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CROOKES’S TUBE OR DISCHARGE TUBE EXPERIMENTS — Passage of electricity through gases at low pressure


Discovery of Electrons and Protons:

We know that air or gases normally do not conduct electricity, unless a very high voltage is applied. The work on the passage of electricity through gases was initiated by a German instrument maker Hein rich Geissler. This work was later extended by W. CROOKES.


To appreciate what happens when an electric current passes through some gas, imagine two metal plates, known as electrodes sealed in a glass tube containing some gas. The tube is provided with an outlet through which gas inside the tube can be evacuated at will. At ordinary pressure, unless very high potential difference is applied, there would be no discharge, and the electric spark cannot be made to pass from one electrode to the other. If the gas inside is gradually pumped out of the tube, the number of molecules of gas present inside is reduced, and fewer molecules inside make it convenient for the spark to pass.


When the pressure inside the tube is reduced to a centimeter of Hg, a potential difference of a few thousand volts would be enough for the spark to pass like a flash of lightening (Fig: 3.1). At further reducing pressure to about a few m.m of Hg, the spark disappears, the two electrodes are seen to glow and the rest of the discharge stream is dark. At about one m.m. of Hg, the tube is mostly filled with a glow extending from the positive electrode and is called positive column. The color of this glow depends upon the gas filled in the tube. As the pressure is continuously lowered, beautiful phenomena are seen. When the pressure inside the tube is lowered to about 0.001 m.m of Hg, the glow disappears and the walls of the glass tube begin to glow with a brilliant green light (Fig. 3.2)


It is obvious that some sort of radiation is passing between the two electrodes. This radiation consisted of a vast swarm of particles emitted by the cathode and were called Cathode Rays. Various experiments were thus performed by various researchers like Hertz, Lenard, Gold stein, Perrin and J.J. Thomson to determine the properties of these cathode rays.



The cathode rays were seen to possess the following properties:


1. The rays travel in straight lines, as they produce sharp shadows of objects placed in their path.


2. The rays emerge normally from the cathode and can be focused by using a concave cathode.


3. The rays penetrate small thicknesses of matter, like aluminum or gold foil without producing any perforations in the foils.

4. The cathode rays are easily deflected by a magnetic field, which can be shown by fringing a magnet close to them.


5. The rays carry a negative charge.


6. The rays can also be easily deflected by an electrostatic field.


7. The rays can exert mechanical pressure, showing they possess kinetic energy.


8. The rays were seen neither to depend upon the material of which the electrodes were made nor upon the gas which is filled in the tube.


9. These rays consist of particles now called Electrons carrying a fixed unit of charge and a fixed mass.


Different discharge tubes with different electrodes and residual gases were tried by a number of workers besides Thomson. All the experiments gave the same value for charge to mass ratio (e/m). This shows that electrons could be produced from any kind of matter and hence perhaps were constituent of all matter.

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