text: If you're talking strictly telegraph, then it's just the tapping of metal on metal.\n\nEvery time the sender presses the key down, he closes an electrical circuit and when he releases the key, it breaks the circuit. It's similar to flipping the light switch on and off.\n\nIf the sender's key is down, it sends electricity through the telegraph wires to the receiver. The electricity energizes an electric-magnet, which pulls down a small hammer on the receiver's side. This results in a "tap" sound. When the sender releases the key, the electricity is shut off, cutting power to the electric-magnet. A spring then pulls the hammer back up to its starting position where it's ready to tap again when the sender presses the key.