Electrical discharge through gases is studied by using a specially designed glass tube commonly called as a discharge tube. It consists of a cylindrical glass tube having a side tube, and two metallic electrodes one at each end. These electrodes can be connected to the respective terminals of a high tension power supply. Air from inside the tube can be pumped out by connecting the side tube to a vacuum pump, and a desired pressure can be maintained inside the tube.
It was found that gases could not conduct electricity even when an electrical potential of about 10,000 volts was applied. But, it was discovered by William Crookes that gases could conduct electricity at low pressures. When, the pressures inside the discharge tube were reduced gradually, the following point was noted.
i) At about 10-2 atm pressure, a glow surrounding the cathode(negative electrode) leaves the electrode surface, and little space is left between it and the electrode. This is called Crooke ‘mass dark space. At this stage, electric current begins flow from one electrode to other.
At sufficiently low pressure (about 10-3 atm), this glow fills whole the tube. The color of the glow depends upon the nature of the gas in the tube, and on the color of the glass used for making the discharge tube.
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ii) When the pressure is lowered to about 10-5 atmosphere, lights emission by the residual air in the discharge tube opposite to the cathode start glowing. At this stage, a stream, called cathode rays is emitted from the cathode.
Thus, we see that when electrical discharge is passed through gasses at very low pressure, cathode rays are produced.
You must have seen bright advertising sign boards of different colors at the Shopping malls and around your city/town. These sign boards consist of many discharge tubes in which neon gas, or a mixture of neon gas with some other gas, is filled at a very low pressure. When very high electrical potential is applied across these tubes, the glows different colors are produced. The color of glow depends upon the nature of the gas in the tube, and the color of the glass used in making the discharge tube. For example, the colors obtained when neon gas or its mixture are used.
Sodium vapor street lamps and television tubes are also based only the same process of electrical discharge through gasses at low pressures.