To ensure a high coefficient of coupling, all fixed transformers are usually wound on a(n) _____________.

ANS:C - common core

Common core means they are wound on the same core. To increase efficiency of coupling the primary and secondary windings are usually wound around the same core. Transformers with high coupling efficiency have reduced leakage flux and inductance between primaries and secondaries, which gives less voltage drop and better regulation when load is applied to the secondary. It also ensures that power dissipated in each set of windings is approximately equal.