In the circuit shown $AB$ is a $10 \Omega$ uniform slide wire $50 cm$ long ${\epsilon }_1$ is $2 V$ accumulator of negligible internal resistance. $R_1$ and $R_2$ are $15 \Omega$ and $5\Omega$ respectively. When $K_1$ and $K_2$ are both open, the galvanometer shows nodeflection when $AJ=31.25 cm$ when $K_1$and $K_2$ are both closed the balance length $AJ=5 cm$.