Thermionic Emmision

  • Thermionic emmision is a process whereby electrons are emitted from a hot metallic surface or filament.

    Thermionic_emmission1.JPG Thermionic_emission2.JPG

  • The hot surface or filament which emits electrons is called the cathode.

  • An electron gun is a device which is used to produce fast moving electrons in a concentrated beam known as the cathode ray.


  • Properties of the cathode ray * It travels in a straight line. * It has momentum. * It has energy. * It produces fluorescence when it hits certain materials like phosphor. * In an electric field, the cathode ray is deflected to the positive plate in a parabolic path. Elect_electricfield.JPG

    * In a magnetic field, the cathode ray is deflected in a circular path in accordance with Fleming's left-hand rule.


Structure and working of a CRO

  • cro_1.gif


  • The electron gun consists of :- * a heated cathode to produce a beam of electrons to form the cathode ray * a grid of variable negative potential between the cathode and anodes to vary the number of electrons passing through it and eventually striking the screen. Hence the grid controls the brightness of the display on the screen. * focussing anodes to focus the beam of electrons to a spot on the screen. * accelerating anodes to accelerate the electrons moving towards the screen.

  • The deflecting system consists of :- * a vertical pair of plates, the X plates which functions to deflect the cathode ray horizontally by means of an electric field applied to them. Normally the X plates act as a time base which controls the speed at which the spot of light sweeps across the screen horizontally from left to right. This is done be means of applying a saw-toothed varying voltage across the X plates.


    The number of times the spot of light sweeps across the acreen per second is called the sweep frequency. Timebase.JPG

    time10msgain1V.gif Timebase 10 ms/div time20msgain1V.gif Timebase 20 ms/div

  • A horizontal pair of plates, the Y plates which function to deflect the cathode ray vertically be means of an electric field applied to them. Small input voltages are usually amplied by built in amplifiers in the Y plates. This leads to an increase in Y deflection. The Y gain control varies the amount of amplification in the vertical direction.

    time10msgain0.5V.gif Y gain 0.5 V/div time10msgain1V1.gif Y gain 1 V/div

  • The fluorescent screen and tube * The fluorescent screen is coated with a fluorescent salt like zinc sulphide. When cathode rays hit the screen, their kinetic energy is converted into light forming a bright spot on the screen. * Air in the tube is removed so as not to obstruct the moving electrons. The earthed graphite coating on the tube beyond the electron gun serves to shield the cathode ray from any electric field outside the tube and to conduct the flow of electrons on the screen to the earth.

  • Uses of CRO

    (a) To measure voltage:-


    * Switch off the time base

    * Connect the voltage to be measured to the Y input terminal

    * Adjust the Y gain until the deflection of the spot of light on the screen is at a measurable distance from the centre.

    * Measure and record the distance of deflection of the spot of light.

    * The measured voltage is given by the following equations:

    For d.c. voltages, V = Y gain x distance of deflection

    For a.c. voltages, peak voltage = Y gain x (distance of deflection /2)

    * Advantages of CRO in measuring voltage include:-

    - its nearly infinite resistance which draws negligible current

    - its use to measure both d.c. and a.c. voltages

    - its immediate response

    (b) To measure short time intervals


    * Set the time base to a suitable value.

    * Connect the source of time intervals to be measured to the Y plates.

    * From the display on the screen, measure and record the horizonral distance between the two points, d

    * The short time interval is given by d x time base

    (c) To display waveforms


    * When a time base is switched on with a suitable frequency, a steady waveform of any input voltage can be displayed on the screen.

    * Waveforms are used in the studies of heart beats, brain waves etc in medical research, the quality and type of sounds made by different musical instruments, varying voltages.

Virtual CRO

Notes to explain the virtual CRO