The Joule effect of the current is used to convert electrical energy into thermal energy to heat the object. Usually divided into direct resistance heating and indirect resistance heating. The former's power supply voltage is directly applied to the object to be heated, and when a current flows, the heated object itself (such as an electric heating ironing machine) heats up. An object that can be directly resistively heated must be a conductor, but with a higher resistivity. Since heat is generated by the object itself being heated, it is internally heated and has high thermal efficiency. The indirect resistance heating needs to be made of a special alloy material or a non-metal material to generate a heating element, and the heating element generates heat energy, which is transmitted to the object to be heated by radiation, convection and conduction. Since the object to be heated and the heat generating component are divided into two parts, the type of the object to be heated is generally not limited, and the operation is simple.
The material used for the heating element of the indirect resistance heating generally requires a large resistivity and a small temperature coefficient of resistance, and is small in deformation at high temperature and is not easily embrittled. Commonly used are metal materials such as iron-aluminum alloy and nickel-chromium alloy, and non-metal materials such as silicon carbide and molybdenum disilicide. The maximum operating temperature of the metal heating element can reach 1000-1500 °C depending on the type of material; the maximum working temperature of the non-metallic heating element can reach 1500-1700 °C. The latter is easy to install and can be replaced by a hot furnace, but it requires a pressure regulating device when working. The life is shorter than that of the alloy heating element. It is generally used in high temperature furnaces, where the temperature exceeds the maximum operating temperature allowed by the heating elements of metal materials and some special occasions.