How did Cockcroft and Walton split the atom in 1932, according to Harry Cliff?

Harry Cliff says Ernest Rutherford challenged physicists in a 1927 Royal Society speech to build machines that could fire particles at will, because natural radioactivity gave too little control. At Cambridge's Cavendish Laboratory, John Cockcroft and his Irish PhD student Ernest Walton built a voltage accelerator several metres high, fired protons at lithium in 1932 and saw alpha particles, showing the lithium nucleus had broken in two.

• The detector was a lead-lined tea chest with a microscope and a scintillating screen, with Walton himself inside counting flashes.
• The experiment was also the first experimental demonstration of E equals mc squared, since breaking the nuclei released energy.
• Rutherford disliked the phrase splitting the atom, because the electron had already been knocked out of atoms in the 1890s.
• Rival teams raced them, including Ernest Lawrence in California with his ring-shaped cyclotron and Merle Tuve in Washington.

Source: our own transcription of the episode. The full conversation is on In Our Time (2026-10-22). Read the whole episode summary →

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