CATHODE RAYS-DISCOVERY OF ELECTRON-DISCHARGE TUBE EXPERIMENT-
CROOK’S TUBE EXPERIMENT
 
INTRODUCTION
 
Gases are bad conductors of electricity. However under reduced pressure and at high potential difference gases conduct electric current.
DISCHARGE TUBE
 
Discharge tube is a glass tube fitted with two electrodes placed opposite to each other. The tube is sealed and contains a vacuum pump. The function of vacuum pump is to reduced or change the pressure inside the tube. The two electrodes are connected to a high voltage battery.
     
THE EXPERIMENT
 
  In discharge tube experiment, at low pressure and at very high potential difference, electric current is passed through the gas.
OBSERVATION
 
Under different pressure different observations were noted;
At 1cm Hg pressure:
When pressure inside the tube is reduced to 1 cm of Hg, at a potential difference of a few thousand volts causes a spark to pass like a flash of light.
At 1 mm Hg:
At 1 mm Hg, the tube is mostly filled with a glow extending from the positive electrode. This is known as "positive column".
At 0.001 mm Hg:
At 0.001 m Hg, the glow disappears and the walls of the glass tube begin to glow with a brilliant green light.
 RESULT
 
These observations indicate that some radiation or rays are emitted from cathode. These rays are known as "Cathode Rays".
PROPERTIES OF CATHODE RAYS
 
ORIGIN
 
These rays originate from cathode.
PATH
 
Cathode rays travel in straight line.
PRESSURE
 
Cathode rays exert mechanical pressure.
SHADOW FORMATION
 
The cathode rays consist of material particles because they produced shadow of objects placed in      the way.
EFFECT OF ELECTRIC FIELD
 
Cathode rays deflect in applied electric field towards the positive terminal.
EFFECT OF MAGNETIC FIELD
 
Cathode rays are deflected by magnetic field.
PENETRATION POWER
 
Cathode rays penetrate small thickness of matter such as aluminum foil, gold foil, etc.
CHARGE
 
These rays carry a negative charge.
DEPENDENCE ON MATERIAL
 
Cathode rays are independent of the material of electrode or the nature of gas in the tube.
e/m RATIO
 
Their e/m ratio was equal to that of an electron.
CONCLUSION
 
Different scientist tried different discharge tubes with different electrodes and different gases but results of all the experiment gave same value for charge to mass ratio. This shows that there is something common in all materials. It was concluded that all the substances in common.
Experiment shows that these particles could be produces from any kind of material. Cathode rays consist of particles which are known as electrons.
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