Thermionic Emission

Nicholas Callan, an Irishman, born in 1799 near Dundalk, had in his time been the Professor of Natural Philosophy in NUI Maynooth. His main claim to fame was for the invention of the induction coil. The device is able to create some very high voltages.

Around 1873 Frederick Guthrie discovered that a red hot conductor would readily lose a negative charge but would hold a positive charge.

Thomas Edison noticed the same effect in 1880. Using the effect he made a device that could conduct electricity in one direction only and filed a patent based on the idea—the first ever patent for an electronic device.

The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that ‘thermionic emission’ could be used to detect radio waves and developed a vacuum tube diode which he patented in 1904

Principle of thermionic emission and its application to the production of a beam of electrons.

Thermionic emission is the giving off of electrons from the surface of a hot metal.

  1. Metal is heated
  2. The particles in the metal gain heat energy
  3. The electrons of the metal are excited to higher energy levels
  4. Some electrons gain enough energy to be released from the metal surface

Cathode ray tube, consisting of heated filament, cathode, anode, and screen.



Accelerating Anode

Deflection Plates

Fluorescent Screen

Use of cathode ray tube to demonstrate the production of a beam of electrons – deflection in electric and magnetic fields.

Photovoltaic cells


• cathode ray oscilloscope

• television.

Use of CRO to display signals:

• ECG and EEG.