groundbreaking work on electrical conduction in gases.

Early life and work

After moving on from an engineering apprenticeship at a local locomotive manufacturer – Sharp-Stewart & Co – at his father’s death, Thomson proceeded to obtain a bachelor’s degree in mathematics from Trinity College at Cambridge in 1880 and a prize-winning Master’s in 1883 on “the motion of vortex rings,” describing the motions of the vortex theory of atoms. In 1884, he was appointed Cavendish Professor of Physics at the said university for his exceptional talent despite limited laboratory work experience.

J.J.Thomson
J.J. Thomson

Having proved an avid interest in atomic structure, Thomson continued to write several research papers on various topics relating to the field, such as electromagnetism. Amongst others, he wrote about Maxwell’s theory of light, introducing the electromagnetic mass of a charged particle, as well as published three books from 1888 to 1895: Applications of dynamics to physics and chemistry (presenting a mathematical description of the transformation of energy), Notes on recent researches in electricity and magnetism (an extension of Maxwell’s Treatise upon electricity and magnetism), and Elements of the mathematical theory of electricity and magnetism (a more popular and vulgarized book).

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Discovery of the electron

Thomson’s initial insight into the electron’s existence was a bold proposition that one of the elementary units of an atom was 1000 times smaller than it, that is, considerably smaller than anything previously envisaged. He discovered this idea through experiments of the properties of cathode rays – streams of electrons observed in discharge tubes – which revealed to travel much further through the air than it would if it were composed of atoms (much bigger than electrons).

In his experiments, he measured that cathode rays were much lighter than atoms and that their mass remained constant irrespectively of the element/atom from which they stemmed. He thus concluded that these unknown particles were very light and negatively charged, and most importantly, found in all atoms. He named them “corpuscles.”

Mass-to-charge ratio

Thomson pushed his experiments further to determine the mass-to-charge ratio of electrons by deflecting a beam of cathode rays by electric and magnetic fields. This more precise information about electrons’ properties allowed Thomson, in 1904, to draft a model of the atom known as the “plum pudding model.” It described a sphere of positive matter that embedded the “corpuscles” or electrons, distributed over a large sea of a positive charge.

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Discovery of isotopes

After studying stream of negatively charged particles (“corpuscles” or electrons) – cathode rays – Thomson now moved on to the experiments on canal rays, streams of positively charged particles. Thomson and his team attempted to measure the beam’s deflection (of neon ions in this case) by placing them through a magnetic and electric field. The results yielded two patches of light on the photographic plates at the end of the beam – two distinct deflection paths – which hinted towards the conclusion that neon had two different atomic masses since the deflection intensity is determined by mass.

Remembering the Work of the Man Who Discovered The Electron & Changed The Course of History. 2
Isotopes of Neon (Image: Chemistry Teaching Resources)

These two different masses are now known as isotopes of the same element (here neon-20 and neon-22 and are explained by different numbers of neutrons for the same number of protons. The method used to separate these isotopes was the first use of mass spectrometry, a laboratory technique that will develop into a widely used method over the centuries.

Achievements

For his investigations’ great merits, he was awarded the Nobel prize in 1906 and two years later, was knighted. 1912, he was appointed to the Order of Merit, and finally, in 1914, became Master of Trinity College at Cambridge University.

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