The experiment. In 1932 Carl Anderson was photographing cosmic rays at the California Institute of Technology. He used a cloud chamber standing on its edge between the poles of a large electromagnet, which gave a field of 15,000 gauss (1.5 T). Thin trails curved one way or the other, and he could not tell whether a trail curving to the left was a positive particle moving up or a negative particle moving down.

The lead plate. To settle the direction, Anderson put a lead plate 6 mm thick across the middle of the chamber. A particle loses momentum going through lead, so its trail curves more tightly after the plate than before it. On 2 August 1932 he photographed a trail that entered the plate with an energy of about 63 MeV and came out with about 23 MeV.

What it meant. The trail was positive, but far too long to be a proton’s. Anderson concluded it was a positive particle with about the mass of an electron, and named it the positron. Paul Dirac’s theory had predicted such an antiparticle of the electron in 1931. The positron was the first antiparticle found, and Anderson shared the 1936 Nobel Prize in Physics for the discovery.

See the original. The Science Museum Group holds a print of the photograph, and Anderson’s paper is The Positive Electron, Physical Review 43, 491 (1933).