Power control for the system can be achieved by a variety of methods. Many latter day units feature thyristor power control which works by means of a full wave AC (alternating current) drive varying the primary voltage to the input transformer. More traditional methods include three phase variacs (autotransformer) or motorised Brentford type voltage regulators to control the input voltage. Another very popular method was to use a two part tank coil with a primary and secondary winding separated by an air gap. Power control was affected by varying the magnetic coupling of the two coils by physically moving them relative to each other.Rudnev, p. 478.
