Apparent Image Effect in Closed-Circuit Magnetic Measurements

Magnetic measurements made under closed-circuit conditions are generally considered to be free from errors associated with the image effect. However, magnetic measurements on permanent magnet sample cylinders clamped between the pole pieces of an electromagnet show an apparent drop in magnetization with increasing applied field that is similar to the image effect found in open-circuit measurements. The closed-circuit effect has been measured for a range of sample geometries and materials, and occurs in both hard and soft magnetic materials. The drop in apparent magnetization is greater, and appears at lower fields, as the saturation magnetization of the sample increases. The effect increases as the length-to-diameter ratio L/D of the sample decreases, and becomes minimal for L/D > 1.8 . In addition to the apparent decrease in measured magnetization, there is a decrease in the measured magnetic field. We attribute the effect to a non-uniform magnetization of the electromagnet pole pieces, where local saturation distorts the magnetic flux distribution around the sample.