The figure shows a device called a Helmholtz coil. It consists of two coils, each with $N$ turns and radius $R$, separated by a distance equal to $R$. The same current $i$ flows through both coils in the same direction.
Calculate the magnetic field $B$ at the midpoint between the two coils and indicate its direction.
Also, calculate the magnetic field $B$ at the center of each coil and indicate its direction.