Triode were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament and a plate (anode). Triodes came about in 1906 when American engineer Lee De Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a grid, between the filament and plate to control current. Von Lieben's partially-evacuated three-element tube, patented
William Petrie (electrical engineer) William Petrie (electrical engineer) William Petrie (1821–1908) was an English electrical engineer, known for the development of the arc lamp. Born at King's Langley, Hertfordshire, on 21 January 1821, he was eldest of four sons of William Petrie (born 1784), a War Office official, and his wife Margaret Mitton, daughter of the banker Henry Mitton, of the Chase, Enfield. In 1829 his father was sent to the Cape of Good Hope, where he acted until 1837 as deputy commissary-general, with as neighbour Sir John Herschel, the astronomer, who was a significant influence on the younger William Petrie's interest in science.