Cathode rays
Almost all discoveries related to particles such as electrons, ions, etc. finds its origins in the experiments conducted by scientists with discharge tubes. J.J. Thomson was the first to show, with the help of experiments with discharge tubes that electrons are same in all substances and that atom is made up of particles which are not indivisible as suggested by Daltonís atomic theory.
Discharge tube is a hard glass tube which is long (about 30 cm or more) and has two electrodes attached at its two ends. The electrodes are made of any metal which is a good conductor, such as copper, aluminum, platinum, etc. The discharge tube has a facility to connect it to a vacuum pump. The electrodes can be connected externally to a high voltage source. The discharge tube is named so because of the gaseous discharge that takes place between the two electrodes. This we shall see in the ensuing discussions.
Let the electrodes have a potential difference applied to them (about 10 kV). The electrode connected to the negative terminal is known as the cathode and the terminal connected to the positive terminal is known as the anode. As the discharge tube is evacuated, measure the pressure inside in terms of millimeters of Hg. You will notice various changes in it. The air inside the discharge tube is a non-conductor of electricity. So initially the tube looks intact. As the air pressure inside reduces, the gas starts ionizing. Since a potential difference is maintained inside the tube, when one gas atom is ionized, the electron escaping from it, ionizes other gas atoms. There a stream of positive ions and negative electrons gets created. These start moving towards cathode and anode respectively. This generates a current. When the pressure is not very low, the gas movement looks like bluish streaks. As the pressure reduces further, the gas inside looks pink. When the discharge tube is evacuated to a high degree, the inside will start looking black, as there is no gas inside to conduct any current. This dark space is called Faradayís dark space. A small glow can be observed at the cathode and the anode. This is due to residual gases.
As the vacuum is reduced further, there will be a greenish glow behind the anode. The reason for this can be inferred from the direction. The rays or particles are coming from the cathode towards the anode. Some of them overshoot the anode and reach the inner surface of the tube. This causes the glow. These rays are called cathode rays.
Since the cathode rays are coming towards anode, they must be negatively charged. It has been proved that cathode rays are nothing but electrons.
The cause for the production of cathode rays can be easily explained. As the discharge tube is evacuated, the electrons at the cathode get attracted to the anode due to the high potential difference. Cathode rays are not seen when the potential difference is low or if the gas pressure is high.
