In telecommunication, a neutral direct-current telegraph system (single-current system, single-current transmission system, single-Morse system) is a telegraph system in which (a) current flows during marking intervals and no current flows during spacing intervals for the transmission of signals over a line, and (b) the direction of current flow is immaterial.
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:This article incorporates:public domain material from the General Services Administration document: "Federal Standard 1037C". (in support of MIL-STD-188)