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Modern telegraph systems are based upon the fact that electric current will flow through a wire which forms a circuit. The circuit can be broken to interrupt the flow of electricity and closed again for a short time by a key to send an electrical impulse through the circuit. When the key is pressed the circuit is closed. Electricity from the battery flows through the wire and earth circuit and causes the buzzer to make a noise. Using the Morse code, or some other code, messages can be sent in this way.
Ancient telegraph systems are based upon the fact that electrons flows through a drum.
The circuit can be interrupted the flow of water.
The circuit can be closed for a short time by a coil.
Electrical impulses will not flow through a wire if the circuit is broken.
