Applying Potentiometer to Measure the Difference between two Cells

Cells are being used for many purposes in the laboratory setup. The potentiometer is a piece of equipment to find the difference between the electromotive force of two cells and create a comparison between the two. It works on the principle of potential difference between two junctures of any cable, which is directly proportional to the height of the cable, in case of a steady flow of current through the cable with a uniform cross-sectional area. Let us understand more about this through this article.

Definitions and concepts

Electromotive force (EMF) and terminal potential difference (V) of any cell are equal in the condition of no electric current. Both the quantities are gauged and estimated in Volts; still, they are not the same kind of quantity.

Electromotive force is the required amount of energy given by the battery of the cell to each coulomb of charge (Q). In other words, it is an amount of energy that leads to the flow of current through the circuit. This is also known as voltage. On the contrary, electroactive force is not a true force.

A voltmeter helps obtain the actual quantity, but it can only be used to calculate the potential difference between the terminals of a cell. However, a potentiometer is another device that helps infer the value of EMF of any cell.

Furthermore, the potentiometer is an instrument used to find the difference between the EMF (electromotive force) of two cells and also measure the internal resistance and potential difference across the resistor.

It comprises a 10 m long wire with a uniform cross-sectional area. The wire should have qualities such as high resistivity and low-temperature coefficient. Both the wires are arranged parallel to each other on a wooden block, and copper bands are used to join the wires. The wooden block also has a metre scale for the readings.

Principal of working: The principle linked with the potentiometer states that the potential difference between two junctures of any cable is directly proportional to the height of the cable if the steady current flows through the cable with a uniform cross-sectional area.

Through experimentation, the EMF of two cells can be determined and compared, which is mentioned below:

​Required equipment

• Potentiometer
• Leclanche cell
• Daniel cell
• Ammeter
• Voltmeter
• Galvanometer
• Battery
• Rheostat
• Resistance box
• Keys (one way and two way)
• Jockey
• Connecting cables and sandpaper

Step-by-step instructions

• Organise the equipment according to the standard circuit diagram.
• Using sandpaper eliminates the insulation from the edges of the connecting copper wires.
• Calculate and compare the EMF of the battery (E) and cells (E₁ and E₂) and observe that E should be greater than E₁ and E₂.
• Use a one-way key to attach the positive and negative pole of the battery (battery should have steady EMF) with the potentiometer’s zero edges (P). Then, connect an ammeter and low resistance rheostat to the potentiometer’s other edge (Q).
• Join the positive pole side of the cells (E₁ and E₂) with the zero-edge (P) of the potentiometer and negative poles with the terminals a and b of the two-way key.
• Now, join the galvanometer (G) and resistance box (R.B.) with the jockey (J) by using the common terminal c of the two-way key.
• Keep the resistance in the rheostat zero and take maximum current from the battery.
• Then, set the plugin the one-way key and between the edges of two keys, a and c.
• Keep the resistance around 2,000 ohms in the resistance box (R.B.).
• Keep the jockey at the zero ends and observe the direction of reflection in the galvanometer.
• Then, again press the jockey at the other edge of the potentiometer cable. Suppose the deflection is in opposite directions as per the first case. In that case, the connections are accurate (in the wrong situation, the deflection direction will be in the same direction as the EMF of the battery is very low).
• Move the jockey over the potentiometer cables till you get no deflection in the galvanometer.
• Keep the 2000 ohms resistance back into the resistance box and find a null point state with accuracy.
• Write down the measurement of the cables for cell E₁. Also, write down the current shown by the ammeter.
• Now, remove the plug of cell E₁ and connect E₂.
• Follow the same conditions for the E₂ cell as well.
• Take the observations alternatively for each cell again and again with the same amount of current.
• You can also increase the current by modifying the rheostat and get three or four observations similarly.
• Write down all the observations.

Possible sources of error

• Maybe there is less or no charge in the battery.
• Maybe the cross-sectional area of the cables is not the same throughout the length.
• Final resistance may not be null.

Conclusion

There are many purposes associated with the cells in the laboratory setup. Electromotive force and potentiometer are crucial to compare the energy associated with the two cells. The potentiometer works on the principle of potential difference between two junctures of any cable, which is directly proportional to the height of the cable in case of a steady flow of current through the cable with a uniform cross-sectional area.