- These are streams of electrons emitted at the cathode of an evacuated tube containing an anode and a cathode.
- They are produced by a set up called a discharge tube where a high voltage source usually referred to as extra high tension (EHT) supply connected across a tube containing air at low pressure thereby producing a luminous electron discharge between the two brass rods placed at opposite ends of the tube.
- These electron discharges are called cathode rays which were discovered by J.J Thomson in the 18 th century.
- They travel in straight lines
- They are particulate in nature i.e. negatively charged electrons
- They are affected by both magnetic and electric fields since they are deflected towards the positive plates
- They produce fluorescence in some materials
- Depending on the energy of the cathode rays they can penetrate thin sheets of paper, metal foils
- When cathode rays are stopped they produce X-rays.
- They affect photographic plates.
- It is a complex equipment used in displaying waveforms from various sources and measuring p.d.
- It comprises of the following main components; - The cathode ray tubes (CRT) – consists of a tube, electron gun, deflection plates and the time base (TB).
- 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.
- 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. Clearly observable when low frequency inputs are applied i.e. 10 Hz from a signal operator.
- 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.
- 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?
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=100/1.8 by =56 Hz or f = 1/T = 1/1.8 × 10-2 = 56 Hz
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.