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The skin effect. Induction's alternating current, unlike DC, concentrates on the surface of the conductor. Hence the outside of a metal object heats up rapidly while the inside stays cool. Given a skin-blistering dose of high-frequency induction current and then quenched in water, metals are hardened on the surface, yet remain resilient and tough inside. Typical parts thus hardened are airplane bearings, tank treads, engine cylinder walls. Often they last ten times longer than without such surface hardening. Noses of armor-piercing shells and bombs are induction-hardened without affecting the rest of the projectile. Among other applications of induction:
> Heating radio vacuum-tubes during sealing to drive gases out of the metal parts.
> Heating vatfuls of chemicals where a flame, spark or arc would set off an explosion.