It uses a resistor of uniform resistance. By varying the position of the sliding contact on the resistor, we take variable voltage from the resistor, but it is not a variable resistor. In a potentiometer entire input voltage is applied across the whole length of the resistor and output voltage is the voltage drop between the fixed and sliding contact as shown below.
That means, unlike rheostat (variable resistance) where one fixed end and the sliding terminal get connected to the circuit, two fixes end terminals of the resistor of a potentiometer are connected across the source voltage. To adjust the output voltage sliding contact gets moved on the resistor.
There are mainly two types of potentiometer depending on the basic constructional features. One is a linear sliding type, and another is a rotary sliding type. The working principle of a potentiometer for both types is the same.
The rotary type potentiometers are used mainly for obtaining adjustable supply voltage to a part of electronic circuits and electrical circuits. The volume controller of a radio transistor is a popular example of rotary potentiometer where the rotary knob of the potentiometer controls the supply to the amplifier.
This type of potentiometer has two terminal contacts between which a uniform resistance is placed in a semi-circular pattern. The device also has a middle terminal which is connected to the resistance through a sliding contact attached with a rotary knob. By rotating the knob one can move the sliding contact on the semi-circular resistance. The voltage is taken between a resistance end contact and the sliding contact. The potentiometer is also named as the POT in short. POT is also used in substation battery chargers to adjust charging voltage of a battery. There are many more uses of rotary type potentiometer where smooth voltage control is required.
The linear potentiometer is basically the same but only difference is that here instead of rotary movement the sliding contact gets moved on the resistor linearly. Here two ends of a straight resistor are connected across the source voltage. A sliding contact can be slide on the resistor through a track attached along the resistor. The terminal connected to the sliding is connected one end of the output circuit and one of the terminals of the resistor is connected to the other end of the output circuit.
This type of potentiometer is mainly used to measure the voltage across a branch of a circuit, for measuring the internal resistance of a battery cell, for comparing a battery cell with a standard cell and in our daily life it is commonly used in the equaliser of a music system, sounds mixing system etc.