The Crookes Tube was developed by English physicist Sir William Crookes to investigate the phenomenon of charged particles given off when a high voltage was connected across the tube and as the air was pumped from the vessel. Wilhelm Roentgen discovered X-Rays whilst experimenting with a Crookes Tube.
The different glowing regions possible in a Crookes Tube as the pressure varies. Image courtesy of Wikipedia.
One circular electrode and the evacuation pip. The support is not sheathed in glass.
The other circular electrode. The glass to metal seal can be seen on the left.
A close-up of one electrode. As a cold cathode device both electrodes are the same.
An X-ray image of the tube. The open tube at the top would be connected to a vacuum pump when in use.
The wide glass tube envelope is 45 mm in diameter, and including the base pins is 425 mm tall.
References: Observation Wikipedia & UK Science Museum.
Updated July 21, 2014.