X-ray are extremely short electromagnetic waves. They are created by bombarding solid materials with electrons. In the case of an X-ray tube this is a metal electrode (anode) with a high electric charge: The higher the charge, the heavier the bombardment and the more intense the radiation. Much heat is realeased in this process. Various solutions have been invented to prevent overheating. Some types therfore had an anode that made contact with a tank with running water.