We shall assume that, in
such cases, it is possible to choose the loop (called
an amperian loop) such that at each point of the
loop, either
(i)
B is tangential to the loop and is a non-zero constant
B, or
(ii)
B is normal to the loop, or
(iii) B vanishes Now, let L be the length (part) of the  loop for which B
is tangential Let Ie  be the current enclosed by the loop Then, Eq