Bunnell 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 made by J. H. Bunnell & Co., 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 and a mark indicating that it was used by the B & O Railroad.
Currently not on view
J. H. Bunnell & Co.
Physical Description
wood (overall material)
metal (overall material)
plastic (overall material)
brass (overall material)
overall: 4 in x 11 5/8 in x 5 1/4 in; 10.16 cm x 29.5275 cm x 13.335 cm
ID Number
accession number
catalog 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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