Discharge tube Experiment (William Crooks Experiment)
(Discovery of Electrons / Cathode rays)
Gases are bad conductors of electricity but it is observed that at low pressure they conduct electricity. For this William purpose Crooks developed a discharge tube or cathode ray tube.
The gas discharge tube consists of :
1. Cylindrical glass tube
2. Two metal electrodes
3. It may contain a gas, air or vapours of any substance
4. Vacuum pump ( The gas can be evacuated to any desired pressure with the help of a vacuum pump)
5. High voltage battery (The metallic electrodes inside the glass tube are connected to a high voltage battery)
6. A slit (used to get a sharp beam of radiation)
Working of Discharge tube:
- William Crooks observed that when an electric current was passed through the gas at a normal pressure in a discharge tube, The gas remained unaffected even at 5000 volts.
- When the pressure inside the gas tube was reduced below 4 torr, a bright glow appeared inside the tube.
- But when the pressure inside the tube was reduced to 0.01 torr by removing gas or air by a vacuum pump and a high voltage of 5000-10,000 volts applied, the original glow of gas disappears and current starts to flow through the gas and the light is emitted.
One the modern examples of a discharge tube are the “ Neon sign”
- At further pressure reduction inside the discharge tube, the light emission by the gas stops. At this point, some rays were produced from the cathode and moved towards the anode.
- These rays were given the name cathode rays as they are emitted from the cathode and travel towards the anode.