Filament also refers to a wire structure found in tubes. When the it is heated by the proper voltage and current, the filament emits electrons. These electrons are attracted to the positive element of the tube, the plate (or anode). In multi-element tubes, the electron stream may pass through grids or beam-forming elements on its way to the plate. In this type of tube, the filament forms part of the circuit and is considered to be negative.

In tubes that have a separate cathode, the filament is placed inside the heater (a metal cylinder) and is used to bring the heater to operating temperature. The filament does not usually form part of the circuit in this type of tube.