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The Geiger-Marsden experiment (also called the Gold foil experiment or the Rutherford experiment) was an experiment done by Hans Geiger and Ernest Marsden in 1909, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester which led to the downfall of the plum pudding model of the atom. They measured the deflection of alpha particles (helium ions with a positive charge) directed normally onto a sheet of very thin gold foil. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees; some were even scattered back toward the source. From this observation Rutherford concluded that the atom contained a very physically-small (as compared with the size of the atom) positive charge, which could repel the alpha particles if they came close enough, subsequently developed into the Bohr model.