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CODED MESSAGES
WITH A BUZZER

     When Thomas Edison worked as a boy on the Grand Trunk Railroad in Michigan, he became familiar with the telegraph system that linked the eastern part of the U.S. with the western part. And recognizing the vital importance of this cross-country communications system, he later made many improvements to it.

     For example, he invented the duplex, the quadruplex, and the multiplex telegraph systems, which increased the number of messages that could be sent simultaneously over the line. Before those inventions, a line could handle only one message at a time and in only one direction.

     Similar in principle to the initial telegraph system (and, in fact, the electric pencil) is the buzzer you are about to build. Only instead of producing a single click when the code key is depressed as did the telegraph, the buzzer gives off a continuous sound.