The circuit for a dimmer switch is very simple. The light bulb acts like a resistor - just put a variable inductor in series with the light bulb and apply an AC voltage.

If the inductance is increased what happens to the light bulb?

1. It gets brighter
2. It gets dimmer
3. It stays the same

Increasing the inductance increases the reactance of the inductor. The overall resistance (the impedance) of the circuit goes up, so the current goes down and the bulb gets dimmer.

Let's play with the frequency and see if anything happens (you can't do this with a dimmer switch at home). Turn down the frequency...what happens to the light bulb?

1. It gets brighter
2. It gets dimmer
3. It stays the same

Decreasing the frequency decreases the inductive reactance and decreases the impedance. The current goes up and the bulb gets brighter.

Variable resistors are easy to come by. Putting a variable resistor in series with the bulb would also give you a dimmer switch...increasing the resistance would decrease the current and the light would get dimmer. Does using a variable inductor offer any advantage over a variable resistor?

1. No
2. Yes (what?)

Using a resistor to reduce current through the lamp means current is passing through the resistor and energy is being dissipated as heat. With the inductor the energy is stored and then given back to the circuit....so, the dimmer switch costs less to operate with the variable inductor.