Clark telegraph relay

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Telegraph relays amplified electrical signals in a telegraph line. Telegraph messages traveled as a series of electrical pulses through a wire from a transmitter to a receiver. Short pulses made a dot, slightly longer pulses a dash. The pulses faded in strength as they traveled through the wire, to the point where the incoming signal was too weak to directly operate a receiving sounder or register. A relay detected a weak signal and used a battery to strengthen the signal so that the receiver would operate.
George C. Maynard, the curator who collected this piece in 1897, showed this and a similar relay to Dr. James J. Clark who, “examined these relays and said they were made by him about 1846 in Philadelphia. [They] represent the first big step in relay construction, after the 'big magnets' first used by [Samuel] Morse. They were originally [mounted] in a box with the adjusting screws projecting. The Clarks (J. J. and his father) made and sold many of these relays. [The Clarks] supplied [Henry] O'Rielly and all the southern lines."
Currently not on view
date made
Clark, James J.
Physical Description
wood (overall material)
brass (overall material)
steel (overall material)
ivory (overall material)
cloth (overall material)
copper (overall material)
overall: 6 in x 7 3/4 in x 3 7/8 in; 15.24 cm x 19.685 cm x 9.8425 cm
ID Number
catalog number
accession number
Credit Line
from Western Union Telegraph Co.
See more items in
Work and Industry: Electricity
Telegraph Relays & Repeaters
Data Source
National Museum of American History


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