Toroid shape Electro-Mag question

[square_imp]

I have a difficult probelm to solve:

There is a dougnut shape (toroidal coil) with N turns of wire wrapped around it. Current I flows in the wire. THe cross section of the 'doughnut' is square with height h. I am meant to use ampere's law to prove that the magnetic field at a radius r from the centre (half way through the coil) is given by:

B = uNI/2pi*r (u = permittivity)

SO far I have that ampere's law says that the integral around a closed path of the Magnetic field is equal to the permittivity times the current enclosed. How does this translate into the above formula? I am probably just missing something really obvious.

 

[Tide]

All you have to do is create a closed path within the toroid that loops around the axis at a fixed distance. You can see that the current passing through the loop you just created is NI.