Last spring break my parents and I went to Oregon to visit my brothers and go snowboarding. On one of the days we went to Mt. Hood and it turned out that is was some sort of family 'western' day and they had a mechanical bull inside the lodge that anyone could ride. So of course we got my brother Haakon to take a turn. In the right upper corner of the picture there is a guy standing at a red podium, this guy was controlling the bull. For little kids, he would make the bull go slow so it would be easier for them. He could control it by hand or just put it on a random setting that would make the bull do random movements. I realize looking back on this picture that the wiring of the controls must of had a variable resistor in order to allow the bull to move faster or slower. A variable resistor increases and decreases the length of the circuit in order to proportionately increase or decrease the the current and thus the speed. Resistance equals resistivity times length over cross sectional area, R= p(L/A). Resistance is directly proportional to resistivity and length, meaning that the larger the length the larger the resistance and vice versa. Resistance is indirectly proportional to cross sectional area. So, when the variable resistor increases the length, it is increasing the resistance and thus decreasing the current, so the bull will move slower. If the length is decreased, the resistance will decrease as well and there will be more current and the bull will move faster.
