Class 12 Physics practical reading To compare the emf of two given primary cells (Daniel and Leclanche cells) using a potentiometer.

Aim

To compare the emf of two given primary cells (Daniel and Leclanche

cells) using a potentiometer.

Apparatus Required 

Potentiometer, galvanometer, a rheostat, a one way key, a two way key, jockey, connecting wires, a Lechlanche cell, a Daniel cell, an ammeter, a secondary cell and a piece of sand paper 

Theory 

Potentiometer is a device used to measure small potential difference accurately and to compare the e.m.f.s of two given cells. It works on the principle that for a conducting wire of uniform cross-sectional area carrying steady current, the potential difference across a given length of the wire is directly proportional to the length. If two cells of emf E1, and E2, are balanced by length l1 and l2 respectively of potentiometer wire, then E1/E2 = l1/l2

Circuit diagram

Procedure

1. Clean the ends of the connecting wire and make the connections as shown in Fig. 

2. Keep keys K1, and K2, open. Use Lechlanche cell as E1, and Daniel cell as E2

3. Close key K1. Insert the plug in the gap between 1 and 3 of the key K2, and place the jockey first near the end A and then near the end B of the potentiometer wire and make sure that the galvanometer shows deflection in opposite directions. If it does not, adjust the rheostat to get the deflection in opposite directions. Repeat the same process by inserting the plug in the gap between 2 and 3 of the key K2 

4. Insert the plug in the gap between 1 and 3 of the key K2, to bring the cell E1, in the circuit and find the balancing point by placing the jockey J at different points along the potentiometer wire. Do not slide the jockey on the wire. Determine the balancing length l1, from the end A. 

5. Without disturbing the Rheostat, take out the plug from the gap between  and 3 and insert it in the gap between 2 and 3 of the key K2, bring the cell E2, in the circuit and find the balancing point. Record the balancing length l2, for the cell E2 6. Repeat the step 4 and 5 for different positions of rheostat slider to get different observations.

Observations

Precautions

1. The ends of the connecting wire should be cleaned before making connections 

2. All the connections should be tight. 

3. The jockey should not be slided on the potentiometer wire as it would change the cross-section of the wire. It should be pressed gently on the wire at different points 

4. The e.m.f of the battery E should be greater than the e.m.f of cells E1, and E2

5. The positive terminal of all the three cells should be connected to the zero end of the potentiometer wire. 

6. While recording one complete set of observation, the current must remain constant. 

7. The current should be passed for minimum possible time. 

Sources of Error

1. Connections are not tight. 

2. Keys are always inserted resulting in the heating of the wire.