How Magnetic Yokes Reduce Flux Leakage
An induction coil produces flux that should pass through the charge. Whatever does not is leakage. Leakage that finds the furnace shell shows up as a hot shell, a noisy furnace, and a power supply working harder than the melt requires.
Silicon steel has high permeability, so the leakage prefers the limb. Grain-oriented sheet makes that preference stronger along the rolling direction, which is why the sheets are arc-stacked onto the coil. Coverage on these yokes is typically 60 to 65 percent of the coil surface, and 72 percent on the vacuum-furnace column.
Stainless plate sitting flat on the laminations can itself carry eddy current. Several yokes leave the cover plate off for that reason, or separate it with phlogopite mica. Copper shield plates beside the water-cooled leads keep the end region from running red.