Western Union type 4-B main-line 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.
“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 type 4-B relay was common in the Western Union system in the early 20th century. The unit bears an inspection stamp "N.Y.R.S. 5 27" indicating that it was serviced at Western Union's New York Repair Shop in May 1927.
Currently not on view
Physical Description
grass (overall material)
steel (overall material)
plastic (overall material)
iron (overall material)
slate (overall material)
overall: 3 3/4 in x 8 in x 4 3/4 in; 9.525 cm x 20.32 cm x 12.065 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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