A photographic image produced by the radiation emitted from a radioactive substance placed on or near the photographic film.
From 'auto-' (self), 'radio' (radiation), and '-gram' (image or record). Developed in the 1920s-30s as scientists used radioactive tracers in biological research and created images from the radiation they produced.
Scientists realized they could place radioactive materials directly on film and let the radiation draw its own picture—it's like the radioactive substance is taking a selfie on the film, which is where 'auto' comes from.
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