Telephone line testing circuit - uses reference resistors in bridge circuit to determine resistance between lines and to earth

摘要

The telephone line testing system comprises several test circuits temporarily connected to the line, to test the resistance between lines and the resistance to earth. The line is connected in one side of a bridge circuit, while reference resistances are connected to the second side. A comparison device comprising a differential amplifier is connected to the diagonal of the bridge. The output of the amplifier is then connected to a warning device indicating whether a satisfactory reading has been obtained.