Two point charges $A$ and $B$ of magnitude $+8\times {10}^{-6}\mathrm{C}$ and $-8\times {10}^{-6}\mathrm{C}$ respectively are placed at a distance $d$ apart. The electric field at the middle point $O$ between the charges is $6.4\times {10}^4 \mathrm{N} {\mathrm{C}}^{-1}$. The distance '$d$' between the point charges $A$ and $B$ is