Telegraph keys are switches in electrical circuits that power on the current. When the operator taps out the signals for a word, the switch finishes a circuit, permitting electricity to continue around it. At the other end of the line, the recipient watches a pointer or dial indicating the different patterns of code, listens to a buzzer or reads the print out from a device.
For a telegraph message to go from one person to another, a wire or cable must run from one location to the next. To get messages from the United States to Europe, this meant that cables had to run beneath the Atlantic Ocean.
The technology behind the telegraph also led to the creation of the telephone, which quickly became more popular. Alexander Graham Bell was actually twiddling with a novel type of telegraph when he spilled something on himself. His assistant heard him calling for help from the next room, but his voice was coming through the wire. Landline telephones use flowing electricity just like the telegraph, but the current carries sounds rather than a code. Speaking into a phone involves the device transforming the speaker's voice into electricity and then converting it back for the person on the other end.