It is used for producing cathode rays and studying their properties.
It is similar to the Geisler ones, but with gas at less pressure (10-5 atm). Different devices can be placed inside it: paddle wheel, a mica layer, an obstacle in the shape of a Maltese cross, etc.
When a very high potential difference is applied between the electrodes, a fluorescence appears situated opposite the cathode and, unlike what happened with Geissler tubes, this fluorescence has always a greenish colour (regardless of the gas used as filler). To explain this phenomenon, the cathode was supposed to emit a radiation which, falling upon the glass of the tube, induced in it a characteristic brightness.
If different accesories are placed between the cathode and the anode, we can determine the properties of these “cathode rays” and eventually conclude that they are formed by very light, negatively charged particles. These experiments gain great importance in the development of the theories about the nature of matter, as they suppose the first empiric proof of the existence of elementary subatomic particles.
Experiments with these tubes were developed by English chemist William Crookes from 1895 on.