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
John Snell (electrical engineer) General Charles Edmund Webber on electrical supply projects in Kensington and a number of country houses, before entering municipal service in London in 1893 as assistant electrical engineer in St Pancras during the construction of the Kings Road power station. In 1896 he transferred to Sunderland as Borough Electrical Engineer, additionally becoming Borough Tramways Engineer in 1896 after which he converted the tramway system to electric power. In 1906 he set up in business as a consulting engineer in Westminster, amalgamating with Preece and Cardew to form Preece, Cardew and Snell. During that time he served as an expert witness