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Thomasina Copeland
5 years ago
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1 TELEPHONE

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3 Telephone Alexander Graham Bell developed the first successful telephone in By the end of the 20th century, more than 800 million telephones were installed in homes, schools, and offices around the world. Today in the United States alone, the average telephone subscriber makes some 3,000 calls a year. History The story of Alexander Graham Bell's invention of the telephone is told in the biography of Bell in this encyclopedia. By 1878 there were 10,755 telephones in service in the United States. And in that year, the first commercial switchboard was built, in New Haven, Connecticut. Since then, many improvements have been made. For example, in Bell's early telephones, the transmitter was also used as the receiver--the same instrument was held first to the mouth and then to the ear. Later designs had a more convenient separate earpiece, with the mouthpiece and the bulk of the set's equipment mounted on the wall. The handset, which combined the receiver and transmitter, was introduced for general use in As telephones improved, so did telephone lines. In the early 1900's, the vacuum tube was invented, and an amplifier making use of it was developed. In 1915, this made it possible to talk across North America from coast to coast. In 1927, commercial radio telephone service spanned the Atlantic Ocean between New York City and London. Undersea telephone cables were first laid in the 1950's, and satellite transmission began in the 1960's. Many changes have also been made in switching. In early switchboards, connections were made by hand. An operator sat in front of a board that had a jack (or socket) for each line served. When a subscriber wanted service, a light would flash at the appropriate jack, and the operator would ask the subscriber what number he or she wished to reach. Then the operator would connect the two lines by plugging the ends of a short wire into the two jacks. In the 1890's, much faster automatic switching devices came into use. Today most telephone connections are made by electronic switches, which were introduced in the 1960's. These devices, which use transistors to perform switching operations in millionths of a second, can perform much more complicated operations than the older electromechanical switches. For example, by dialing a code, users can transfer calls from their homes to any other place where they may be. Conference calls can be set up between three or more telephones. Jespersen, James. "Telephone." The New Book of Knowledge Grolier Online. 29 Mar < nbk.grolier.com/cgi-bin/article?assetid=a h>.

4 In 1861, German Johann Phillip Reis [German: ] tried to develop a telegraph that transmitted sounds, which he called a telephone. Alexander Graham Bell in the 1870s had a similar idea; one telegraph wire could carry sounds of different pitches representing several messages at once. Experimenting in 1875, Bell noticed that variations in the amount of current moved thin pieces of metal in his apparatus. He redesigned the device to use the sound to vary current; at the other end, the changing current reproduced the sounds. This simple device was patented by Bell in 1876 as the telephone. Bell and his assistant Thomas Watson soon improved the telephone by using a diaphragm to move permanent magnets that altered levels of current. The same apparatus was used to send and to receive sounds. In 1882, a version with a sending mechanism held near the mouth and a receiver placed on the ear was introduced. Bell's telephone system connected a small group of phones to a central operator who signaled individual phones to indicate incoming calls. Each phone had its own pattern of rings, but picking up any receiver allowed anyone in the group to participate in the call, a system known as the "party line." Improvements included the first way to send several calls at once over the same wires (1887), a mechanical switching system to replace central operators (1892), the rotary dial telephone (1896), and automatic switching systems (1912). In 1983, the first cellular telephone networks in the United States allowed subscribers to use wireless battery-powered telephones. "Telephone." The Blackbirch Encyclopedia of Science & Invention. 4 vols. Blackbirch Press, Reproduced in Kids InfoBits. Detroit: Gale,

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Final Project for Summer Physics Institute Electricity and Magnetism Instructor: Dr. Meera Chandrasekhar University of Missouri-Columbia

Build Your Own Telephone Hixson Middle School Runnrz26@aol.com Marion Count R-II Rrm009@mail.connect.more.net Holman Middle School williams.dane@webster.k12.mo.us Final Project for Summer Physics Institute