Consider a transformer where the primary coil (with N₁ turns) is connected to a battery. The secondary coil (with N₂ turns) is connected to a resistor.

At first there is a switch on the secondary side. When the switch is closed what is the current through the resistor?

1. zero - there is no current
2. constant and equal to N₁/N₂ times the current in the primary
3. constant and equal to N₂/N₁ times the current in the primary
4. there is a brief current and then the current drops to zero

The switch is then moved to the primary side of the circuit. When the switch is closed what is the current through the resistor?

1. zero - there is no current
2. constant and equal to N₁/N₂ times the current in the primary
3. constant and equal to N₂/N₁ times the current in the primary
4. there is a brief current and then the current drops to zero

Transformers require a varying flux to work. They are therefore perfect for AC power, but do not work at all for DC power, which would keep the flux constant. The ease with which voltage and current can be tranformed in an AC circuit is a large part of the reason AC power, rather than DC, is distributed by the power companies.