The spectacular transport revolution of the 20th century was accompanied by a communications revolution quite as dramatic, although technologically springing from different roots. In part, well-established media of communication like printing participated in this revolution, although most of the significant changes—such as the typewriter, the Linotype, and the high-speed power-driven rotary press—were achievements of the 19th century. Photography was also a proved and familiar technique by the end of the 19th century, but cinematography was new and did not become generally available until after World War I, when it became enormously popular.
The real novelties in communications in the 20th century came in electronics. The scientific examination of the relationship between light waves and electromagnetic waves had already revealed the possibility of transmitting electromagnetic signals between widely separated points, and on December 12, 1901, Guglielmo Marconi succeeded in transmitting the first wireless message across the Atlantic. Early equipment was crude, but within a few years striking progress was made in improving the means of transmitting and receiving coded messages. Particularly important was the development of the thermionic valve, a device for rectifying (that is, converting a high-frequency oscillating signal into a unidirectional current capable of registering as a sound) an electromagnetic wave. This was essentially a development from the carbon-filament electric lightbulb. In 1883 Edison had found that in these lamps a current flowed between the filament and a nearby test electrode, called the plate, if the electric potential of the plate was positive with respect to the filament. This current, called the Edison effect, was later identified as a stream of electrons radiated by the hot filament. In 1904 Sir John Ambrose Fleming of Britain discovered that by placing a metal cylinder around the filament in the bulb and by connecting the cylinder (the plate) to a third terminal, a current could be rectified so that it could be detected by a telephone receiver. Fleming’s device was known as the diode, and two years later, in 1906, Lee De Forest of the United States made the significant improvement that became known as the triode by introducing a third electrode (the grid) between the filament and the plate. The outstanding feature of this refinement was its ability to amplify a signal. Its application made possible by the 1920s the widespread introduction of live-voice broadcasting in Europe and America, with a consequent boom in the production of radio receivers and other equipment.
This, however, was only one of the results derived from the application of the thermionic valve. The idea of harnessing the flow of electrons was applied in the electron microscope, radar (a detection device depending on the capacity of some radio waves to be reflected by solid objects), the electronic computer, and in the cathode-ray tube of the television set. The first experiments in the transmission of pictures had been greeted with ridicule. Working on his own in Britain, John Logie Baird in the 1920s demonstrated a mechanical scanner able to convert an image into a series of electronic impulses that could then be reassembled on a viewing screen as a pattern of light and shade. Baird’s system, however, was rejected in favour of electronic scanning, developed in the United States by Philo Farnsworth and Vladimir Zworykin with the powerful backing of the Radio Corporation of America. Their equipment operated much more rapidly and gave a more satisfactory image. By the outbreak of World War II, television services were being introduced in several countries, although the war suspended their extension for a decade. The emergence of television as a universal medium of mass communication is therefore a phenomenon of the postwar years. But already by 1945 the cinema and the radio had demonstrated their power in communicating news, propaganda, commercial advertisements, and entertainment.
