An impulse entering the coil of the magnet caused an armature to rotate and close an independent circuit actuated by a battery. This action sent a fresh pulse of current into the line, and this pulse in turn activated successive relays until the receiver was reached. A few years after Morse developed his receiving instrument and demonstrated it successfully, telegraph operators discovered that it was possible to distinguish dots and dashes by sound alone, and the Morse recording apparatus was therefore discarded. The other fundamental principles of the Morse system continued in use in wire-telegraph circuits, however.

Because telegraphy was too expensive for widespread use, several means of sending several messages simultaneously over a single line were developed. In duplex telegraphy, the earliest advance of this kind, one message can be transmitted simultaneously in each direction between two stations. In quadruplex telegraphy, invented in 1874 by the American engineer Thomas Edison, two messages were transmitted in each direction simultaneously. In 1915 multiplex telegraphy came into use, permitting the transmission of eight or more messages simultaneously. Because of this and the development of teleprinting machines during the mid-1920s, the Morse manual telegraph system of code and key was gradually discontinued for commercial use and replaced by automatic wire and wireless radio-wave methods of transmission.

III. Automatic Telegraph Systems

There are two basic systems of modern telegraphic communication: the teleprinting system (teletype), which is still in use, and the facsimile reproduction system, which became obsolete in the 1980s.

A. Teleprinting

In teleprinting, the message is received in the form of typed words on a paper form. In this system each letter of the alphabet is represented by one of 31 combinations of five equal-interval electronic impulses, with the sequence of used and unused intervals determining the letter. The start-stop printing code uses seven pulses for each character, the first pulse indicating the beginning and the seventh pulse the end of the letter.

The transmitter or teletypewriter consists of a typewriterlike keyboard and may or may not record the message on tape before it is transmitted. The receiver is basically like a typewriter without a keyboard that prints the message on a tape or a paper form. Most machines in the start-stop system are both transmitters and receivers. News organizations were among the major users of the teletype and similar communications systems.