Time and …? Th Is Apparatus, Held in London’S Science Museum, Has Some Signifi Cant Purpose — Or Curiosity Value — in the History of Physics

ENDGAME Time and …? Th is apparatus, held in London’s Science Museum, has some signifi cant 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. Th omson’s electron-diff raction camera

In 1937, George Paget Th omson 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. Th omson had fi rmly opposed. Ten years earlier, G. P Th omson had asked one of his students to study electron diff raction. When the student fi red electrons through a thin fi lm of celluloid, surprisingly, diff use halos were produced on the observing screen. Th ese halos looked like diff raction patterns made by waves, but it was impossible to be sure, because the structure of celluloid was not known then. So Th omson checked the results using metals — aluminium, platinum and gold — whose structure was both simple and well known, and he too saw clear diff raction patterns. Th e diff raction apparatus was G. P. Th omson’s work was with wavelengths given by De based on the same discharge- similar to that of Clinton Joseph Broglie’s equation. Th us quantum tube principle as that used by Davisson (who shared the Nobel theory, which was still being J. J. Th omson in discovering the Prize with Th omson) and Lester pieced together in the 1920s, electron. In this picture of G. P. Halbert Germer at the Bell was shown to have some basis Th omson’s diff raction camera, Telephone Laboratory in the in reality, and was not simply a electrons fl owed from the negatively same year (Nature 119, 558–560; mathematical construct. charged electrode at the left , 1927). Th ey investigated the through a small hole in the metal angular distribution of electrons JANE WESS disc to focus them, then through scattered from a sample of Jane Wess is a curator at the a thin fi lm of celluloid or metal nickel, and calculated that the Science Museum, Exhibition onto the screen at the right. Th e electrons were scattered by the Road, South Kensington, London diff raction pattern on the screen surface atoms at the exact angles SW7 2DD, UK. could then be photographed. predicted by Bragg’s formula, www.sciencemuseum.org.uk

66 nature physics | VOL 1 | OCTOBER 2005 | www.nature.com/naturephysics