Three telegraph sounders and sounder-screens, 1850-1950

Morse code was the standard code for communicating by telegraph. The code uses a series of short and long connections in the electric current, usually called 'dots' and 'dashes'. These dots and dashes could then be decoded to reveal the message being transmitted. A telegraph sounder comprises a spring-loaded metal arm, pivoted near the middle. At one end is an electromagnet and at the other, an anvil. When a current passes, the electromagnet pulls the arm down, making a loud 'clunk'. When the current ceases the arm springs back against the anvil with another clunk. A dash is about three times as long as a dot, so the time interval between clunks indicates the dot or dash. The arrangement freed the operator to write down the message as it was being received. The default status for sounders was 'on', so that operators always knew it was working, and could easily identify if there was a fault in the wire. The sounder screens were used to project the sounds towards the operator, which was particularly useful in noisy offices.

A telegraph sounder is an electromagnetic device for receiving telegraphs. When the sounder is activated by a telegraph key, the metal bar is released, making an audible 'click'. When the current from the telegraph key ends, the metal bar is pulled back to its resting position, it makes a 'clack'. This differentiation was useful for enabling operators to distinguish between the long and short signals of Morse code. The default status for sounders was 'on', so that operators always knew it was working, and could easily identify if there was a fault in the wire. The sounder screens were used to project the sounds towards the operator, particularly useful in noisy offices.