CARL ANDERSON / SCIENCE PHOTO LIBRARY CARL ANDERSON / SCIENCE PHOTO LIBRARY
The discovery photograph of the positron, the positive version of the electron & the first antimatter particle to be discovered. The cloud chamber photo was taken in 1932 by US physicist Carl Anderson. It shows the track of a positive particle that enters the chamber from below. The particle is known to be positive because of the way it bends in the chamber's magnetic field; & it is proved to be moving up the picture because it loses energy & curves more in the magnetic field after traversing the 6 mm thick lead plate in the middle. But the track is too faint to be caused by a proton; it was exactly like an electron's track; it had to be the predicted positron.
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