Why are coils of resistance box made of doubled wire?

To make a coil for the resistance box, the wire is first doubled on itself and then wounded on a plastic bobbin. By doing so current flows in one direction in half portion of the wire and in the opposite direction in the other half portion. Consequently, nearly zero magnetic field is produced by the current and therefore, such a coil has a negligible inductance. No induction effects will be observed on making or breaking the circuit containing such a resistance box.