Flux tube
= Flux tube
{wiki}
A flux tube is a bundle of <magnetic field lines> bounded by a surface everywhere tangent to the <magnetic field>. <Gauss's law for magnetism> makes the <magnetic flux> through any transverse cross-section constant along the tube. For a narrow tube with nearly uniform field magnitude $B$ and transverse area $A$, this gives $BA=\mathrm{constant}$.