Old X-ray machine from museum of Wilhelm Conrad Röntgen in Würzburg. The bulb to the right applied cooling water to the anode. The electrode to the left is the cathode filament. The lower surface of the glass envelope facing the anode is blackened with metal sputtered from the anode, indicating heavy use. These Coolidge x-ray tubes began to be used in the 1920s.
Source: Aida