In this note, the question of the effect of assuming infinite permeability in the iron on the accuracy of calculations of the interpolar fields in electric machines is studied. One can do this very easily by replacing the saturated by non-saturated poles, covered by an infinitely thin sheet of current giving the same tangential component of the field along the surface of the iron. The field produced by this sheet of current, then, represents the difference between the exterior fields of the saturated circuit and of the non-saturated circuit. In addition to this indirect method of estimating the influence of saturation, the sketch of the field between the poles and in the air-gap can also be directly developed by determining with the aid of the differential field, the point of indifference of the saturated circuit. A comparison of sketches obtained in this way shows that, for the same useful flux, the interpolar fields are almost the same in both cases. From these sketches and others which will appear shortly in the Revue Générale de l'Electricite, it is evident that the sketches and functional curves given by Messrs. Stevenson and Park, and Wieseman, in their very valuable work, can still be used even though the poles are saturated.