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the other end. The only problem with this technique was channel separation: Because the frequency bands were relatively close to each other, they would eventually interfere with one another, causing problems at the receiving end of the circuit. For the telegraph, therefore, harmonic multiplexing was deemed to not be practical. All three inventors Gray, Edison, and Bell were in a dead heat to create the first device that would transport voice across a long-distance network. None of them had really tested their inventions extensively; they had been used in the laboratory only. Nevertheless, on February 14, 1876, Alexander Graham Bell filed a notice of invention on his own untested device. Simultaneously, Elijah Gray filed a notice of invention on his device. There is some argument as to who filed first; many believe that Gray actually filed first. Whatever the case, three days later Bell became the first person to transmit voice electronically across a network. And in 1877, following numerous experiments and tweaks to his original device, Bell founded the Bell Telephone Company. Meanwhile, Thomas Edison continued to work on his own device, patented in 1875, and in 1876 was hired by Western Union. The arrival of the telephone was seen as a major problem by the company but they had no intention of sitting idly by while Gray and Bell destroyed their business with their new-fangled invention. In 1877 Edison crafted a very good transmitter based on compressed carbon powder (still in widespread use today, by the way; see Figure 3-2) that would eventually replace the far less capable transmitter invented by Bell. For the
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Figure 3-2 Bell s carbon microphone, still in use today
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experimenters among the readers out there, it s a fun exercise to build a compressed carbon microphone. In a mortar and pestle (OK, with a plastic bag and a hammer) grind two charcoal briquettes to a fine powder. Pour the powder into a jar lid until the powder is even with the lip of the lid. Stretch a piece of plastic wrap tightly over the lid and secure it with a rubber band. Next, drill a hole in each side of the lid and insert an insulated wire that has been stripped back about a half inch to expose the copper wire. Next, connect the wires to a speaker and battery. When you speak into the microphone, the sound waves impinging upon the stretched plastic wrap cause the carbon powder to be compressed and expanded, which in turn changes the resistance between the two conductors inserted into the powder. Naturally, tensions ran high between the Bell Telephone Company and Western Union during this period. In 1877, Bell offered to sell his patent to Western Union for $100,000. They turned down Bell s offer, deciding instead to buy Gray and Edison s patents and form the American Speaking Telephone Company. Bell sued Western Union for patent infringement in 1878, and the case was settled in 1879 to everyone s satisfaction. Bell won the rights to Western Union s telephone patents, provided he pay royalties for the duration of the 17-year patent life of each device. Meanwhile, Bell Telephone Company agreed to stay out of the telegraph business and Western Union agreed to stay away from telephony. Once the legal wrangling had ended, telephone service moved along rather quickly. By the spring of 1880 the United States had 138 exchanges and 30,000 subscribers. By 1887, 146,000 miles of wire had been strung to connect 150,000 subscribers to nearly 750 main offices and 44 branch offices. As we described in 2, there were no switches initially, so telephones were sold in pairs, one for each end of the connection. Wires were strung in a haphazard fashion, attached to the outside of buildings, and strung across neighbors rooftops; they threatened to fill the sky, as shown in Figure 3-3. Obviously, this one-to-one relationship was somewhat limiting. A technique was needed that would allow one phone to connect to many. The answer came in the form of the central office. Connections were installed from all the phones in a local area to the central exchange, where operators could monitor the lines for service requests. The customer would then tell the operator whom they wanted to speak with, and the operator would set up the call using patch cords. When the parties had finished speaking, the operator would pull down the patch cord, free-
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