Comparing the EMF of Two Primary 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, rheostat, one-way key, two-way key, jockey, connecting wires, Leclanche cell, Daniel cell, ammeter, secondary cell, and sandpaper.

Theory

A potentiometer is an instrument used to measure small potential differences accurately and to compare the EMFs of two cells.

It works on the principle:

For a wire of uniform cross-section carrying steady current,
the potential difference across it is directly proportional to its length.

If two cells of EMF (E_1) and (E_2) are balanced at lengths (l_1) and (l_2) respectively, then:

[ \frac{E_1}{E_2} = \frac{l_1}{l_2} ]

Circuit Diagram

Procedure

1. Clean the ends of the connecting wires and make the connections as shown in the circuit diagram.
2. Keep keys (K_1) and (K_2) open. Use the Leclanche cell as (E_1) and the Daniel cell as (E_2).
3. Close key (K_1). Insert the plug between points 1 and 3 of key (K_2).
   Place the jockey near end A and then end B of the potentiometer wire.
   Ensure the galvanometer shows deflections in opposite directions.
   If not, adjust the rheostat.
4. Insert the plug between 1 and 3 of (K_2) to bring cell (E_1) into the circuit.
   Find the balancing point by placing the jockey at various points on the wire.
   Do not slide the jockey.
   Record balancing length (l_1) from end A.
5. Without changing the rheostat setting, switch the plug to between 2 and 3 of (K_2) to bring cell (E_2) into the circuit.
   Find and record balancing length (l_2).
6. Repeat steps 4 and 5 for different positions of the rheostat slider to obtain multiple readings.

Observations

Precautions

1. Clean the ends of connecting wires before making connections.
2. All connections must be tight.
3. Never slide the jockey along the potentiometer wire. Press gently at points.
4. EMF of the battery (E) must be greater than EMFs of (E_1) and (E_2).
5. Positive terminals of all cells must connect to the zero end of the potentiometer wire.
6. Current must remain constant during one full set of observations.
7. Current should be passed for the minimum possible time.

Sources of Error

1. Connections may not be tight.
2. Keys may be left inserted for long durations, heating the wire.