Variable resistor controlling a bulb

 Here we took a rheostat that has 3 terminals and only connected 2 to make a variable resistor. Above, the variable resistor is set to zero so that all of the potential difference is across the bulb. But below, the variable resistor is on its highest resistance setting. You can tell because the current reading in the left hand meter has gone down. There is still a potential difference across the bulb, shown by the other meter. The bulb is off so you would have though that the potential difference would be zero. That's the problem with a variable resistor as a way of controlling a bulb. No matter what you do, the potential difference of the bulb will never go down to zero. If you want that when, for example, doing IV characteristics, you have to connect up all 3 terminals to wire the rheostat up as a potentiometer.