How did the Quadruplex Telegraph Work?

In 1874, inventors were already experimenting with ways to make the telegraph more efficient. These inventors include Julius Wilhelm Gintl and J. B. Stearns, who focused on sending one message in each direction down one wire at the same time. Neither one had made the technology commercially viable.

Their invention was known as the duplex telegraph, and Edison developed his own version. He took it one step further, and added the ability to send two messages in each direction at the same time (known as the diplex telegraph), leading to four simultaneous messages.

“The bridge configuration produces the duplex action (excitations are prevented from driving the local sounders), and diplex (two messages in same direction simultaneously) is accomplished by sending one message with current polarity and the other with current amplitude (small or big).”

- Don Fulton (Electrical Engineer)

Edison’s Quadruplex Telegraph Patent, 1876, US Patent Office

The duplex telegraph worked by comparing voltage levels at the incoming wire and the local telegraph key. This allows the telegraph to distinguish between the message it is sending and a message that it is receiving. The diplex used multiple polarities to encode two messages simultaneously. Edison realized that one device could use both methods at once, leading to his invention.

“The invention was tested a week ago in Boston, with the greatest sucess, and is certain to mean in the near future practically doubling the message carrying capacity of all the telegraph lines in use.”

- Chicago Tribune

Robert Nies, Bridge Duplex Telegraph Simulation Video, 2020

This video shows a model of the duplex telegraph. It is based on the "bridge duplex" style, which was later replaced by a more efficient design in Edison's quadruplex telegraph.