How a telegraph key turns a circuit into a message
A simple electrical circuit explains the key, but different telegraph systems used different ways to represent information.
The simplest model
Imagine a power source, a wire, a switch and a receiver connected in a circuit. Pressing the switch closes the path so that current flows. Releasing it breaks the path. At the far end, an electromagnetic device can respond to the changing current. This is an educational model, not a wiring diagram for every historical installation.
The circuit does not know the meaning of a message. Sender and receiver need an agreed relationship between a detectable change and a symbol. A trained operator, recording mechanism or indicating instrument makes that relationship useful.
More than one design
Cooke and Wheatstone's needle system in Britain and the coded system associated with Morse and Vail took different approaches. In the former, needle positions could indicate letters. In a Morse system, combinations of timed signals represent characters. Calling every early electric telegraph a “Morse machine” hides this difference.
The Library of Congress preserves the 24 May 1844 message sent from Washington to Baltimore. It is strong evidence for that demonstration, not proof that no one had proposed electrical telegraphy earlier.
From wire to wireless
Early wireless telegraphy could also send coded messages, but the signal crossed a radio path instead of relying on a continuous wire between stations. Voice broadcasting came through another set of developments. Try our Morse playground to explore timing without needing batteries, wires or a transmitter.