Amperes Circuital Law states that the line integral of the magnetic field H around a closed  path equals  the  total  current enclosed.

∫ H       dl          ∑ I

Applying this to the special case of a circular  path   around  a   single   current carrying  conductor allows  a simple calculation to be made to determine H at a radius r from the wire:

H=I/2r

Similar analyses may be applied to other examples that lend themselves to a closed form integration. More  complicated examples usually  involve  computer solution.Fortunately, many practical cases exist where a simple calculation is possible. We will only be concerned with these cases.

H.dl around a closed path containing thoseN turns will be equal to NI.First  suppose we have a coil of N turns carrying a current I. Then the integral of 

NI i.e.: