Dimmer switch. This is the inside of the switch. The working components of a dimmer switch consist of a rheostat (variable resistor, lower left). Turning a circular knob increases the resistance in the rheostat, thereby lowering the amount of voltage available to the light and dimming it. This is a somewhat innefficient solution, since a lot of power is lost as heat energy from the resistor. More efficient modern systems rapidly switch the electrical supply on and off to effect the same result.