The radial electric field is determined by the ambipolar condition in helical systems according to the neoclassical theory of the cross field transport. In general, the ambipolar condition has three roots. It is known that one of them is unstable and the others are stable. The transition from one stable root to the other can occur across a particular magnetic surface. The model of the ambipolar equation is assumed to be a cubic of the radial electric field. The weak inhomogeneity of the plasma is taken into account. It is found that the radial electric field equation has a stationary and step-like solution. The characteristic scale length of the solution is estimated. The linear stability of the solution is also studied. The condition for the transition is obtained by solving the ambipolar equation numerically when the realistic expressions of fluxes are employed.