The paper describes the uses to which an additional conductor on an overhead transmission line can be put if the conductor is lightly insulated at each support or embedded in insulation within the earth wire and earthed at the ends. One such use is to provide a cheap and reliable communication channel; another is to employ the induced currents from the main conductors in new types of discriminating protection, and in the measurement of fault locations on straight or teed feeders. Formulae are derived for calculating the current in the signal conductor under various conditions of operation and fault, and the calculated values are compared with those observed.