Potentiometer:

Potentiometer valve position indicators was show in the below figure gives an accurate indication of position by the travel of a valve or control rod. An extension is physically attached to a variable resistor. As the extension moves up or down then the resistance of the attached circuit changes and changing the amount of current flow in the circuit.  An amount of current is proportional to the valve position.

Figure: Potentiometer Valve Position Indicator

Potentiometer valve position indicator failures are generally electrical in nature. An electrical open or short would cause the indication to fail at one extreme or the other. Erratic indicated valve position occurs if an increase or decrease in the potentiometer resistance occurs.