A 12-volt dimmer switch is used to control light levels. You can make the light fixture give off a brighter light or dim the light by degrees using a dimmer switch. The light level is determined by how many watts the light fixture receives at any given time. When you use the dimmer switch to brighten the light, more watts are being used. As you dim the lights, fewer watts are reaching the fixture.
Watts are a unit of measure that measure power, or wattage. Wattage is determined by two factors: voltage and amperage. Multiply the two factors and the result is the number of watts received by the fixture, or bulb. A dimmer switch, then, allows you to control the number of watts the fixture receives.
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The variable resistor used in a 12-volt dimmer switch is more mechanical than electrical. A variable resistor is a piece of electrically resistant material within the dimmer switch. At one end of the resistant material is a stationary arm. Within the switch is also a movable arm. These two arms work together to control the amount of voltage that reaches the bulb. When you raise the light level, the movable arm decreases the electrical resistance that in turn increases the voltage, making the light brighter. Dim the light and the movable arm, moving away from the stationary arm, increases the electrical resistance and so decreases the voltage.
Alternative to the variable resistor is the turn-on value. This method employs a system by which the resistors control the electrical flow by very quickly turning the circuit on and off. The circuit to the light bulb will go on and off many times every second. The "disruption" of electricity goes unnoticed because it happens too fast for the human eye to perceive.
When you turn your dimmer switch to brighten the light, the circuit will switch off then on again at the level indicated by the dimmer switch itself. Conversely, if the switch is set to a lower light level, the amount of times the light turns on and off is lessened, because not as much electrical flow, or voltage, is called for.