CATHODE RAYS AND THEIR PROPERTIES
At normal atmospheric pressure, the gases, are bad conductors of electricity. But when a very high potential difference of 20000 V is applied between 2 electrodes which are separated by few millimeters distance, a series of sparks passes between the electrodes. Experimentally, when the pressure of gas is reduced, the conduction of electricity takes place comparatively l at low potential difference. This is called as electric discharge. The reason is that when a discharge is initiated the gasgets ionized.
When a discharge tube is exhausted to a low pressure of 10-2 mm to
10-3mm, the positive column disappears and a dark space called Crooke dark space forms and fills the whole tube . At this stage the walls of the glass tube fluorescence is with green color This is the stage when bluish streamers travel across the tube from cathode to anode. These streamers are called Cathode Rays.
Cathode rays are independent of the nature of the gas or electrodes employed in the discharge tube. In fact, Cathode rays are beams of electrodes which are common to all elements. When these rays pass through gas,fresh ions are produced by collision and discharge is maintained.
Following are the properties of Cathode rays
1. Cathode rays are emitted from the cathode surface normally irrespective of the position of anode .
2. They travel in straight lines and cast shadows of objects placed in their path.
3. They produce heat when allowed to fall on matter.
4. They produce fluorescence on some substances on which they strike .
5. They possess mechanical energy or they exert mechanical pressure .
6. Cathode rays can penetrate through small thickness of matter .
7. They carry negative charge.
8. They ionize the gas through which they pass and make it conducting.
9. They affect a photographic plate.
10. Cathode rays are deflected by electric field.
11. Cathode rays are also deflected by magnetic fields.
12. When Cathode rays strike a solid substances of large atomic weight , they produce X-rays .
The Cathode rays are the precursor of X-Rays. We are aware the usefulness of X- Rays in medical science particularly in Orthopedics. In Engineering the X= Rays are used to detect internal cracks in Castings. If cracks are not detected before assembly may result in severe cracking up and ultimately machine break down and production losses.