Telegraph sounder

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.