Polar accentuation was originally developed by Thomas Edison in his 1874 quadruplex telegraph – this accustomed 4 signals to be beatific forth a brace of lines, 2 in anniversary direction. Sending a arresting in anniversary administration had already been able earlier, and Edison begin that by accumulation amplitude and appearance accentuation (i.e., by arctic modulation), he could bifold this to 4 signals – hence, quadruplex.
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