The thermionic valve consists of a glass casing (with some models being made of metal or ceramic), in which a vacuum was created. Inside the casing there is a metal filament which is led to incandescence (between 1,000 and 3,000 ° C) with the use of an electric current. Inside the casing there are also one or more metal elements (in the form of grids or screens), which can be connected from the outside. The metallic filament, or rather a small metal tube that wraps it, in the case of indirect heating, is called a cathode. The outermost metal element is called an anode. Any intermediate element is called grid.