Williams-Farmer duplex telegraph patent model

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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 the model that inventor Moses Farmer of Salem submitted along with the application that resulted in US Patent #160,581. Issued in 1875 the patent for an "improvement in Duplex-Telegraph Apparatus." A duplex telegraph sends two messages simultaneously through a single wire. Farmer found that on long lines a static charge built up and interfered with the operation of the circuit. He designed this apparatus with a relay, a duplex transmitter, a rheostat and special induction coils to counter that static charge. It is unclear how much of this device Farmer himself actually constructed since the relay bears the marking of Charles Williams, Jr., a noted telegraph and telephone maker in Boston.
Currently not on view
Date made
associated person
Farmer, Moses G.
Woodman, Asa F.
Charles Williams, Jr.
Physical Description
wood (overall material)
metal (overall material)
cloth (overall material)
ivory (overall material)
overall: 5 in x 12 in x 12 in; 12.7 cm x 30.48 cm x 30.48 cm
ID Number
catalog number
accession number
patent number
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Work and Industry: Electricity
Telegraph Relays & Repeaters
Data Source
National Museum of American History


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