SHIRLEY GRIFFITH: I'm Shirley Griffith.

STEVE EMBER: And I'm Steve Ember with EXPLORATIONS in VOA Special English. Today we tell about the history of radio and the latest technology.

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SHIRLEY GRIFFITH: Our story begins in Britain in 1873. A scientist named James Maxwell wrote a mathematical theory about a kind of energy. He called this energy electromagnetic waves.

His theory said this kind of energy could pass unseen through the air. James Maxwell was not able to prove his idea. Other scientists could not prove it either until German scientist Heinrich Hertz tried an experiment around 1887.

STEVE EMBER: Hertz's experiment sounds very simple. He used two pieces of metal placed close together. He used electricity to make a spark jump between the two pieces of metal. He also built a simple receiver made of wire that was turned many times in a circle or looped. At the ends of the loop were small pieces of metal separated by a tiny amount of space. The receiver was placed several meters from the other device.

Heinrich Hertz proved that James Maxwell's idea was correct. Electromagnetic waves or energy passed through the air from one device to the other.

SHIRLEY GRIFFITH: Later, Hertz demonstrated the experiment to his students in a classroom. One of the students asked what use might be made of this discovery. But Hertz thought his discovery was of no use. He said it was interesting but had no value.

He was wrong. His experiment was the very beginning of the electronic communications we use today. In recognition of his work, the unit of frequency of a radio wave, one cycle per second, is named the hertz.

STEVE EMBER: Radio waves became known to scientists as Hertzian Waves. But the experiment was still of no use until Guglielmo Marconi improved on the device that created Hertzian Waves. He began his experiments in Italy in 1894.

Marconi was soon able to transmit sound across a distance of several kilometers. He tried to interest Italian government officials in his discovery, but they were not interested.

Marconi traveled to Britain. His invention was well received there. In 1897, he established the Wireless Telegraph and Signal Company. The company opened the world's first radio factory in Chelmsford, England in 1898.

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SHIRLEY GRIFFITH: Very quickly, people began sending and receiving radio messages across long distances using equipment made by Marconi's company.

Ships at sea needed the device. Before Marconi's invention, they had no communication until they arrived in port. With radio, ships could call for help if they had trouble. They could send and receive information.

All of Marconi's radios communicated using Morse code. An expert with Morse code could send and receive 30 or 40 words a minute. Marconi's radio greatly increased the speed of communications.

STEVE EMBER: On December 24th, 1906, radio operators on ships in the Atlantic Ocean near the American coast began hearing strange things. At first it was violin music. Then they heard a human voice. The voice said "Have a Merry Christmas."

That voice belonged to a man named Reginald Fessenden. He had been working on producing a device that could transmit the human voice or music using radio. He decided to try it for the first time on December 24th. It was the first time a human voice had been heard on radio.

SHIRLEY GRIFFITH: Improvements in radio technology now came more quickly. Large companies became interested. Broadcasting equipment and radio receivers were improved.

Fourteen years after Reginald Fessenden's voice was heard by radio operators at sea, the first real radio broadcast was transmitted. It came from the city of Pittsburgh, Pennsylvania.

The radio program was transmitted on radio station KDKA on the evening of November 2nd, 1920. The man speaking on the radio was Leo Rosenberg. He was announcing the early results of the presidential election between James Cox and Warren Harding.

STEVE EMBER: Those first KDKA broadcasts led to the success of the radio industry. People began buying the first radios. Other companies decided radio could make a profit. Only four years after the first KDKA broadcast, there were 600 radio stations in the United States. Radio stations also began to broadcast in other countries.

Radio stations began selling "air time" as a way to pay their workers and to pay for needed equipment. A few minutes of air time were sold to different companies so they could tell about their products to the radio station's listeners. This method of supporting radio and later television is still used today.

SHIRLEY GRIFFITH: Radio changed the way people thought and lived. It permitted almost everyone to hear news about important events at the same time. Political candidates could be heard by millions of listeners. The same songs were heard across the country.

The work by British scientist James Maxwell and German scientist Heinrich Hertz led to the development of modern communications technology. This includes television broadcasts, satellite use, cellular telephones, radio-controlled toys and much more.

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STEVE EMBER: Now we will explain electromagnetic waves. We will begin with Heinrich Hertz's experiment. You can also try this experiment. First, move the controls on your radio to an area where no station is being received.

Now, you will need a common nine-volt battery and a metal piece of money. Hold the battery near the radio and hit the top of the battery with the coin. You should hear a clicking noise on the radio.

Your coin and battery are a very simple radio transmitter. This radio will not transmit very far. However, if you know a little of Morse code, you could communicate with this device.

SHIRLEY GRIFFITH: Electromagnetic energy travels almost like an ocean wave – up and down, up and down. It also travels at the speed of light – 299,792,458 meters each second.

Scientists have learned how to separate radio waves into different lengths called frequencies. This permits many radio stations to broadcast at the same time and not interfere with each other.

STEVE EMBER: You may be hearing our broadcast on what is called short wave. These are frequencies between 3,000 and 30,000 kilohertz. They are often called megahertz. One megahertz is the same as 1,000 kilohertz.

Short wave is good for broadcasting very long distances. The short wave signals bounce off the ionosphere that surrounds the Earth, back to the ground and then back to the ionosphere.

The first radio broadcasts were made using amplitude modulation. AM radio can be sent over larger distances, but the quality of the sound is not as good as a later kind of radio signal processing, called frequency modulation. FM radio stations transmit in a range of frequencies between 88 and 108 megahertz. AM radio is between 535 and 1,700 kilohertz.

SHIRLEY GRIFFITH: Radio technology continues to improve. Today, VOA broadcasts to satellites in space that send the signal back to stations on the ground that transmit programs with a clear signal.

In the United States, people who want satellite radio programs can buy the services of the company Sirius XM. The company provides listeners with programs about music, news, sports, weather, politics and much more. Many of these programs do not include commercial advertising. People can use these satellite radio services in their cars, homes or on portable devices. More and more radio stations are also broadcasting using digital radio technology.

STEVE EMBER: The Internet is also adding to the expansion of radio programming. Radio stations around the world can put their programs on the Internet for listeners everywhere to hear. The website Pandora began its Music Genome Project to create a database of song descriptions. Pandora's Internet radio is able to predict what songs listeners will like based on their earlier musical choices. Listeners can create their own personalized radio programming with this and other forms of Internet radio.

We think Heinrich Hertz would look at all these developments and be very proud of the device he made that he thought would never be of any use.

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SHIRLEY GRIFFITH: This program was written by Paul Thompson. It was produced by Dana Demange. I'm Shirley Griffith.

STEVE EMBER: And I'm Steve Ember. Our programs are online with transcripts and MP3 files at voaspecialenglish.com. And you can find us on Twitter, Facebook and YouTube at VOA Learning English. Join us again next week for EXPLORATIONS in VOA Special English.

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（來源：VOA 編輯：陳丹妮）