Electronics is that branch of applied physics which deals with the control of motion of electrons using different devices. Electronic devices being more effective and reliable have revolutionized the fields of telecommunication and information technology. This chapter aims at providing basic concepts of electronics
16.1 THERMIONIC EMISSION
In the 1850's, physicists started to examine the passage of electricity through a vacuum by putting two electrodes in a sealed vacuum tube. Some kind of rays were emitted from the cathode or the negative electrode. These rays were called cathode rays. J.J. Thomson in 1897 observed the deflection of cathode rays by both electric and magnetic fields. From these deflection experiments, he concluded that cathode rays must carry a negative charge. These negatively charged particles were given the name electrons.
The process of emission of electrons from the hot metal surfaces is called thermionic emission. Metals contain a large number of free electrons. At room temperature electrons cannot escape the metal surface due to attractive forces of the atomic nucleus. If the metal is heated to a high temperature, some of the free electrons may gain sufficient energy to escape the metal surface.
Thermionic emission can also be produced by electrically heating a fine tungsten filament. Typical values of the voltage and current used are 6 V and 0.3 A respectively. Now we examine some important experiments performed for discovering the properties of the electrons.