This is a picture of one of the first positron tracks observed by Anderson in 1933. It was taken in a cloud
chamber in the presence of a magnetic field (so the particle paths are curved). A cloud chamber contains
a gas supersaturated with water vapour. In the presence of a charged particle (such as a positron), the
water vapour condenses into droplets - these droplets mark out the path of the particle.
The band across the middle is a lead plate, which slows down the particles. The radius of curvature of the
track above the plate is smaller than that below. This means that the particle is travelling more slowly
above the plate than below it, and hence it must be travelling upwards. From the direction in which the
path curves one can deduce that the particle is positively charged. That it is a positron and not a proton
can be deduced from the long range of the upper track - a proton would have come to rest in a much shorter
Carl Anderson won the 1936 Nobel Prize for Physics for this discovery.
Picture taken from C.D. Anderson, Physical Review 43, 491 (1933).