I've just read this http://www.allaboutcircuits.com/vol_1/chpt_6/1.html and i understand what a potentiometer is. My doubt is: why not a two way potentiometer? It should work with the wiper connected to the source, or maybe i didn't understand at all.

F.

 

When you connect the center terminal of a potentiometer to one end of the potentiometer it is called a rheostat, and it works as a variable resistor with only 2 available terminals.