A copper wire of length 3m and area of cross-section $1m{m}^2$ , passes through an arrangement of two frictionless pulley,  $P_1$ and  $P_2$. One end of the wire is rigidly clamped and a mass of 1 kg is hanged from the other end. If the Young’s modulus for copper is  $10\times {10}^{10}N/m^2$, then the elongation in the wire is_______  $\times {10}^{-4}m.$
 $(g=10m/s^2)$