This Unit is about the Cathode Ray Oscilloscope and how it operates.


Cathode rays are highly energetic electrons emitted by metal surfaces when heated to very high temperatures. They more from the cathode to the anode.

Production of cathode rays


The electrons are produced at the cathode by thermionic emission and are accelerated towards the screen by the anode which is connected to the terminal of the extra high tension battery. The thermionic emission is the process whereby metal surfaces emit electrons when heated.

The tube is evacuated to avoid electrons interacting with any particle before they reach the screen .When the cathode rays hit the fluorescent screen, the screen glows. This shows that electrons posses momentum and therefore have mass.

Properties of cathode rays

  1. i) They are negatively charged.
  2. ii) They travel in a straight line.

Iii) They are deflected by both magnetic and electric fields (this proves they carry change).

  1. iv) They cause fluorescence in certain materials.
  2. v) They are electrons moving with high speeds.
  3. vi) When cathode rays are stopped by heavy metals, X rays are emitted.

Verification that electrons travel in a straight line


If an opaque object (Maltese cross) is placed in the path of the cathode rays, a sharp shadow of the Maltese cross is cost on the screen.

Cathode Ray Oscilloscope (CRO)

This is an instrument  used  for  studying  the  current and voltage wave  forms  in  various  circuits .the  the  chief  feature  of the  oscilloscope is the vacuum  containing three  main parts.

-Electron Gun

-the deflecting system

-The florescent screen

Uses of the parts

  1. Electron Gun

This consists of a heater    G, a cathode a control Grid and the cylindrical [Ring-like] anodes.

     a.Evacuated glass tube

The glass tube is evacuated to prevent scattering of the electron beam when electrons collide with air molecules.

    b. Cathode;

Emits electrons by thermionic emission

   c. The control Grid

It consists of hollow metal cylinder with small hole at the end. it is held  at variable  negative  potential  to the cathode  by means of the potential  divider   R1 .The  Grid  serves two  purposes  namely

-It determines   the brightness of the spot on the screen.

-It refocuses the electron beam so that the beam emerges from the hole as a narrow beam.

Anodes A and A2; – These are held at a positive potential relative to the cathode. The  anode accelerates  the electron  beam  along  the tube  and  also focuses  the electron beam into fine  beam  by means  of the  potential divider  R2.

  1. The Deflecting system

This deflects the beam either horizontally or vertically. It has two pairs of plates, a horizontal pair called the Y-plates and vertically pair called X plates.

X-plates, X and X these are vertical plates but they deflect the beam horizontally when a p.d is applied across them.

  1. Fluorescent screen

This is coated with fluorescent material such as zinc supplied. It enters light when struck by electron beam.

-Graphite coating

Enables light to be seen only on the screen as the graphite coating absorbs the electron’s kinetic energy.

-Power supply:

This is a smoothened rectified a.c fed through a chain of resisters.


The brightness of the spot depends on the rate at which the electron strikes the screen. The number of electron will increase if there is increase in the heater current and the grid is less negative.

Operation of the CRO

Suppose the x-plates where shunted and a.d.c. voltage was applied to the y-plates. The electron spot would be deflected vertically.


If the x-plates are shunted and an a.c voltage is applied to y – plates, the electron beam is drawn into a vertical line.


When no signal is applied to the y – plates, the voltage v, causes the electron beam to sweep horizontally to and from on the screen as shown.


To observe the ware from of the applied voltage to y – plates, the frequency of the time bass is synchronized with the frequency of the signal applied to y – plates. For an a.c signal applied across the y-plates and time base on the x-plates, one observes the wave from shown below;


Uses of a CRO

  1. Displaying of wave forms:
  2. Measurement of voltage:

An unknown voltage is applied across the y-plates. If the time base is switched off, a vertically line is obtained on the screen.

Comparison of frequencies of two wave forms;

Comparison of CRO with a moving coil voltmeter

  • The CRO has very high impendence. It gives accurate voltages than a moving coil voltmeter.
  •  A CRO can measure both d.c and a.c voltage. A moving coil voltmeter measures only D.C voltages unless a rectifier is used.
  •  A CRO has negligible inertia as compared to a moving coil voltmeter. The CRO respond almost in state onerously
  •  CRO doesn’t give direct voltage readings.

Photoelectric effect

When some metals held at a negative potential are illuminated by electromagnetic radiations electrons are emitted. This called photoelectric emission.

Demonstration of photoelectric effect


When light falls on a metal cathode, a galvanometer shows a deflection, indicating flow of current. However when the plates are covered, more current flows. Energy of the incident light is absorbed by the electrons and instantly an electron jumps out. Such effected electrons are called photoelectrons.

Experimental observation on photo electric effect

  1. There is negligible time delay between irradiation of metal surface and emission of electrons by the surface.
  1. The photo current (number of photo electrons per second) is proportional to the intensity of the incident radiation.
  1. The maximum kinetic energy of photoelectrons increases linearly with the frequency or the incident radiation but is independent of the intensity of the radiation.
  1. For each metal surface there is a minimum frequency of the incident radiation below which no electrons are emitted however high is the intensity. The frequency is called These hold frequency of the metal surface.



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