Originally Posted by kontrol
Why the other guy in his article use a 2k then?
Because with an LM317 it isn't the value of either resistor that sets the voltage, it's the ratio between them. I was using a 220 ohm fixed resistor, so a 2.5 kohm pot allowed for a range of ~11 V.
With a potentiometer wired in series, all you're doing is sinking some of the current into the pot and dissipating it as heat. Whatever is "left over" powers the motor.