If a solenoidal fieldBcan be separated into a fundamental componentB0with closed field lines and a perturbationB1of the order &egr;, it is possible to integrate the differential equations for the field lines up to the second order in &egr;. It turns out that the rotational transform is of the order &egr; if the flux of the fieldB1through a surface limited by any closed field line ofB0does not vanish. When the transform is of the order &egr;2one can derive an explicit expression for the magnetic surfaces.