A simple saturable inductor circuit operated from a sinusoidal source is shown to be a useful test circuit for ferromagnetic cores. The pulse-field/pulse-duration characteristic of the material, which is necessary for the design of devices that depend upon reactance switching, can be ascertained. The pulse-field curves enable the probable time the flux in a lamination takes to attain a specific value (the ‘core time’) to be determined. The circuit is shown to have uses for grading batches of cores and for indicating interlamination faults.