A long horizontal wire $P Q$ of mass $m$ and carrying a steady current $i_1$ is free to move in the vertical plane. It is in equilibrium at a height $d$ above another horizontal long wire $R S$ of mass $m$ and carrying a current $i_2$. The wire $R S$ is fixed and not free to move. Which of the following statements is/are correct?

• Correct answer
  A

  The magnetic force on the lower wire is in the vertically downward direction.
• Correct answer
  B

  The magnitude of the magnetic force per unit length on the upper wire is $\frac{\mu_0 i_1 i_2}{2 \pi d}$
• Correct answer
  C

  If the upper wire as a whole is displaced slightly upward, it will perform simple harmonic motion with time period $T=2 \pi \sqrt{d / g}$.
• Correct answer
  D

  If the current in the upper wire is reversed, the wire will no longer be in equilibrium.

Hence, the correct answer is options: 'A', 'B', 'C', 'D'.