For the case where Magnetic Field B is 

  always directed along the tangent to perimeter of a closed curve, and 

   constant in magnitude all along the perimeter with a value B, the Ampere circuital

      laws states:

 

       B x (perimeter of closed circuit) =  o x ( Total current passing through any 

                                                         surface of which the closed circuit is the 

                                                         perimeter)