Hall duplex 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.
This duplex relay was made by Thomas Hall of Boston and used in experiments by Moses Farmer. Duplex relays operated in a system designed to send two messages simultaneously over the same wire. Someone, Farmer himself perhaps, removed the electromagnet coils prior to the donation of the relay in 1899.
Currently not on view
date made
ca 1875
Farmer, Moses G.
Hall, Thomas
Physical Description
wood (overall material)
brass (overall material)
steel (overall material)
overall: 4 in x 9 3/4 in x 5 3/4 in; 10.16 cm x 24.765 cm x 14.605 cm
ID Number
catalog number
accession number
Credit Line
from Sarah J. Farmer
See more items in
Work and Industry: Electricity
Telegraph Relays & Repeaters
Data Source
National Museum of American History


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