A long straight wire is fixed horizontally and carries a current of $40\ \text{A}$. A second long straight wire, parallel to it and directly above it, carries $25\ \text{A}$ in the opposite direction, as in the figure. The upper wire has a mass of $8.0\ \text{g}$ per metre. Smooth vertical guides at its far ends let it move up and down only, and it floats at rest at some height above the fixed wire. It is then pushed down a little and let go. Ignore the air and the thickness of the wires. Take $\mu_0 = 4\pi \times 10^{-7}\ \text{T m/A}$ and $g = 9.8\ \text{m/s}^2$. What is the period of its small up-and-down oscillations? Give the answer in $\text{ms}$, to the nearest whole number.

Fig. 1: End view of two long horizontal wires, one directly above the other. The lower wire is fixed and carries 40 A into the page. The upper wire carries 25 A out of the page and has a mass of 8.0 g per metre; dashed vertical guides at its far ends let it move only up and down.