main-line telegraph relay

main-line telegraph relay

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Usage conditions apply
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.
“Main line” relays like this unit were one of the most common types of relay and were typically made with a resistance of 150 ohms. As the name suggests, main line relays served on major intercity circuits several hundred miles long. This unit has a 122 ohm rating indicating its use on a somewhat shorter line than usual. We know a little bit more than usual about the past history of this unit, something called "provenance" in museum-speak. The relay came to the Smithsonian from the former Western Union Museum in 1972. There's a business card from J. F. Calahan, Manager, Western Union Co., Parkersburg, WV taped to bottom. The curator at the WU Museum wrote on the card: "Rec'd Feb. 24, 1954 from J.F.C. / Donated by Carl B. Wetzel / Eureka Pipe Lines Co. / 218 - 5th St., Parkersburg." Where Mr. Wetzel found the relay remains a mystery.
Currently not on view
Object Name
telegraph relay
date made
ca 1870
Physical Description
wood (overall material)
metal (overall material)
brass (overall material)
plastic (overall material)
overall: 3 3/4 in x 8 1/2 in x 4 1/2 in; 9.525 cm x 21.59 cm x 11.43 cm
ID Number
accession number
catalog number
collector/donor number
Credit Line
from Western Union Corporation
See more items in
Work and Industry: Electricity
Telegraph Relays & Repeaters
Data Source
National Museum of American History
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