For example, in most vacuum tubes the cathode is a nickel tube heated by a tungsten filament inside it, and the heat from the filament causes the outside surface of the tube to emit electrons.
The cathode is a nickel tube. This is in most vacuum tubes. This is for example. This nickel tube is heated by a tungsten filament inside it. The heat from the filament causes the outside surface of the tube to emit electrons. This is for example.