Clockface pole-changer 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 distinctive-looking relay was known as a “clockface pole-changer” and was designed for use on polar duplex circuits powered by wet-cell batteries. Duplex circuits allowed for simultaneous transmission of two messages on the same line. The circuit design was such that operators could not tell which pole of the battery at a distant station might be on line and mismatched polarities interfered with reception. The clockface pole-changer could switch the polarity of the battery at the receiving station to eliminate the problem.
Currently not on view
Edison, Thomas Alva
Physical Description
wood (overall material)
brass (overall material)
glass (overall material)
metal (overall material)
plastic (overall material)
overall: 5 in x 8 1/2 in x 4 1/2 in; 12.7 cm x 21.59 cm x 11.43 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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