English: A flux tube, a graphical aid for visualizing a portion of a magnetic field, It consists of a cylindical surface which is everywhere parallel to the magnetic field lines. No magnetic flux passes through the side walls of the flux tube. Since a magnetic field has zero divergence (${\displaystyle \operatorname {div} B=\nabla \cdot B=0}$) the amount of magnetic flux passing through any cross section of the tube is the same, and the magnetic flux entering through surface ${\displaystyle S_{1}}$ is equal to the flux leaving through ${\displaystyle S_{2}}$