Gray telegraph relay 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 relay was constructed early in 1867 by noted electrical inventor Elisha Gray to accompany a patent application. He received US Patent 69,424 later that year for an improved telegraph relay. Gray's idea was to use a second set of electromagnet coils rather than a spring to retract the armature after a signal pulse. He wrote that the pulse's strength could vary so much that a relay's spring needed constant adjustment. His design compensated automatically for the variation in pulse strength.
Currently not on view
date made
Gray, Elisha
Physical Description
wood (overall material)
brass (overall material)
plastic (overall material)
cloth (overall material)
overall: 4 1/2 in x 11 1/2 in x 5 3/8 in; 11.43 cm x 29.21 cm x 13.6525 cm
ID Number
catalog number
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patent number
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Work and Industry: Electricity
Telegraph Relays & Repeaters
Data Source
National Museum of American History


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