X-Ray tubes have three main parts. Which is NOT one of them?

X-ray tubes consist of three main components: the cathode, the anode, and the glass envelope or tube, which is often made of Pyrex. The cathode's role is to produce electrons when heated, while the anode serves as a target for these electrons, allowing the conversion of their kinetic energy into X-rays. The glass envelope is essential because it houses the cathode and anode, providing a vacuum environment that is critical for the efficient operation of the tube.

The transformer, while an essential component in an overall X-ray machine setup for generating high voltage, is not actually part of the X-ray tube itself. It functions to step up the voltage necessary for the operation of the X-ray tube, but it does not constitute one of the primary components of the tube structure. Thus, the correct identification of the transformer as not being one of the main parts of the X-ray tube accurately reflects the functionality and structural classification of the tube's components.