The effect of the helical instability on the internal magnetic field in the positive column, is investigated theoretically under the assumptions that the densities of electrons and ions are perturbed independently and that the perturbed densities have finite amplitudes. The internal magnetic field is found as a function of the amplitude of the superimposed helix for the helical instability (m=1) and the phase shift &dgr;mbetween the perturbed densities of electrons and ions. The theoretical results are compared with experimental data and found to be in fair agreement with experimental values. It is concluded that the direction of the internal magnetic field produced by the helical instability is parallel to that of the applied magnetic field.