One of the first cloud chamber photographs showing the track of a cosmic ray, photographed by Dmitry Skobeltzyn in his Leningrad laboratory in 1927. Skobeltzyn was studying electrons knocked from atoms by gamma rays. He subjected his cloud chamber to a powerful magnetic field, causing the tracks of electrons to curve tightly, as demonstrated by most of the tracks in the picture. But a few photographs had straight tracks like the one at top centre. These tracks gave values of electron momentum & energy enormously greater than from any known source, & Skobeltzyn concluded they were fast-moving electrons knocked from atoms by cosmic gamma rays traversing his chamber.