Expert: James J. Kovalcin - 9/20/2005

Question
Could you exlain how moving a slider on a variable resistor makes a lamp dimmer or brighter please?

Answer
It really does matter HOW the slide wire resistor is attached to the light bulb. If the light bulb is connected in a simple series circuit with a battery and the slide wire resistor, sliding the wire so as to increase the resistance of the adjustable wire will decrease the current flow in the circuit, thus making the light bulb dimmer.
If the battery is connected across the endpoints of the slide wire resistor and the light bulb is connected between either end of the slide wire resistor and the slide itself, sliding the slide connection will change the voltage across the light bulb. as the resistance of teh the slide wire in paarallel with the light decreased, the bulb will become dimmer. In this configuration the slide wire resistor is being used as a "voltage divider".