The Sounder - A simple way of interpreting the received signal. The Sounder was a device for interpreting the signals coming off the telegraph cable. Every time a Morse key was pressed, the sounder at the other end of the line made a corresponding 'click.'

How it worked - The sounder consisted of two electromagnets, with a soft iron armature which was held a short distance away from their cores by a spring. When a current flowed through the electromagnets, a magnetic field was generated which attracted the armature. The armature then hit a small plate, creating a 'click.' When the current stopped flowing, the spring brought the armature back up to its original position, creating the second 'click.' Early sounders were very quiet and what little sound they made was described as 'irritating' so later designs tried to solve these problems.

The Relay - A method of switching on a large current from a small one. Relays like this were used to detect the weak current in telegraph cables (10 - 20 mA). The relay translates the signal on the telegraph line into an equivalent signal in a local circuit.

How it worked - A current passes through the electromagnets generating a magnetic field, which moves the small, magnetized armature between them. This action brings two contacts together, allowing current to flow through the local circuit.

The Resistance Box - A device for producing standard resistances. Resistance boxes were used to put known values of resistance into circuits. They came in several different, but equivalent, formats.

How it worked - When the thick brass plugs on top were in place, they allowed current to flow without significant resistance. When one was removed, the current was forced to flow through the thin wire coil inside, which was of a measured resistance.

The Single Current Morse Key - A basic 'on-off' switch for sending a signal. This design, which originates from the late 1830s, is the simplest form of Morse Key.

How it worked - When pressed the key completed a circuit which then sent a current down the cable. The key was held down briefly to represent a Morse 'dot' and approximately three times longer to represent a Morse 'dash.'