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Potentiometer is a device mainly used to measure the emf of a given cell and to compare emfs of cells. It is also used to measure the internal resistance of a given cell.

Circuit diagram

Potentiometer consists of a long resistive wire AB of length L (about 6 m to 10 m long) made up of mangnine or constantan and a battery of known voltage e and internal resistance r called supplier battery or driver cell (Fig. 9). Connection of these two forms primary circuit.
One terminal of another cell (whose emf E is to be measured) is connected at one end of the main circuit and the other terminal at any point on the resistive wire through a galvanometer G. This forms the secondary circuit. Other details are as follows:
Fig. 9
J = Jockey
K = Key
R = Resistance of potentiometer wire
ρ = Specific resistance of potentiometer wire
Rh = Variable resistance which controls the current through the wire AB
  • The specific resistance (ρ) of potentiometer wire must be high but its temperature coefficient of resistance (α) must be low.
  • All higher potential points (terminals) of primary and secondary circuits must be connected together at point A and all lower potential points must be connected to point B or jockey.
  • The value of known potential difference must be greater than the value of unknown potential difference to be measured.
  • The potential gradient must remain constant. For this, the current in the primary circuit must remain constant and the jockey must not be slided in contact with the wire.
  • The diameter of potentiometer wire must be uniform everywhere.

Potential gradient (x)

The potential difference (or fall in potential) per unit length of wire is called potential gradient, i.e., 75309.png, where 75317.png
So 75323.png
Potential gradient directly depends upon
  • the resistance per unit length (R/L) of potentiometer wire.
  • the radius of potentiometer wire (i.e., area of cross section).
  • the specific resistance of the material of potentiometer wire (i.e., ρ).
  • the current flowing through potentiometer wire (i).
    Potential gradient indirectly depends upon
  • the emf of battery in the primary circuit (i.e., e)
  • the resistance of rheostat in the primary circuit (i.e., Rh)

Working of potentiometer

Suppose jockey is made to touch a point J on wire then potential difference between A and J will be V = xl. At this length (l), two potential differences are obtained: V due to battery e and E due to unknown cell (Fig. 10).
Fig. 10
  • If V > E, then current will flow in galvanometer circuit in one direction
  • If V < E, then current will flow in galvanometer circuit in opposite direction
  • If V = E, then no current will flow in galvanometer circuit this condition to known as null deflection position, length l is known as balancing length.
    In balanced condition, E = xl
    or 75329.png
    If V is constant, then L  l ⇒ 75335.png

Standardization of potentiometer

The process of determining potential gradient experimentally is known as standardization of potentiometer.
Fig. 11
Let the balancing length for the standard emf E0 is l0 then by the principle of potentiometer E0 = xl0
⇒ x = E0/l0

Sensitivity of potentiometer

A potentiometer is said to be more sensitive if it measures a small potential difference more accurately.
The sensitivity of potentiometer is assessed by its potential gradient. The sensitivity is inversely proportional to the potential gradient.
In order to increase the sensitivity of potentiometer,
  • the resistance in primary circuit will have to be decreased.
  • the length of potentiometer wire will have to be increased so that the length may be measured more accurately.
Table 2 Difference between voltmeter and potentiometer
Its resistance is high but finite.
Its resistance is infinite.
It draws some current from source of emf.
It does not draw any current from the source of unknown emf.
The potential difference measured by it is lesser than the actual potential difference.
The potential difference measured by it is equal to actual potential difference.
Its sensitivity is low.
Its sensitivity is high.
It is a versatile instrument.
It measures only emf or potential difference.
It is based on deflection method.
It is based on zero deflection method.

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