State the principle of potentiometer. Draw a circuit diagram used to compare the e.m.f. of two primary cells. Write the formula used. How can the sensitivity of a potentiometer be increased ?

The principle of a potentiometer is based on the idea that the potential difference (voltage) across a uniform wire is proportional to the length of the wire. This means that when you have a constant current flowing through a wire of uniform cross-section, the voltage drop (or potential difference) along the wire can be measured accurately by the length of the wire that corresponds to that voltage. This property allows us to compare the electromotive forces (e.m.f.) of different cells or batteries without drawing current from them, which is key in many electrical experiments.

Understanding the Circuit Diagram

To compare the e.m.f. of two primary cells using a potentiometer, we typically set up a circuit that includes a potentiometer wire, a standard cell of known e.m.f., and the two primary cells whose e.m.f. we want to compare.

Circuit Diagram

Here is a simplified description of the components in the circuit:

• A long, uniform wire representing the potentiometer.
• A standard cell connected in series to provide a reference voltage.
• The two primary cells connected to the potentiometer via a galvanometer to measure the voltage.

In the circuit, one end of the potentiometer wire is connected to one terminal of the standard cell, and the other end is connected to the negative terminal of the primary cells. The slide or jockey is moved along the wire to find a point where the galvanometer shows zero deflection, indicating a balanced condition.

The Formula Used

When the galvanometer shows zero deflection, the e.m.f. of the primary cell (E1 or E2) can be expressed using the ratio of the lengths of the wire:

E1/E2 = L1/L2

Where:

• E1 = e.m.f. of the first primary cell
• E2 = e.m.f. of the second primary cell
• L1 = length of wire corresponding to E1
• L2 = length of wire corresponding to E2

Increasing Sensitivity of a Potentiometer

To improve the sensitivity of a potentiometer, several methods can be employed:

• Use a longer wire: A longer potentiometer wire increases the available length for measurement, allowing for finer resolution in voltage differences.
• Decrease the current: Reducing the current flowing through the potentiometer wire decreases the voltage drop across it, enhancing the accuracy of voltage readings.
• Higher resistance wire: Utilizing a wire with higher resistance can help in achieving a larger voltage drop for a given current, which can lead to improved sensitivity.
• Calibrated scale: Having a more finely divided scale on the measuring device helps to read smaller voltage differences accurately.

By applying these techniques, one can significantly enhance the performance and accuracy of measurements made with a potentiometer. This is particularly useful in laboratory settings where precise voltage measurements are essential for experiments and analysis.