In 1897, the British physicist J.J. Thomson was investigating streams of particles given off by metal electrodes placed under high voltage in a vacuum. These particles turned out to be much smaller than atoms and, unlike neutral atoms, negatively charged. The discovery of these "electrons" put paid to the idea that the atom was uniform and indivisible. To maintain the atom's overall electrical neutrality, Thomson suggested that electrons were embedded inside it like plums in a "pudding" of positive charge. By 1908, New Zealander Ernest Rutherford, working with his assistant Hans Geiger at the University of Manchester, UK, had revealed a different picture. When fired from a radioactive source, positively charged "alpha particles" -themselves later revealed to be the atomic nuclei of helium - largely passed through metallic foils placed in their way, deflected by just a few degrees. The atom, it seemed, incorporated a large amount of empty space. Follow-up experiments by Geiger and a research student, Ernest Marsden, delivered an even greater surprise. Some alpha particles bounced straight back, turned by up to 180 degrees. It was, as Rutherford later said, "as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you".
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