100. Mutual Inductance

The secondary of a 1:1 transformer is hooked through a switch to a light bulb. The primary is hooked in series with another light bulb to the 120 V. line. When the switch is open neither bulb lights; the inductive reactance of the primary keeps the current small in the primary circuit. But when the switch is closed, both bulbs light! Now the secondary circuit is drawing current, so the mutual inductance of the two coils reduces the effective inductive reactance of the primary and enough current flows in the primary circuit to light its bulb.

Alternatively, conservation of energy tells us that the primary current must increase so that energy can be delivered to the secondary bulb through the magnetic fields linking the coils.