Passage of Electricity through Gases at Low Pressure:
Gases are bad conductor of electricity but it is observed that at low pressure they conduct electricity.
For this William purpose Crooks developed discharge tube or cathode ray tube in 1875.
But when the pressure inside the tube was reduced to 0.1mmHg by removing gas or air by vacuum pump, an electric discharge took place i.e. flow of current started and also the gas started glowing.
The color of glow depends upon composition of glass and nature of gas. On further reducing pressure to 0.01mmHg original glow of gas disappears. Some rays were produced which created fluorescence on the glass wall opposite to the cathode. These rays were given the name as cathode rays as they are emitted from cathode and travel towards anode.
Further investigations were made by performing different experiments, some are following:
Hittorff discharge tube
- Hittorff (in 1869) modified discharge tube into shape as shown in figure. In this of opaque object placed in their path. This experiment reveals that:
a) cathode rays are capable of casting shadows of opaque objects.
b) cathode rays travel in straight line.
Crooks discharge tube
(ii) Crooks (in 1870) performed an experiment in discharge tube as shown in figure. He placed a small paddle wheel in the path of cathode rays. The cathode rays made paddle wheel rotating.
This shows that
a) cathode rays are capable of doing work.
b) they possess energy and momentum.
c) They are material particles.
J. Perrin (in 1895) discharge tube
(iii) J. Perrin (in 1895) used a discharge tube (cathode ray tube) as shown in figure. He passed cathode rays through electric field.
The rays on passing through electric plates, were bent towards +ve plate.
This experiment shows that cathode rays are negatively charged.
An experiment similar to J. Perrin experiment was carried out. When cathode rays were passed through magnetic field ,they were bent perpendicular to poles of magnet.
This experiment also proves that cathode rays are charged rays.