John Ambrose Fleming, professor at University College London, investigates the 'Edison effect' (electrical conduction inside a vacuum bulb from an incandescent filament to a metal plate) starting in the 1880s. In 1904 he builds such a bulb and uses it to rectify high-frequency oscillations, thereby detecting wireless signals. He patents the device the same year, later known as the 'Fleming valve' — the first thermionic diode, a direct precursor to all 20th-century vacuum-tube electronics.