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Cathode Rays

A cathode ray is defined as a stream of electrons leaving the negative electrode (or cathode) in a discharge tube that contains gas at low pressure or the electrons emitted by the heated filament in certain electron tubes containing an anode and a cathode.

Production Of Cathode Rays

The Cathode ray  was invented by  J. J. Thomson  during his experiments with cathode ray tubes. During this experiment he  took a tube made of glass containing two pieces of metal as an electrode.

The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

1. Thomson designed a glass tube that was partly evacuated, i.e. all the air had been drained out of the building. He then applied a high electric voltage at either end of the tube between two electrodes.

He observed a particle stream (ray) coming out of the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray and the entire construction is called a cathode ray tube.·  

 Properties Of Cathode Rays

1. Like a wave, the cathode rays travel in straight lines and form a shadow when obstructed by the objects.
2. They are particulate in nature i.e. negatively charged electrons
3. They are affected by both magnetic and electric fields since they are deflected towards the positive plates
4. They produce fluorescence in some materials
5. Depending on the energy of the cathode rays they can penetrate thin sheets of paper, metal foils
6. When cathode rays are stopped they produce X-rays.
7. They affect photographic plates.

 Cathode Ray Oscilloscope (CRO)

It is a complex equipment used in displaying waveforms from various sources and measuring  potential difference (p.d) It comprises of the following main components;

The cathode ray tubes (CRT) –A cathode-ray tube (CRT) is a vacuum tube in which an electron beam is deflected and modulated by applied electric or magnetic field to produce a image on a fluorescent screen or a projector .

The picture may reflect electrical wave forms (oscilloscope), photographs (television, computer monitor), echoes of radar-detected aircraft, and so on. The single electron beam can be processed to show movable images in natural colours.

Generally, the  function of the cathode ray tube is to convert an electrical signal into a visual display.

 CRT consists of a tube, electron gun, deflection plates and the time base (TB).

1. The tube is made of strong glass to withstand the pressure difference between the outside atmospheric pressure and the vacuum inside. It has a square grid placed in front of it to allow measurements to be made.
2. The electron gun produces the electrons with main parts consisting of a filament, a cathode, a grid and the anode. Electrons are produced by the cathode when heated by the filament.

 The grid is a control electrode which determines the number of electrons reaching the screen therefore determining the brightness of the screen.

 The Y-deflection plates deflects the beam up or down.

The X-deflection plates are used to move the beam left or right of the screen at a steady speed using the time base circuit which automatically changes voltage to an a.c. voltage. When time base control is turned the speed can be adjusted to produce a waveform.

The television set (TV) is a type of a CRT with both Y and X-deflection plates which control the formation of a picture (motion) on the screen.

The colour television screen is coated with different phosphor dots (chemicals) which produce a different colour when struck by an electron beam.

·Question

1. If the time base control of the CRO is set at 10 milliseconds per cm, what is the frequency of the wave traced given wavelength as 1.8 cm?

 Solution

 Wavelength = 1.8 cm.

 time for complete wave= period = 1.8 × 10 milliseconds / cm

•  = 18 milliseconds
•  = 1.8 × 10^-2 seconds.

      Frequency ‘f’, is given by f = 1 / T

• = 1 / 1.8 × 10^-2 
• = 100 / 1.8
• = 56 Hz.