Blackett was born in London on November 18, 1897. He attended the naval colleges at Dartmouth and Osborne and, when World War I began, joined the Royal Navy. At the conclusion of the war, Blackett enrolled in Magdalene College at Cambridge University, where he studied physics under Ernest Rutherford. He received his bachelor's degree in 1921 and his master's in 1923.

Blackett's earliest research involved the use of C. T. R. Wilson 's cloud chamber, invented in 1911. The cloud chamber contained air saturated with a vapor. When the chamber was suddenly expanded, the vapor cooled and condensed on any charged particles present in the chamber. Since radiation produces such particles in its "wake," the cloud chamber created a vapor trail surrounding the path of the radiation which could be photographed.

The problem with Wilson's original design was that if the chamber was not expanded at the same time radiation happened to pass...