clock face pole-changer telegraph relay
notes: label: Description
content: 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.
label: Description
content: 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. This particular relay has a resistance of 8 ohms. On 24 October 1905, "C" of Western Union inspected the relay and marked it "Ok."
label: Location
content: Currently not on view
setName: label: See more items in
content: Work and Industry: Electricity
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content: Communications
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content: Telegraph Relays & Repeaters
creditLine: label: Credit Line
content: from Western Union Corporation
dataSource: label: Data Source
content: National Museum of American History
identifier: label: ID Number
content: EM.332687
label: accession number
content: 294351
label: catalog number
content: 332687
objectType: label: Object Name
content: pole changer relay
label: Object Name
content: relay
label: Object Name
content: telegraph relay
physicalDescription: label: Physical Description
content: wood (overall material)
label: Physical Description
content: brass (overall material)
label: Physical Description
content: slate (overall material)
label: Physical Description
content: steel (overall material)
label: Physical Description
content: glass (overall material)
label: Measurements
content: overall: 5 in x 8 1/4 in x 7 7/8 in; 12.7 cm x 20.955 cm x 20.0025 cm