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"With a long main-line and many instruments in circuit, the resistance may be so great as to render the main-battery current so feeble that it cannot operate the sounder with sufficient energy to render the signals distinctly audible. This difficulty is met by introducing a 'local battery,' and a 'relay' at each station on the line." -Avery 1895

Telegraph Relay

"With a long main-line and many instruments in circuit, the resistance may be so great as to render…