Radio and TelevisionRadio and television were major agents of social change in the 20th ce

Radio and Television

Radio and television were major agents of social change in the 20th century. Radio was once the center for family entertainment and news and television enhanced this revolution by adding sight to sound. Both opened the windows to other lives, to remote areas of the world, and to history in the making. News coverage changed from early and late editions of newspapers to broadcast coverage from the scene. Play-by-play sports broadcasts and live concerts enhanced entertainment coverage. For many, the only cultural performances or sports events they would ever hear or see would come from the speakers or the screens in their living rooms. Each has engaged millions of people in the major historical events that have shaped the world.

If people could look at the sky and see how it is organized into frequency" bands used for different purposes, they would be amazed. Radio waves crisscross (十字形) the atmosphere at the speed of tight, delivering incredible amounts of information—navigational data, radio signals, television pictures—using devices for transmission and reception designed, built, and refined by a century of engineers.

Key figures in the late 1800s included Nikola Tesla, who developed the Tesla coil, and James Clerk Maxwell and Heinrich Hertz, who proved mathematically the possibility of transmitting electromagnetic signals between widely separated points. It was Guglielmo Marconi who was most responsible for taking the theories of radio waves out of the laboratory and applying them to practical devices. His "wireless" telegraph demonstrated its great potential for worldwide communication in 1901 by sending a signal—the letter "s"—in Morse code a distance of 2 000 miles across the Atlantic Ocean. Radio technology was just around the comer.

Immediate engineering challenges addressed the means of transmitting and receiving coded messages, and developing a device that could convert a high-frequency oscillating (振荡的) signal into an electric current capable of registering as sound. The first significant development was "the Edison effect", the discovery that the carbon filament (灯丝) in the electric light bulb could send out a stream of electrons to a nearby test electrode if it had a positive charge. In 1904, Sir John Ambrose Fleming of Britain took this one step further by developing the diode (二极管) which allowed electric current to be detected by a telephone receiver. Two years later, American Lee De Forest developed the triode (三极管), introducing a third electrode (the grid) between the filament and the plate. It could amplify a signal to make live voice broadcasting possible, and was quickly added to Marconi's wireless telegraph to produce the radio.

Radio development was prevented by restrictions placed on airwaves during World War I. Technical limitations were also a problem. Few people had receivers, and those that did had to wear headphones. Radio was seen by many as a hobby for telegraphy fans. It would take a great deal of engineering before the radio would become the unifying symbol of family entertainment and the medium for news that was its destiny.

In the mid-1920s, technical developments expanded transmission distances, radio stations were built across the country, and the performance and appearance of the radio were improved. With tuning circuits, capacitors, microphones, oscillators, and loudspeakers, the industry blossomed in just a decade. By the mid-1930s almost every American household had a radio. The appearance of the transistor in the 1950s completely transformed its size, style, and portability.

Both television and radar were logical byproducts of the radio. Almost 50 years before television became a reality, its fundamental principles had been independently developed in Europe, Russia, and the United States. John Baird in England and Charles Jenkins in the United States worked indepen

A．whether they can broadcast cultural performances

B．whether they may attract millions of people in the great historical events

C．whether they open the windows to remote areas of the world

D．whether they may offer sights to the people