Abstract: A loop closure detector for a two-wire telephone line comprises a four-arm resistive bridge, in which the four arms are constituted respectively by a first battery feed resistor, the two-wire line in series with a second battery feed resistor, and two resistors forming a potential divider across the battery voltage which is coupled across one diagonal of the bridge. A voltage sensing circuit, constituted by a comparator or a light emitting diode, is connected across the other diagonal of the bridge to provide an output representing a loop closure state of the line. A ring-ground detector is also described using a similar bridge and voltage sensing arrangements.