Apparatus:
Potentiometer, battery (3 volts), leclanche cell, galvanometer, rheostat, one way key, two way key, voltmeter, connecting wires and sand paper
Theory
When k1of the two way key is closed, the emf E1 of cell is proportional to the balance length l1
when K2 of the two way key is closed, the unknown emf Eof cell is proportional to the balance length l1
Dividing eq. (ii) by eq.(1)
Procedure:
1. Draw the circuit diagram and make the connection accordingly.
2. Measure the emf of leclanche cell with voltmeter.
3. Close the key k of the potentiometer’s main circuit and k of the two way key. Touch the jockey to the potentiometer wire near the end a and the near the end b. The deflection in galvanometer should be reversed at these ends.
4. Find the balance point p when kl is closed and note the balance length fa. p this balance length is proportional to the emf of leclanche cell,
5. Now open the key k and close k of the two ways key. Find the balance point p and note the balance length 1 app. this balance length is proportional to the unknown emf e of the Daniel cell.
6. Find the emf of the deniell cell by formula
7. Take the reading by adjusting the rheostat.
Observation and calculation
emf of leclanche cell = E1=……………volts
Mean emf =………volts
Precautions:
1. The emf of the battery should always be greater that e and e.
2. Measure the balance length from a where the positive terminal of battery, leclanche cell and danial are connected
3. Remove the plugs from key as the readings are noted.
4. Rheostat should be adjusted to minimum resistance.
Viva voce
Q.1: What is potentiometer?
Ans: It is an instrument used to compare and measure the potential accurately because it does not draw any current from the circuit
Q.2: What is the principle of the potentiometer?
Ans: The potential difference across any length of the potentiometer wire of uniform area of cross-section is directly proportional to the length when constant current flows through it.
Q.3: What is the basic difference between a potentiometer and rheostat?
Ans: the potentiometer is a used as voltage control device while rheostat is used as a current control device.
Q.4: Can we use rheostat as potential divider?
Ans: Yes, rheostat can be used as a potential divider.
Q.5: What is a primary cell?
Ans: It is a device which converts the chemical energy into electrical energy.
