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Home Physics Class 12 Physics To compare the EMF of two given primary cells using a potentiometer
Class 12 Physics Lab Experiments

To compare the EMF of two given primary cells using a potentiometer


To compare the E.M.F.’s of two given primary cells using a potentiometer.


A potentiometer, a laclanche cell, a Daniel cell, an ammeter, a voltmeter (0-5v), a galvanometer, a battery (or battery eliminator), a rheostat of law resistance, a resistance box, a one-way key, a two-way key, a jockey, a set square, connecting wires and a piece of sand paper.

Circuit Diagram


Range of voltmeter: 5V

Least count of voltmeter: 0.05V

E.M.F. of battery E: 3V

E.M.F. of Laclanche Cell, E1: 1.45V

E.M.F. of Daniel Cell, E2: 1.125V

Table for Lengths:

S.No. Balancing length when E 1(Leclanche Cell) is in the circuit (cm) (l1) Balancing length when E 2(Leclanche Cell) is in the circuit (cm) (l2) Ratio E1/E2 = I1/I2
1 558 437 558/437 = 1.277
2 789 617 1.278
3 848 670 1.266
4 893 706 1.265
5 662 521 1.270


Mean E1/E2 = 1.271 (Unit less)



  1. The connections should be neat, clean & tight.
  2. The positive poles of the battery E and cells E 1 and E 2 should all be connected to the terminals at the zero of the wires.
  3. The jockey should not be rubbed along the wire. It should touch the wire gently.

Sources of Error:

  1. The auxiliary battery may not be fully charged.
  2. The potentiometer wire may not be of uniform cross-section and material density throughout its length.
  3. Heating of potentiometer wire by current, may introduce some error.