To compare EMF of two given primary cells using potentiometer

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 .

Theory

When we keep key (K₁) closed and (K₂) open , let the null point found be l_AJ₁

E₁=Kl_Aj₁ (1)

When we keep K₁ open and K₂ closed , let null point obtained by l_AJ₂ .

E₂=Kl_Aj₂ (2)

(1)/ (2)

E₁ / E₂ = Kl_AJ₁ / Kl_Aj₂

E₁/E₂ =l₁/l₂

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

Procedure

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

Result

The ratio of emf E₁/E₂ ~0.87

Precaution

• • 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 E_x 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.

Diagram

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 .

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