A balanced Wheatstone bridge arrangement yields zero galvanometer deflection. The condition for this balance is:
$\frac{R_1}{R_2} = \frac{R_3}{R_4}$
Here, R1, R2, R3, and R4 denote the resistance in each arm. Specifically, R1 and R2 are the resistances of wire segments AJ and JB. R3 is 8 Ω and R4 is 12 Ω.
Given the length of AB is 40 cm, the resistance ratio is $\frac{R_1}{R_2} = \frac{8}{12} = \frac{2}{3}$
The ratio of resistances is also expressed as $\frac{R_1}{R_2} = \frac{x}{40 - x}$, where x is the length of AJ. Thus, $\frac{x}{40 - x} = \frac{2}{3}$
Solving for x: 3x = 80 − 2x. This simplifies to 5x = 80, yielding x = 16 cm.
The distance of J from B is calculated as 40 cm − 16 cm = 24 cm.