Internal Resistance Of a Given Primary Cell Using a Potentiometer

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To determine the internal resistance of a given primary cell using a potentiometer.


A potentiometer, a battery (or battery eliminator), two one-way keys, an id resistance, a galvanometer, a high resistance box, a fractional resistance box, an ammeter,  voltmeter, a cell (say Leclanche cell), a jockey, a set square, connecting wires and sandpaper.




When Key K2 is open and K1 is closed,
Let null point be obtained at a distance l1 from A
Therefore E = Kl1 –> (1)
When Key K2 is closed and K1 is open,
Let null point be obtained at a distance l2 from A
Therefore V = Kl2 –> (2)
(1) divided by (2)
E/V = l1/l2
(S+r)/S  = l1/l2
r = S*(l1-l2)/l2

Where S : the shunt resistance in parallel with given cell.
l1 and l2 : Balancing lengths without and with shunt respectively.
r : internal resistance of the cell.


  1. Make the connections as shown in circuit diagram.
  2. Clean the ends of the connecting wires with sandpaper and make tight connections. Tighten the plugs of the resistance box.
  3. Check the E.M.F. of the battery and cell and see that E.M.F. of the battery is more than that of the given cell otherwise null or balance point won’t be obtained. (E’ > E)
  4. Take maximum current from the battery, making rheostat resistance small.
  5. Insert the plug in Key K1 and adjust the rheostat so that a null point is obtained or the fourth wire of potentiometer.
  6. Insert the 2000 Ohm plug in its position in resistance box and obtain a null point by slightly adjusting the jockey.
  7. Measure the balancing length L1.
  8. Take out the 2000 Ohm plug from the resistance box. Introduce the plugs in Key K1, as well as in Key K2. Take act a small resistance from the resistance box R connected in parallel with the cell. Slide the jockey along the potentiometer wire and obtain a null point. Insert the 2000 Ohm plug back in its position in R.B. and make a further adjustment for a sharp null point.
  9. Insert the 2000 Ohm plug back in its position in R.B. and make a further adjustment for a sharp null point.
  10. Measure the balancing length l2 from end P.
  11. Remove the plugs keys K1 and K2. Wait for some time and respect the activity for the same current.
  12. Record your observations.


  • Range of voltmeter
  •  Least count of voltmeter
  •  E.M.F. of battery (or battery eliminator)
  •  E.M.F. of cell





The internal resistance of the given cell is…………… ohm.



  1. The e.m.f. of the battery should be greater than that of the cell.
  2. For one set of observation, the ammeter reading should remain constant.
  3. Current should be passed for a short time only while finding the null point.



You can also get Class XII Practicals on BiologyPhysics, and Physical Education.

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