Potentiometer is a device used to compare the EMF (electromotive force) of two cells, to measure the internal resistance of a cell, and the potential difference across a resistor. It consists of a long wire (10 m) having a uniform cross-sectional area. The wires are stretched parallel to each other on a wooden board.

The wires are joined in series by using thick copper strips. A meter scale is also attached to the wooden board. The potentiometer works on the potentiometer principle, that is when a constant current flows through a wire of uniform cross-sectional area, the potential difference between its two points is directly proportional to the length of the wire between the two points.

If a cell of EMF 𝐸 and internal resistance 𝑟, is connected to an external resistance 𝑅, then the circuit has the total resistance (𝑅 + 𝑟). The current 𝐼 in the circuit is given by, $$I=\frac{E}{R+r}$$

$$E=I R+I r$$

We know that 𝑉 = 𝐼𝑅. So, $$E=V+I r \Rightarrow V=E-I r$$

Using a potentiometer, we can adjust the rheostat to obtain the balancing lengths 𝑙₁ and 𝑙₂ of the potentiometer for open and closed circuits, respectively.

Then, 𝐸 = 𝑘𝑙₁ and 𝑉 = 𝑘𝑙₂; where 𝑘 is the potential gradient along the wire.

Now, we can modify the equation for getting the internal resistance of the given cell, by using the above relations as;$$r=R \frac{l_1-l_2}{l_2}$$

Requirements for this Science Experiment

⦁ Potentiometer ⦁ Leclanché Cell ⦁ Resistance Box ⦁ Jockey ⦁ Galvanometer ⦁ Ammeter ⦁ Galvanometer ⦁ Plug key