he original devices that they created performed the multiplexing task mechanically by vibrating metallic reeds at different frequencies. The frequencies would pass down the line and be received correctly at Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Telephony the other end. The only problem with this technique was channel sepa- ration: 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 origi- nal 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 wide- spread use today, by the way; see Figure 3-2) that would eventually replace the far l