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 act of notability was for the invention of the induction coil. The device is able to create 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

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

Principle of thermionic emission

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.

Heater

The Heater heats the metal. This gives the electrons in the metal an energy that allows them to climb to higher orbits (electron shells). Being more removed from the nucleus allows them to be attracted away from their atom. The heater is usually powered by a normal (lower than the accelerator) voltage.

Cathode

This cathode is the metal plate and is held at 0V (zero potential) with respect to the Accelerating Anode. This plate is heated by the heater, and thus becomes the donor of the electrons that make up the beam.

Accelerating Anode

A cylinder that has a cylindrical tube bored through it. It is held at a High Positive Potential (e.g. 5000V). This potential difference attracts the weakly held electrons of the Cathode, and they accelerate, aquiring an energy of qV (where q is the charge on an electron, and V the potential difference between the cathode and Anode.

Deflection Plates

Variable voltages control the positioning of the beam on the screen, if the beam is to go up, the y-axis deflection plates would aquire a potential difference that attracts the electrons up, (i.e. positive voltage on the top plate), left- right is controlled by the x-axis deflection plate.

Fluorescent Screen

a transparent screen coated on one side with a phosphor that fluoresces when exposed to X-rays or cathode rays

https://www.thefreedictionary.com/fluorescent+screen 200126

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

Photovoltaic cells

Applications

• cathode ray oscilloscope

• television.

Use of CRO to display signals:

• ECG and EEG.