One end of a spring of negligible unstretched length and spring constant k is fixed at the origin (0, 0). A point particle of mass m carrying a positive charge q is attached at its other end. The entire system is kept on a smooth horizontal surface. When a point dipole $\overrightarrow p$ pointing towards the charge q is fixed at the origin, the spring gets stretched to a length l and attains a new equilibrium position (see figure below). If the point mass is now displaced slightly by $\Delta$ l << l from its equilibrium position and released, it is found to oscillate at frequency ${1 \over \delta }\sqrt {{k \over m}}$. The value of $\delta$ is $\underline{\hspace{2cm}}$.