Final answer:
The function of a headlamp dimmer switch is to switch headlamps between separate low- and high-beam circuits, allowing the driver to change the level of brightness as needed without adjusting voltage or applying resistance.
Step-by-step explanation:
The function of a headlamp dimmer switch is to switch headlamps between separate low- and high-beam circuits. It does not apply resistance to the headlamps to reduce brightness, nor does it reduce or increase voltage for low-beam or high-beam operation, respectively. Instead, it simply changes the circuit path for the electrical current to flow through either the high beam filament or the low beam filament within the headlamp.
This mechanism is akin to having two separate lamps within one headlamp casing, each designed for a different level of brightness—one for standard visibility and the other for enhanced visibility under specific conditions such as on a poorly lit road at night. The low beam provides sufficient illumination for ordinary driving conditions, while the high beam is used when there are no oncoming vehicles to avoid dazzling other drivers.
When a vehicle's headlamp is cold, it initially has a lower resistance and draws more current. As the filament in the headlamp heats up, its resistance increases, which is a common behavior of resistive materials (detailed in the topic of Resistance and Resistivity). This influences the current and power usage initial upon turning on the headlamp.