 Image Number 8 for United States Patent #4057783.
A device for transmitting the condition of a switch comprising a first assembly consisting of a switch, a first pair of coils serially connected when the switch is closed, and a second assembly consisting of a second pair of coils, magnetically coupled to each other. A first coil of the second pair generates an alternating magnetic field. A detector is coupled to the second coil of the second pair for producing different outputs respectively when the second coil of the second pair is excited only from the field of the first coil of the second pair and when the second coil of the second pair is also excited from the second coil of the first pair.
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