As a result of recent work on switchmode power converters I needed the ability to measure the saturation properties of inductors. This simple instrument was built to allow visualisation of the current ramp as an inductor charges from a constant voltage, the current vrs time gradient being an indication of inductance (as the current through a perfect inductor rises linearly with time when charged from a constant voltage). When an inductor’s magnetic material saturates the inductor suffers a drop in inductance; essentially the matter in the core can no longer contribute to the magnetisation, only the vacuum in the field remains capable of further magnetisation. This rapid loss of inductance is visible as a steepening knee in the current curve; the instantaneous inductance drops and current rises more rapidly with time.
Via the comments.