ldl dB R r=R+x X P b boi r cos ad/ 2 Rc2兀R db= do ldz B dl R dB uo I cos adl B 4兀 2(x2+R2) 32x x R * p 2 0 cos d 4π d r I l Bx   =  = l r I l B 2 0 cos d 4π   2 2 2 cos r R x r R = +  =  = R l r IR B 2π 0 3 0 d 4π  2 3 2 2 2 0 （2 x R ） IR B + =  2 0 d 4π d r I l B  =    o B  d r I l  d