8 V Fig 18.10: Circuit diagrams for a potential divider
it drops in the ratio of the resistances. For example, in the circuit shown in Fig 18.10 there is a 1 resistance followed by a 2 resistance.This means that of the potential
1 3
C A B
drops across the first resistor and across the second resistor. A voltmeter across resistor A shows a potential of 4 V, while across B is 8 V.
2 3
A second circuit could be set up in parallel to the 1 resistor as shown, this would now operate off a 4 V emf. By varying the ratio of the resistors we can vary the division of the potential for the second circuit. The set-up described above involved dividing the potential in a fixed ratio.
A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider.
B C C Input voltage A B Output voltage
Two forms of rheostat showing terminals
A Fig 18.11: Potentiometer
A rheostat can be used as a potentiometer. Place the rheostat in a circuit with a fixed power supply, with the contacts at either end of the full length of wire (points A and B). A second circuit contains the rheostat, with the contacts at A and the movable contact C. By varying the position of the movable contact, we are varying the ratio of length (and therefore the resistances) from A to C and C to B.When the movable contact is at A, the voltage across the second circuit is zero; as it is moved across to B the voltage increases to the same size as the input voltage.
