Unipolar Telegraphy

a direct-current telegraphic method in which the coded signals consist of unipolar current pulses, for example, of a positive polarity, and of intervals in which a flow of current is absent. A start-stop multivibrator telegraph apparatus is used as a transmitter, and a nonpolar electromagnet serves as a receiver. In unipolar telegraphy in which signals are transmitted over open-wire lines, the electromagnet is more sensitive to current leakage and to interference that are induced in the line that it would be in a bipolar system. These effects are especially noticeable in damp weather. Thus, unipolar telegraphy is used with open-wire lines only if the lines are comparatively short—from 200 to 350 km. It is also used in lines that connect telegraph apparatus to telegraph offices in cities.