A long straight wire carrying a current of $200 A$ runs through a cubical box, entering and leaving through holes in the centres of opposite faces as shown in figure. Each side of the box is of $20 \mathrm{cm}$.

Consider an element $PQ$ of the wire $1 \mathrm{cm}$ long at the centre of the box. Calculate the magnetic field produced by this element at the point $A, B, C$ and $D$. The points $A, B$ and $C$ are the centres of the faces of the cube and $D$ is the midpoint of one edge.