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The difference between the magnetic field strength in the center and that in a measuring point in a sector type of slim electromagnet with ordinary iron pole pieces had been measured under a degree of measuring precision of 20-4in such a way as not to cause the eddy-current effect and the magnetic after-effect. Important sources producing the inhomogeneities of magnetic fields in the pole pieces with a sharp-cornered profile are the saturation and the hysteresis effects in the central ray, and the saturation effect and the magnetic asymmetry of the geometry of pole pieces in the radial direction. The saturation effect is the superimposition of the internal and the leakage fluxes from the edges to the center in the pole pieces. The hysteresis effect is that different parts along the central ray never draw the same magnetization curve because of the saturation in the pole pieces. The magnetic asymmetry of the geometry of pole pieces is the difference between leakage fluxes in the pole-piece sides with a small and a large curvature radii. The B-constant design of pole pieces has excellent effects in avoiding the field inhomogeneities caused by the above sources, although a small cycling effect due to the residual magnetization remains in the pole pieces.