Diagram depicting the passage of current in a needle telegraph system. One of the two main types of electromagnetic telegraphs, needle telegraphs use indicating needles moved electromagnetically as a means of displaying messages.
Ward, Lock. “Passage of Current in Needle Telegraph System.” Wonders of Electricity and the Elements, Being a Popular Account of Modern Electrical and Magnetic Discoveries, Magnetism and Electric Machines, the Electric Telegraph and the Electric Light, and the Metal Bases, Salt, and Acids. London, England, 1870–1900. https://digital.sciencehistory.org/works/9k7bd7y.
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