An expression is derived for the magnetic scalar potential due to helical currents in toroidal surfaces. Nonlinearity of helical windings and noncircularity of the cross-section are taken into account as the work was carried out during an attempt to quickly evaluate the spectra of sinusoidal modes produced by instability currents in Tokamaks. Taking the inverse aspect ratio as the order of magnitude reference parameter, a successive approximations method is developed that provides a solution with any desired accuracy.