Discovery Of Electron

DISCOVERY OF ELECTRON
“The cathode ray tube”
Electrons were discovered in a device called a cathode Ray tube. William crook discovered the cathode Ray Tube in 1895. A cathode ray tube is made of glass, contain, two metallic electrodes called as ”cathode” and” Anode”. It may contain gas or vapors or air at any desired pressure. The tube is connected to a VACUME PUMP so that the pressure can be maintained at any desired level. Sir William Crooks was experimenting in 1895 on the discharge of electricity though gases at low pressure. Which resulted into the discovery of Electron. A simple cathode ray tube used for the study on the discharge of electricity through gases is shown in figure.

We can also study gas discharge at various pressures. At first the pressure is kept at normal and a high voltage (10KV) is applied. No conduction of electricity can be noted. But when pressure is lowered, electricity can pass through and produce sparks. When the pressure in further reduced to almost 0.1 torr, the conduction took place and a uniform glow appeared inside the tube instead of sparks. Under this condition gas is said to have conducted the electric current.When the pressure is further reduced to 0.01 torr, the glow disappeared and some kind of rays are produced. These rays, when strike the opposite side of the tube produce fluorescence. These rays are called as CATHODE RAYS.The properties of cathode ray were determined using different gases in the tube. It was found that properties remained the same regardless of nature of gas used.

PROPERTIES OF CATHODE RAYS

Various experiments on cathode rays showed the following properties: -

Cathode rays always travel in a straight line, when same opaque object is placed in its path, it produce shadow as shown in fig:

Cathode rays possess the properties of particles. Cathode rays possess momentum and have kinetic energy. This can be demonstrated by placing a very light pinwheel in their path. When cathode rays fall on the pinwheel, the pinwheel will start rotation. This shows that the particles of cathode rays falls on the paddle of the pin wheel pushing it forward, this causing rotation into it, this experiment can be explain by he following figure:

An electric field is applied across the cathode ray tube. Electric field consists of a positively charged plate and a negatively charged plate. When sharp beam of cathode ray is passed though his electric field it is observed that this ray would deflect toward the positive plate of the applied electric field. This deflection towards the positive plate shows the negatively charged nature of the cathode ray tube.

When the cathode rays are passed though the applied magnetic field, they will defect at right angle to the axis joining the north and south poles of magnet.

This shows the charged nature of the cathode rays.
When a small metal fail is placed in the path of the cathode rays, it will heat up the metal foil.
Mosley discovered that when cathode rays strike a metal. It generates certain rays called as x-rays. Metal used are usually with high atomic mass.
Keeping in view the above experiments, one can suggest that cathode rays behave like stream of particles, which are negatively charged deflected by electric and magnetic filed. G.J Stoney named these particles as “Electrons” in 1891.