Thomas Edison’s quadruplex telegraph of 1874 is the most famous of a series of inventions of the late 19th century intended to increase the efficiency and profits of telegraph companies. A central goal of inventors in the telegraph field during and after the 1870s was to learn how to transmit and receive multiple messages simultaneously on a single telegraph line. Running additional wires, which would have accomplished the same thing, was becoming too expensive.
In 1853, Julius Wilhelm Gintl in Vienna, Austria discovered a way to send two telegraph messages in opposite directions down the same wire. This technology was not commercially successful until 1871, when it was improved by the duplex system of inventor J. B. Stearns. Within a year or two, Western Union and other telegraph companies were using diplex telegraphs on their busiest lines in major cities. In 1874, Thomas Edison invented the first quadruplex telegraph, which was capable of sending two messages simultaneously in each direction. He accomplished this by having one message consist of an electric signal of varying strength, while the second was a signal of varying polarity. Western Union adopted the invention and had 13,000 miles of quadruplex lines by 1878. Years later, quadruplex telegraphs were displaced by two new inventions, multiplex telegraphy capable of eight or more simultaneous transmissions, and the use of teletype machines, which did not require their operators to know Morse Code.