Western Union box 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.
Box relays like this unit used by Western Union were special relays most often used by linemen or station operators for testing purposes or where a local battery was not available. The covering box acted as a resonator that amplified the sound of the relay’s light-weight armature, making the signal audible without a sounder. The relay includes a built-in telegraph key.
Currently not on view
Physical Description
wood (overall material)
brass (overall material)
metal (overall material)
cloth (overall material)
plastic (overall material)
overall: 3 in x 9 1/4 in x 5 1/4 in; 7.62 cm x 23.495 cm x 13.335 cm
ID Number
accession number
catalog number
Credit Line
from Western Union Corporation
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Work and Industry: Electricity
Telegraph Relays & Repeaters
Data Source
National Museum of American History


I believe that the relay in the photograph would properly be called a drum relay because of the size of the sound box used. Also the Relay and Key On Board (KOB) makes it a wrecking or caboose set. Box and Drum relays were used for the combination of sensitivity and readability in that they respond well to week current and yet they are loud enough to be copied by ear. Caboose Sets are used by conductors who must communicate with dispatch in order to obtain permission to use some portion of the Right of Way that has a high likelihood of encountering other rail traffic. Wrecking Sets were used to coordinate the movement of Maintenance Of Way (MOW) equipment on the rails around the site of a wreck or major maintenance operation. I just thought that you might want to know that I have seen these terms that I offer here in old Railway Telegrapher Manuals. You could readily check my usage by asking the folks at the Morse Telegraph Club. Their membership includes many retired railway telegraphers.

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