from utility frequency (50 or 60 Hz) to 400 kHz or higher, usually depending on the material being melted, the capacity (volume) of the furnace and the melting speed required. Generally, the smaller the volume of the melts, the higher the frequency of the furnace used; this is due to the skin depth which is a measure of the distance an alternating current can penetrate beneath the surface of a conductor. For the same conductivity, the higher frequencies have a shallow skin depth—that is less penetration into the melt. Lower frequencies can generate stirring or turbulence in the metal. A preheated, one-ton furnace melting iron can melt cold charge to tapping readiness within an hour. Power supplies range from 10 kW to 42 MW, with melt sizes of 20 kg to 65 tons of metal respectively. An operating induction furnace usually emits a hum or whine (due to fluctuating magnetic forces and magnetostriction), the pitch of which can be used by operators to identify whether the furnace is operating correctly or at what power level. Refractory lining There is a disposable refractory lining used during casting. See also Electric arc furnace—for another type of electric furnace, used in larger foundries and mini-mill steelmaking operations References Further reading External links "How Induction Furnace Are Making It Hot For The Axis", Popular Science, November 1943. Detailed article on the basics with numerous illustrations Industrial furnaces Furnace, Induction hr:Plasma Induction ja:高周波炉