${({\text{d}} = \sqrt 3 \ell }$    ${{\theta _1} = 0)}$

${ \Rightarrow \frac{{{\mu _0}{\text{i}}}}{{4\pi \sqrt 3 \ell }}\left( {\sin 0 + \frac{\ell }{{2\ell }}} \right)}$

${ \Rightarrow \frac{{{\mu _0}{\text{i}}}}{{4\sqrt 3 \pi l}}\left( {\frac{1}{2}} \right)}$

${\boxed{{{\text{B}}_{\text{p}}} \Rightarrow \frac{{{\mu _0}{\text{i}}}}{{8\sqrt 3 \pi l}}}}$