August 2, 1932

Positron

Carl Anderson photographs the decisive positron track

A lead plate six millimetres thick showed which way the invisible particle had been travelling, letting Anderson rule out an ordinary electron moving the other way.

2 August 1932, in a Caltech cloud chamber

Carl D. Anderson photographed a cosmic-ray particle track in a magnetized cloud chamber at Caltech on 2 August 1932. It became the first clearly identified positive electron.

The track curved in the direction corresponding to positive charge, and its length and ionization ruled out a proton of the observed curvature.

Why the plate mattered

A lead plate across the chamber slowed the particle, so its path curved more sharply after it crossed. That change established that the particle was moving upward rather than being an ordinary electron moving downward.

In the published photograph the plate was 6 mm thick, and the particle entered at about 63 MeV and left at about 23. Without it, the track could have been read as an ordinary electron travelling the other way through the picture.

Five weeks of silence

The image recorded decisive evidence, but nothing was announced that day. Anderson's first short report appeared in Science on 9 September 1932.

His definitive paper, in Physical Review the following March, reported 15 positive light-particle tracks among 1,300 cosmic-ray photographs and constrained the particle's mass to less than 20 electron masses if it carried one positive elementary charge.

Sources

Researched 23 Aug 2026 6 sources 2 audit passes, 1 issue open

How this was checked

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What the sources leave uncertain

The Caltech archival record dates the decisive negative specifically to August 2, 1932. Broader institutional histories sometimes say only “August 1932,” but no comparably authoritative source consulted contradicts the archive's exact date.
The August 2 image was the decisive observation, not the publication date. Anderson's preliminary announcement appeared September 9, 1932, and the paper that introduced “positron” appeared March 15, 1933.