Low-frequency heating works by circulating a high current at very low frequency (typically a few hertz) directly through the transformer windings. At low frequency, the inductive reactance is small, so you can drive large currents at relatively low voltage. This produces uniform I²R losses inside the copper conductors, which heat the windings from the inside outward. This is effective for drying because moisture diffusion in thick paper is slow, and internal heating creates a strong temperature and vapor pressure gradient that pushes water toward the surface, where it can be removed by vacuum. High-frequency or external heating cannot achieve this same uniform internal heating in massive windings.