In a potentiometer arrangement, a cell of emf 1 . 20V gives a...

In a potentiometer, null point or balance point is defined as the point where the deflection of the galvanometer is zero. If emf of the unknown battery is $ε$ then,

$ε =$(Potential Gradient)$\times$(Balancing length $l$). Therefore,

$\dfrac{{ε}_{1}}{{ε}_{2}} = \dfrac{{l}_{1}}{{l}_{2}}$

$⇒ \dfrac{1 . 20}{1 . 80} = \dfrac{36}{{l}_{2}}$

$⇒ {l}_{2} = 18 \times 3 = 54cm$

Hence, difference in balancing length is ${l}_{2} - {l}_{1} = 54 - 36 = 18cm$