A  $\mathrm{U}-$shaped wire $\mathrm{PQRS}$ of mass m carrying a current I is stationary above the surface of the earth in the region of a uniform magnetic field B directed into the page as shown in figure. Then:

 

Refer to figure. It follows from Fleming's left hand rule that the wire segment $\mathrm{PQ}$ experiences a force ${\mathrm{F}}_1=\mathrm{BI}{\mathcal{l}}_1$ directed to the right and the wire segment SR experiences an equal force ${\mathrm{F}}_1=\mathrm{BI}{\mathcal{l}}_1$ but directed to the left. These forces cancel each other. The wire segment QR experiences a force $\mathrm{F}=\mathrm{BI}{\mathcal{l}}_2$ directed vertically upward. The wire frame will remain stationary in air if $\mathrm{F}$ equal the weight mg of the wire frame, i.e.

$\mathrm{BI}{\mathcal{l}}_2=\mathrm{mg}$

$\mathrm{I}=\frac{\mathrm{mg}}{{\mathcal{l}}_2\mathrm{B}}$