Properties Of Cathode Rays

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Properties of Cathode Rays

During the latter half of the nineteenth century, it was found that while normally dry gases do not conduct an electric current, they do so under very low pressure and then patches of light are seen. The passage of electricity through gases as studied by a number of physicists, particularly by Faraday, Davy, Crookes and J.J. Thomson.

When a current of high voltage (10,000 volts) is passed through a gas of air kept at a very low pressure (0.01 0.03 mm) blue rays are seen emerging from the case. These rays are called Cathode Rays.

Some of the important properties of the cathode rays studied by Sir J.J. Thomson and others are given below:
• Cathode rays come out at right angles to the surface of the cathode and move in straight lines.
• Their path is independent on the position of the anode.
• They produce phosphorescence on certain salts like ZnS and fluorescence on glass.
• They blacken photographic plates.
• The rays pass through thin sheet of metals. If the metal sheet is too thick to be penetrated the rays cast a shadow.
• They produce X-ray when they strike a metal.
• The rays ionize a gas through which they pass.
• They heat a substance on which they fall.
• They rotate a light wheel placed in their paths. This shows that cathode rays contain material particles having both mass and velocity.
• The mass of a particle present in cathode rays is found to be 1/1837 of H-atom. This shows that the particle is of sub-atomic nature.
• Cathode rays are deflected by a magnetic or an electric field showing the particle to be electrically charged, the direction of deflection shows that they are negatively charged.
• Cathode rays contain the smallest unit of negative charge.
• No cathode ray was produced when the tube was completely evacuated.
• Different gases produce same cathode rays as they have the same e/m (charge/mass) ratio. This indicates that the particles present in cathode rays are fundamental constituent of all matter.

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