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The amount of power flowing through a high-voltage transmission line is proportional to its voltage and the current flowing through it. To increase power flow without modifying the line to support increased voltage, the current must be increased. There is a limit to this, however, since increasing the current causes the conductor temperature to increase and, hence, the conductors to elongate and sag. In this paper, a method to simply and inexpensively measure the amount of conductor sag and, through simple calculation, the average conductor core temperature is described, and the results of a field test are summarized. The method involves attaching two ends of a grounded wire of high electrical resistance to an appropriate location on each of two transmission-line towers and measuring the current induced on the wire by the nearby transmission-line conductors. Information from this measurement is a critical input to any method for dynamically rating transmission lines.