POTENTIONMETER | GRADE 12 | IMPORTANT TOPIC

 Potential difference is usually measured by an instrument

called a voltmeter. The voltmeter is connected across the two

points in a circuit between which potential difference is to be

sured . It is necessary that the resistance of the voltmeter

he large compared to the circuit resistance across which the

voltmeter is Connected. Otherwise an appreciable current will

Ch ow through the voltmeter which will alter the circuit current and

a the potential difference to be measured. Thus the voltmeter can

read the correct potential difference only when it does not draw

n any Current from the circuit across which it is connected. An

ideal voltmeter would have an infinite resistance.

A However, there are Some potential measuring instruments

h such as digital voltmeter and cathode ray oscilloscope which

ie practically do not draw any current from the circuit because of

their large resistance and are thus very accurate potential

measuring instruments. But these instruments are very

expensive and are difficult to use. A very simple instrument

which can measure and compare potential differences

accurately is a potentiometer.

A potentiometer consists of a resistor R in the form of a wire

on which a terminal C can slide (Fig. 13.24 a). The resistance

between A and C can be varied from 0 to R as the sliding

contact C is moved from A to B. If a battery of emf  is

connected across R (Fig. 13.24 b), the current flowing

through it is I = E/R. If we represent the resistance between A

and C by r, the potential drop between these points with be

rl=r E/R. Thus as Cis moved from A to B, r varies from 0 to R

and the potential drop between A and C changes from 0 to E.

Such an arrangement also known as potential divider can be

used to measure the unknown emf of a source by using the

circuit shown in Fig. 13.25. Here R is in the form of a straight

wire of uniform area of cross section. A source of potential,

say a cell whose emf , is to be measured, is connected

between A and the sliding contact C through galvanometer

G. It should be noted that the positive terminal of E, and that of

the potential divider are connected to the same point A. If, in the loop AGCA, the point C and the negative terminal of E, an

at the same potential then the two 

galvanometer will be at the same potential and no

wire. Therefore, the unknown emf is also given by

terminals

flow through the galvanometer. Therefore, to measure

potential E, the position of C i is so adjusted that th

galvanometer shows no deflection. Under this condition, th

emf E, of the cell is equal to the potential difference between

cross section, the resistance is proportional to the length of the

and C whose value Er/ Ris known. In case of a wire of uniform

E,-E E

of

E,=E and E,=E

Dividing these two equations, we get

lengths.

where L is the total length of the wire AB and is its length from

A to C, after C has been adjusted for no deflection. As the

maximum potential that can be obtained between A and Cis &

so the unknown emf E, should not exceed this value, often with

the null condition will not be obtained. It can be seen that the

unknown emf E, is determined when no current is drawn from

and therefore, potentiometer is one of the most accurate

methods for measuring potential.

Current W

The method for measuring the emf of a cell as described

above can be used to compare the emfs E, and E, of two

cells. The balancing lengths , and, are found separately for

the two cells. Then,

E,

(13.28)

So the ratio of the emfs is equal to ratio of the balancing

(13.29)