Cathode rays

When the pressure in the discharge tube is less than 10^-4 mm of Hg, the discharge tube starts showing fluorescence. When this fluorescence was investigated, it was found that the fluorescence consisted of beams of negatively charged electrons. These electrons emanate normally from the cathode. As these emanate from the cathode, the rays are called the Cathode Rays.

We know that hydrogen atom is the lightest atom. In 1869, Sir J.J. Thomson found that the electrons had mass far less than of even the hydrogen atom. Experiments showed that mass of the electron is approximately 1/1860 of mass of hydrogen atom.

The cathode, in the discharge tubes used by Sir Thomson, was cold cathode. It needed a high voltage of the order of 30,000 volts. Modern cathode ray tubes have hot cathodes which require much less voltage, nearly 3000 volts.

Cathode Ray Oscilloscope

The discovery of cathode rays led to a vast field of practical application in Electronics. The glow produced by the fast moving electrons on a fluorescent screen led to its use in radar and television.

The oscilloscope has many points in common with the discharge tube. In the oscilloscope, the electrons are emitted by a hot cathode which is situated in a highly evacuated tube. At a short distance from the cathode is an anode having a central hole in it. A potential difference of some hundreds of volts is applied between cathode and anode. As a result, the electrons accelerate across the gap between the electrodes and a narrow stream of the electrons emerges from the hole in the anode.

Such arrangement of electrodes where a stream of electrons is produced is often known as the electron gun. On leaving the gun, the electron stream passes across the tube and eventually hits the screen at the far side. The screen is coated with phosphorus.

If necessary the stream of emerging electrons can be deflected in its passage between the gun and the screen. This deflection is produced by two pairs of parallel plates arranged at right angles. Usually, the potential difference applied to the X-plates makes the spot move across the screen at a uniform speed. If we alter the potential difference between the Y-plates, the beam is deflected upwards or downwards on the screen.

(a) Cathode rays travel in straight lines.

Cathode rays consist of an invisible stream of negatively charged particles.

When the plates X and Y are given +ve and -ve potentials respectively, the cathode ray beams gets attracted to the positive plate. This proves that they are made up of negatively charged particles.

Figure (c) above shows a beam of cathode rays being deflected by the magnetic field. By applying ‘Fleming’s left hand rule’ we can also prove that they are negatively charged particles.

Cathode rays travel with a great velocity nearly 9/10th of the speed of light and hence they possess great kinetic energy. When cathode rays are made to fall on a mica paddle wheel, the wheel starts rotating. This experiment proves that the rays possess great amount of kinetic energy.

Cathode rays can ionize gases.

Cathode rays can penetrate through thin sheets of aluminium and silver without perforating them.

They can produce fluorescence in many substances.

When they impinge on a metal of high atomic weight, X-rays are produced.

Measurements of their deflection by electric and magnetic fields show that they consist of charged particles whose specific charge is (charge to mass ratio) 1.76 x 10¹¹ C kg^-1. In fact they are electrons which are constituent of all matter.

Uses of Cathode Ray Tubes

They are widely used in science research laboratories by scientists for converting electrical signals into visual signals and television tubes.

Doctors use them for converting electrical impulses corresponding to heart beats into visual signals.