ABCD is a square loop made of a uniform conducting wire. The current enters the loop at A and leaves at D. The magnetic field is

$\mathrm{At} \mathrm{the} \mathrm{centre} \mathrm{of} \mathrm{the} \mathrm{loop} , {\mathrm{B}}_1\propto 3\times \frac{\mathrm{I}}{\mathrm{r}}\left(\mathrm{Field}\mathrm{due}\mathrm{to}\mathrm{AB},\mathrm{BC}\mathrm{and}\mathrm{CD}\right)$

${\mathrm{B}}_2\propto \frac{3\mathrm{I}}{\mathrm{r}}\left(\mathrm{Field}\mathrm{due}\mathrm{to}\mathrm{AD}\right)$

Clearly, the two fields are equal in magnitude and the opposite in direction.