假设长直导线中通有电流I,则可得B=μ0I/2πl
dφ=BdS=Badl=μ0Iadl/2πl
dΨ=Ndφ=μ0NIadl/2πl
Ψ=∫dΨ=∫(b~2b)μ0NIadl/2πl=μ0NIaln2/2π
M=Ψ/I=μ0Naln2/2π