English: Cloud chamber photograph of the first positron ever observed. The thick horizontal line is a lead plate. The positron entered the cloud chamber in the lower left, was slowed down by the lead plane, and curved to the upper left. The curvature of the path is caused by an applied magnetic field that acts perpendicular to the image plane. The higher energy of the entering positron resulted in lower curvature of its path.
Original caption: A 63 million volt positron (H_ρ = 2.1×10⁵ gauss-cm) passing through a 6 mm lead plate and emerging as a 23 million volt positron (H_ρ = 7.5×10⁴ gauss-cm). The length of this latter path is at least ten times greater than the possible length of a proton path of this curvature.