It is consist of long uniform wire of manganin or constantan fixed parallel to one another connected in series by copper strips. A meter is fixed parallel to wires (each one meter) and two ends are fixed with binding screws. The potentiometer jockey can be touched to wire at any location. A battery E connected across A and B sends the current through the wire which is kept constant by using rheostat Rh.
Principle: The working of potentiometer is based upon the fact that fall of the potential across any portion of the wire is directly proportional to the length of the wire provided wire has uniform cross section area and constant current flowing through it.
Consider a length of wire l and current flowing through it is I. Cross section area is A and ρ is Resistivity and R is resistance. Then we can say potential across the wire length is
if I and A is constant then
V/l =K = constant, which also called potential gradient that is fall of potential per unit length of wire.
It is called potentiometer because it is used to measure potential difference.