A telegraph sounder is a device which produces an audible sound when connected to an operating electrical telegraph.
It is similar in form to a relay.
When current flows through an electromagnet, the resulting magnetic field attracts an armature that is held up by a weight. When the current is switched on, the arm is pulled down to the electromagnet, resulting in a "click" When the current ends, the weight pulls the armature back to its resting position, resulting in a "clack." Thus, as the Telegraph key (at the sending end) makes and breaks a contact, the sounder echoes the up and down state of the key.
It is important that a sounder makes a sound both when the circuit is broken and when it is restored. This eases the process of identifying long and short presses of the key to send characters in morse code.
- Morse Telegraph Club, Inc. (The Morse Telegraph Club is an international non-profit organization dedicated to the perpetuation of the knowledge and traditions of telegraphy.)