• This apparatus, held in London's Science Museum, has some significant purpose — or curiosity value — in the history of physics. Can you guess what it is?

      They wait for no man, but using this device a noble physicist could predict how long one of them would stay. Answer next month.
    • Last month, on our website: G. P. Thomson's electron-diffraction camera

      In 1937, George Paget Thomson followed in his father's footsteps to win the Nobel Prize for Physics. Paradoxically, his work made apparent the wave nature of electrons — a concept J. J. Thomson had firmly opposed.

      Ten years earlier, G. P Thomson had asked one of his students to study electron diffraction. When the student fired electrons through a thin film of celluloid, surprisingly, diffuse halos were produced on the observing screen. These halos looked like diffraction patterns made by waves, but it was impossible to be sure, because the structure of celluloid was not known then. So Thomson checked the results using metals — aluminium, platinum and gold — whose structure was both simple and well known, and he too saw clear diffraction patterns.

      The diffraction apparatus was based on the same discharge-tube principle as that used by J. J. Thomson in discovering the electron. In this picture of G. P. Thomson's diffraction camera, electrons flowed from the negatively charged electrode at the left, through a small hole in the metal disc to focus them, then through a thin film of celluloid or metal onto the screen at the right. The diffraction pattern on the screen could then be photographed.

      G. P. Thomson's work was similar to that of Clinton Joseph Davisson (who shared the Nobel Prize with Thomson) and Lester Halbert Germer at the Bell Telephone Laboratory in the same year (Nature 119, 558–560; 1927). They investigated the angular distribution of electrons scattered from a sample of nickel, and calculated that the electrons were scattered by the surface atoms at the exact angles predicted by Bragg's formula, with wavelengths given by De Broglie's equation. Thus quantum theory, which was still being pieced together in the 1920s, was shown to have some basis in reality, and was not simply a mathematical construct.