(3)电流在回路之外 B -Lo B=ol 2π1 2兀 do B1·dh1=-B2d2 do 2兀 B d,+B.dl =0 B·dl=0I l   d 2π d d 0 1 1 2 2 I B  l = −B  l = −     B1 dl 1 + B2 dl 2 = 0     d = 0  B l l   （3）电流在回路之外 2 0 2 1 0 1 2π 2π r I B r I B   = ， = d d 1 l  1 r 2 r 2 dl  B1  B2 