Bunnell polar 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.
Relays required adjustment to compensate for changing conditions on the line. Older designs used adjusting screws and springs to change the position of the coils and the sensitivity of the armature–a tricky task. Polarized or “polar” relays like this unit made by J. H. Bunnell & Co., used a special coil-mount to eliminate the springs and coil adjusters. The coils were mounted to one end of a permanent magnet and the armature connected to the other end, so the coils and the armature had opposite magnetic polarity. Without an incoming signal the armature, attracted equally by both coils, sat balanced between them. The coils were wound in such a way that an incoming signal reinforced the magnetic field of one coil and reduced the field in the other, attracting the armature to one side to make contact and activate the relay.
Currently not on view
J. H. Bunnell & Co.
Physical Description
wood (overall material)
metal (overall material)
brass (overall material)
plastic (overall material)
relay: 5 1/4 in x 8 1/2 in x 4 3/4 in; 13.335 cm x 21.59 cm x 12.065 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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