Potentiometer

# “POTENTIOMETER”:-
The potentiometer is an instrument which is used to measure exact potential difference between two point in an electric circuit or to measure the EMF of a cell. With the help of this instrument the internal resistance of a cell can also be determined.

* PRINCIPLE OF POTENTIOMETER:

In above figure shows the principle of potentiometer. In which we can from primary circuit first, for this positive terminal of accumulator cell C is connected with the one end of potentiometer wire A and its negative terminal is connected with the other end of potentiometer wire B through rheostat (Rh) and key (K).
                       For the secondary circuit the positive terminal of experimental cell is joined with first end of a potentiometer wire and its negative terminal is joined with the sliding jockey (J) through galvanometer (G).
              On pressing the key the current flowing through the potentiometer wire by the accumulator cell from A to B.
Thus, the potential at end A is maximum and the potential at other end B is minimum. Jockey (J) is placed at end A and it slides smoothly on potentiometer wire towords end B till the deflection produced in the galvanometer is to be zero. Thus, the point D is obtained on potentiometer wire. This is balancing point and the length AD is called balancing length.

“The potential difference across the balancing length of potentiometer wire is equal to the EMF of an experimental cell”.

Let,
the length of potentiometer wire, AB = L cm
Balancing length, AD = r cm

and
resistance of per unit (1cm) length = x ohm

Hence, resistance of wire AB is R = L.x ohm
And
Resistance of wire AD is R’ = r.x ohm

If  I  ampere current flows through the potentiometer wire then by ohm's law-

Potential difference across the potentiometer wire AB is V = R.I
                     V = (L.x)I -------(1)
And
P.D across balancing length AD is V' = R'.I
                V’ = (r.x)I

But V’ = E (EMF of experimental cell)
Hence, E = r.x I  --------(2)

Dividing eqn(2) by eqn(1)

E/V  =  rx I / Lx I

E = (V/L)×r

Here the quantity (V/L) is a constant, this is known as the potential gradient (K) of a potentiometer wire.

Hence, E = K.r

Or, EMF of a cell = potential gradient ×
                                   Balancing length


# SENSITIVITY OF POTENTIOMETER:
It is defined as the length of potentiometer wire which produces unit potential difference. It is represent by S.

S = (length of wire) / (P.D across wire)

S = L/V

* SOME WAYS TO INCREASE THE SENSITIVITY:
1) The length of potentiometer wire must be more, that's why the length of potentiometer wire is kept along

2) The EMF of a cell used in primary circuit must be less but must be more than the EMF of experimental cell.

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