Cockcroft and Walton's Accelerator, 1932.

In April 1932, at Cambridge, John Cockcroft (1897-1967) and Ernest Walton used this machine to accelerate protons to disintegrate lithium nuclei. In 1928 Gamow had explained, using quantum mechanics, how a particle escaped from a nucleus by tunnelling out through the barrier holding it in. Cockcroft realised that comparatively low-energy particles might be able to tunnel into a nucleus, causing it to disintegrate. They built this accelerator to prove it, and received the Nobel Prize in 1951.