Double-current telegraph key, probably made and used by the Post Office, British, 1830-1900.

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. This example is similar to those used with the Bright's bell, or acoustic telegraph.