Bunnell "double-balanced" 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.
This is a special type of relay made by Bunnell called a "double-balanced" relay and was designed for use in a burglar alarm circuit. Each coil is wound to 150 ohms and are in electrical series on the telegraph line. One coil activates the alarm bell if the current drops, the other coil activates the alarm if the current rises, making it difficult to quietly tamper with the circuit.
Currently not on view
date made
ca 1900
Physical Description
wood (overall material)
brass (overall material)
steel (overall material)
plastic (overall material)
overall: 4 1/8 in x 7 5/8 in x 4 1/4 in; 10.4775 cm x 19.3675 cm x 10.795 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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