Direct resistance heating (or ohmic heating) is based on the flow of an electrical current through the body to be heated, which is directly connected to an electrical supply. The heating can be realized by DC or AC currents, but the industrial applications are mostly done with the alternating ones.
Materials with fairly high electrical resistivity, such as carbon and low alloy steels and nickel alloys, are readily heated by direct resistance. With low resistivity materials such as copper and aluminum, the process is often not cost-effective.
Direct resistance heating is used in the iron and steel industry: for heating rods and billets prior to rolling and forging for ferrous and non-ferrous annealing; either alone or i^l combination with other fuels for melting glass; in electrode boilers, for water heating and steam raising; and in salt baths for the surface heat treatment of metallic components.
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