Answer: telegraphs

AnswerCrossword Clue
telegraphsA system for transmitting messages from a distance along a wire, esp. one creating signals by making and breaking an electrical connection
telegraphsA device for transmitting messages in such a way
telegraphsTELEGRAPH, to transmit by telegraph, a combination of apparatus for transmitting information to a distance
telegraphsA system for the rapid transmission of messages by relayed visual signals