In August 1914 on the outbreak of World War I Blackett was assigned to active Service as a midshipman. He was transferred to the Cape Verde Islands on HMS Carnarvon and was present at the Battle of the Falkland Islands. He was then transferred to HMS Barham and saw much action at the Battle of Jutland. While on HMS Barham, Blackett was co-inventor of a gunnery device on which the Admiralty took out a patent. In 1916 he applied to join the RNAS but his application was refused. In October that year he became a sub-lieutenant on HMS P17 on Dover patrol, and in July 1917 he was posted to HMS Sturgeon in the Harwich Force under Admiral Tyrwhitt. Blackett was particularly concerned by the poor quality of gunnery in the force compared with that of the enemy and of his own previous experience, and started to read science textbooks. He was promoted to Lieutenant in May 1918, but had decided to leave the Navy. Then, in January 1919, the Admiralty sent the officers whose training had been interrupted by the war to Cambridge University for a course of general duties. On his first night at Magdalene College, Cambridge he met Kingsley Martin and Geoffrey Webb, later recalling that he had never before, in his naval training, heard intellectual conversation. Blackett was impressed by the prestigious Cavendish Laboratory, and left the Navy to study mathematics and physics at Cambridge.