The document discusses magnetic hysteresis curves (B-H curves) of ferromagnetic materials. It explains that B-H curves plot the flux density (B) versus the field strength (H) and show how B increases with H until saturation. The curves also demonstrate retentivity, where some residual magnetism remains after H is removed. Drawing the full curve as H is cycled from positive to negative produces a magnetic hysteresis loop. The size of the loop indicates the energy lost to hysteresis during the cycling of H. Materials with smaller loops incur lower hysteresis losses and are preferred for applications with frequent field reversals like transformer cores.