In later life ......... |
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| Rutherford showing 'JJ' to his seat for the Cavendish Laboratory annual photograph (1934). |
Joseph John Thomson must have been an engaging character. He was appointed to lead the Cavendish Laboratory in 1884 when still a young man of 28. He occupied the last twenty or so years of his life as master of Trinity College, where he died at 84.
J J Thomson and his student Ernest Rutherford were the first to demonstrate the ionization of air by X rays, but Thomson is best known for deflecting cathode rays [1897] with a perpendicular electric field - showing that they were negative particles! He measured the charge to mass ratio with a magnetic field, and the speed of the negative particles with crossed electric and magnetic fields. The laboratory made their own tubes and power supplies. It was ground-breaking work.
Cathode rays in a magnetic field.
Cathode rays in an electric field.
The diagrams show the ray. Thomson's tube had no gas left inside. The cathode ray was not visible. The only thing he saw was the spot on the glass at the far end due to fluorescence.
> Clip
Ratio e/mMagnetic deflection only: Remembering that for a circular path in a perpendicular magnetic field, the centripetal force is given by mv2/r ... gives.... ... where r is the path radius. The energy of the electron is given by ... Combining the two equations to eliminate the velocity gives.... |
Particle velocityCrossed fields: When no deflection takes place the magnetic force equals the electric force. Canceling e and rearranging gives the speed v as the ratio.... ... where V is the plate voltage |
Some years later Millikan measured the charge on the electron, and as a consequence, the electron mass. In modern terms, when we know what to look for, the math is easy. It was not so easy when nobody knew what to expect.
Thomson was not finished with the electron. In 1913 he noted that neon atoms have two different atomic weights [20 and 22] using equipment he called a "positive-ray" apparatus. He did not realize it at the time, but he had invented a crude mass spectrometer.
In 1920 F W Aston, following on Thomson's work, discovered a third isotope of neon with atomic weight 21. Aston devoted the remainder of his life to refining and improving the mass spectrometer. He described 212 of the 287 naturally occurring isotopes.
> IB multiple choice questions
J J Thomson was awarded the 1906 Nobel Prize in physics. His son, George P Thomson, some years later, received the Nobel Prize for experimentally confirming the wavelike properties of electrons.
In later life ......... |
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| Rutherford showing 'JJ' to his seat for the Cavendish Laboratory annual photograph (1934). |
For J J Thomson's experiment repeated with a Teltron tube in the Lab at ISB....