Which telegraph system did Thomas Edison develop to send four messages on one wire?

Thomas Edison developed the quadruplex telegraph system to send four messages on one wire.

The system combined two forms of simultaneous signaling so that two messages could travel in each direction over a single telegraph wire. Edison developed it in the 1870s while working for Western Union, and it greatly increased the capacity of existing telegraph lines.

The quadruplex was commercially important because telegraph companies could transmit more traffic without building an entirely new network of wires. Edison’s work therefore produced value not only through a new device but also through more efficient use of infrastructure.

It is often confused with the duplex telegraph, which handles two simultaneous messages, one in each direction. “Quadruplex” specifically refers to four messages, making the name a useful clue to the system’s capacity.