Morse key with send-receive switch, made by the General Post Office, British, 1900-1920.

With a single-current telegraph key the position of the relay is restored to the normal position when current ceases by a magnetic bias or by a spring in tension. In other words, signal currents only flow in the line when the key is depressed. In the double-current system a current in the reverse direction is used to restore the relay to the spacing side. The 'send-receive' switch is necessary to disconnect the key from line while signals are being received, otherwise a spacing current flows along the line, preventing signals from being sent from the other end.