Daryl McHenry, age 14, of Rock Island, I11., for his question

HOW DOES AN ELECTRIC FURNACE RUN?

An electric furnace is an electrically heated device that is used industrially for melting metals or firing ceramics. It is also called an electrothermic furnace.

The simplest type of electric furnace is called the resistance furnace. Heat is generated by passing a current through a resistance element surrounding the furnace or by utilizing the resistance of the material being heated. The heating element in an externally heated furnace may take the form of a coil or metal wire wound around a tube of refractory material or it may be a tube of metal or other resistive material such as carborundum.

Resistance furnaces are particularly useful in applications in which a small furnace with precisely controlled temperatures is needed. Small resistance furnaces are widely used in laboratories and in shops for the heat treatment of tools

Larger furnaces are used for firing ceramics and melting brass.

The electro arc furnace is the most widely used type of electric furnace for the production of quality alloy steels and range in capacity from 500 pounds to 181 metric tons. In these furnaces the heat is generated by an arc struck between the metal being heated and one or more electrodes suspended above the metal. A typical form of arc furnace has three electrodes, fed by a three phase power supply, giving three heating arcs. The electrodes are made of either graphite or of carbon.

The most recently developed type of electric furnace is the induction furnace consisting of a crucible in which a metallic charge is heated by eddy currents induced magnetically. Around the crucible is wound a coil through which high frequency alternating currents are passed. The magnetic field of this coil sets up eddy currents in the metal in the crucible.

Induction furnaces have a number of advantages, chief among them being the speed at which metal can be melted.

A special type of furnace called an electrolytic furnace, is used in the production of aluminum, magnesium and sodium. In the electrolytic furnace, a salt is fused by the heat generated by the passage of a large electric current and is at the same time electrolyzed so that the pure metal is deposited at one electrode.

The highest temperature at which resistance furnaces are operated in the manufacture of graphite is in the neighborhood of 7,366 degrees Fahrenheit. That s hot!

At comparatively low frequencies the induced eddy currents in the induction furance exert a stirring action on the molten metal. Because the higher frequencies are the most effective for heating, some induction furnaces have two coils, one for high frequency current and one for low frequency.

The earlier types of induction furnaces operated at frequencies between 60 and 60,000 cycles per second, but some modern furnaces are designed to use frequencies of 1 million cycles or more per second.