william stanley, jr. designed one of the first practical devices to transfer ac power efficiently between isolated circuits. using pairs of coils wound on a common iron core, his design, called an induction coil, was an early precursor of the modern transformer. the system used today was devised in the late nineteenth century, largely by nikola tesla. contributions were also made by george westinghouse, lucien gaulard, john dixon gibbs,wilhelm siemens and oliver shallenger. ac systems overcame the limitations of the direct current system used by thomas edison to distribute electricity efficiently over long distances.