Telegraph: is a system of transmitting the message over a distance by electrical impulses through wires. By pressing a key at the transmitting station, a circuit is closed and a current flows through the conducting wire to the receiver. The dots and dashes of the Morse code are obtained by changing the length of time for which the current flows. At the receiving station, the feeble electrical impulses are made to operate a relay. It closes a local circuit carrying a large current. This current causes the dots and dashes to be automatically recorded.