Two small beads, each of mass $1\ \text{g}$, carry charges of $+1\ \mu\text{C}$ and $-1\ \mu\text{C}$. They are fixed to the ends of a light insulating rod of length $2\ \text{cm}$. The rod can turn freely in a horizontal plane about a vertical axis through its midpoint. A uniform horizontal electric field of $1\times10^5\ \text{N/C}$ is switched on. When the rod is turned slightly away from its equilibrium position and released, it oscillates. The time period of these small oscillations is closest to