English physicist and electrical engineer John Ambrose Fleming, who had worked with Thomas Edison’s company in London, invented and applied for the patent for the two-electrode vacuum-tube rectifier on November 16, 1904. He filed the complete specification on August 15, 1905 and received British patent no. 24,850 on September 21, 1905 for "Improvements in Instruments for Detecting and Measuring Alternating Electric Currents." Fleming had been aware since 1884 of the “Edison effect,” more commonly known as thermionic emission, of “unilateral flow of particles from negative to positive electrode, and he repeated some of the experiments, with both direct and alternating currents, beginning in 1889. . . . [In 1904] he returned to his experiments on the Edison effect, with a view to producing a rectifier that would replace the inadequate detectors then used in radiotelegraphy. He named the resulting device a ‘thermionic valve,’ for which he obtained a patent in 1904. This was the first electron tube, the diode, ancestor of the triode and the other multielectrode tubes which have played such an important role in both telecommunications and scientific instrumentation” (Dictionary of Scientific Biography).
Fleming's first written document on the valve was the British patent. However, his first distributed publication on the topic was "On the Conversion of Electric Oscillations into Continuous Currents by Means of a Vacuum Valve," Proceedings of the Royal Society 74 (1905) 476-487, which appeared in the issue of the Proceedings dated March 16, 1905. Fleming’s patent, and this scientific paper introducing the basic principle of the two-electrode vacuum tube or diode, marked the beginning of electronics.
Aside from its multitude of users in radio, radar and other devices, before the development of the transistor the vacuum tube became the first switch used in the earliest electronic computers. Using vacuum tubes as switches, the first general purpose electronic computer, the ENIAC, operated 10,000 times the speed of a human computer. By comparison, the Harvard Mark 1, which used electromechnical relays as switches, computed at 100 times the speed of a human computer.
Carter & Muir, Printing and the Mind of Man (1967) no. 396 (Proc. Roy. Soc. paper)