The Quadruplex telegraph is a type of electrical telegraph

Electrical telegraph

An electrical telegraph is a telegraph that uses electrical signals, usually conveyed via telecommunication lines or radio. The electromagnetic telegraph is a device for human-to-human transmission of coded text messages....

 which allows a total of four separate signals to be transmitted and received on a single wire at the same time (two signals in each direction.) Quadruplex telegraphy thus implements a form of multiplexing

Multiplexer

In electronics, a multiplexer is a device that selects one of several analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output...

.

The technology was invented by American inventor Thomas Edison

Thomas Edison

Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

, who sold the rights to Western Union

Western Union

The Western Union Company is a financial services and communications company based in the United States. Its North American headquarters is in Englewood, Colorado. Up until 2006, Western Union was the best-known U.S...

 in 1874 for the sum of $10,000.

The problem of sending two signals simultaneously in opposite directions on the same wire had been solved previously by Julius Wilhelm Gintl

Julius Wilhelm Gintl

Julius Wilhelm Gintl was an Austrian physicist. He was notable as the developer of an early form of duplex electrical telegraph, which allowed two messages to be transmitted on a single wire, in opposite directions. This "duplex" communication was an early specific case of the general practice of...

 and improved to commercial viability by J. B. Stearns; Edison added the ability to double the number in each direction.

To send two signals in a single direction at the same time, the quadruplex telegraph used one signal to vary the absolute strength or voltage of the signal (amplitude modulation

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

) and the other signal to vary the phase (polarity) of the line (phase modulation

Phase modulation

Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...

), i.e., the direction of current flow imposed upon the wire.

Today this concept is known as polar modulation

Polar modulation

Polar modulation is analogous to quadrature modulation in the same way that polar coordinates are analogous to Cartesian coordinates. Quadrature modulation makes use of Cartesian coordinates, x and y. When considering quadrature modulation, the x axis is called the I axis, and the y axis is...

, considering amplitude and phase as radius and angle in polar coordinates.