To compare EMF of two given primary cells using potentiometer

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Aim : To compare EMF of two given primary cells using potentiometer

Apparatus : potentiometer , a leclanche cell , a daniel cell , an ammeter , a voltmeter , a galvanometer , a battery , (battery eleminator), a rheostat , of low resistance , a resistance box , a one way key , a two way key , a jockey , a set square , connecting wire , a piece of sand paper .


When we keep key (K1) closed and (K2) open , let the null point found be lAJ1

E1=KlAj1 (1)

When we keep K1 open and K2 closed , let null point obtained by lAJ2 .

E2=KlAj2 (2)

(1)/ (2)

E1 / E2 = KlAJ1 / KlAj2

E1/E2 =l1/l2

Where E1 and E2 are the emf of two given cells .


  1. Arrange the apparatus as shown in the circuit diagram .
  2. Connect the positive poles of the cells to the terminal and the negative poles to the terminal a and b of the two way key .
  3. Insert the plug in the key K and also in between the terminals a and c of the two way key .
  4. Slide the jockey gently over the potentiometer wires until you obtain a point of no deflection .
  5. Note the length l1 at the point.
  6. Repeat this with E2 by disconnecting E1 and inserting plug into gap a and c of two way key .
  7. Record l2 at null point .
  8. Repeat this different resistance .

Observation table

Balancing Lengths E1 / E2 = lAJ1 / lAj2
L1 for cell E1 (cm) L2 for cell E2 (cm)
327 376 0.86
323.5 371 0.87
321.5 369 0.87
312.5 352.5 0.88


The ratio of emf E1/E2 ~0.87


      1. The connections should be neat, clean and tight.
      2. The plugs should be introduced in the keys only when the observations taken.
      3. The positive poles of the battery E and cells Ex and E2 should, all be connected to terminal at the zero of the wires.
      4. The jockey key should not be rubbed along the wire. It should touch the
      5. The e.m.f. of the battery should be greater than the e.m.f.’s of the either cells.
      6. Some high resistance plug should always be taken out from resistant the jockey is moved along the wire.

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. throughout its length.
  3. End resistances may not be zero.


compare emf cell

VIVA Question

What is emf of a cell?

Electromotive force of a cell is the potential difference across the terminals of the cells when it is in open circuit i.e. no current is drawn from the circuit .

What is potentiometer ?

An instrument to measure potentiometer difference or emf of a cell .

What is the principle of potentiometer ?

For a constant current , fall of potentiometer along a uniform wire is directly proportional to its length .

What is preferred for making potentiometer wire ?

Magnum , due to its low temperature coefficient of resistance and high resistivity .

What will you conclude if the deflection of the galvanometer is in the same direction ?

The positive terminals aren’t connected at one point

Potential difference between wires ends is less than emf of cells to be measured .

Emf of driving cells is less than that of cells to be measured .

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


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