Small diameter Tungsten wires (usually two of different size) that are spring-wound like coils.

These wires are placed into a focusing cup which is the main component of the cathode assembly.

The size of the filament wire and the placement in the focusing cup determines the focal spot size and the resulting resolving power of the tube.

Power is applied, via the high tension cables, causing the selected filament to heat up (small or large).

The heat releases electrons from the wires during X-ray production.

Back to Anatomy of an X-Ray Tube

Principles of X-Ray Tubes

How Tubes Work