The geometry of a Rowland's ring is usually a toroid of magnetic material around which is closely wound a magnetization coil consisting of a large number of windings to magnetize the material, and a sampling coil consisting of a smaller number of windings to sample the induced magnetic flux. The electric current flowing in the magnetization coil dictates the magnetic field intensity H in the material. The sampling coil produces a voltage proportional to the rate of change of the magnetic field B in the material. By measuring the time integral of the voltage in the sampling coil versus the current in the magnetization coil, one obtains the hysteresis curve.