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RADIO ENGINEERING AND ELECTRONICS
Electron Emission
There is little doubt that wireless, radio, and television are among the greatest
miracles of modern science. Traveling with the speed of light, code signals, the
human voice and music can be heard around the world within the very second they
are produced in the broadcasting studio. Through television, world events can be
observed in full color at the same moment they occur hundreds of miles away.
The more we learn of the fundamental principles of radio and its operation the
more amazing does their reality become. The heart of a tube is the source of
electrons. There are several ways in which free electrons are obtainable.
Thermionic Emission. The velocity of electrons and atoms, as they move about
within the confines of the material they comprise, is dependent on the temperature.
At a temperature of absolute zero all molecular activity is supposed to cease. As
the temperature is increased, the activity of electrons and atoms increases until a
point is reached where the electrons have sufficient velocity to enable them to
break through the potential barrier of the material. This evaporation of electrons
from the body of a solid at high temperature is known, as thermionic emission. The
emission or evaporation of electrons takes place at lower temperature than does
that of atoms. The mass of electrons "being smaller, it reaches the higher velocities
necessary for evaporation at low temperatures than does the heavier atom. The
temperature becoming high enough for the atoms to evaporate, the material or solid
that they compose rapidly disintegrates.
Secondary Emission. One knows to a high degree of certainty*1 that being
accelerated to a sufficiently high velocity an electron may have enough kinetic
energy imparted to it to knock one or more electrons out of any material it comes
in contact with, either a metal conductor or an insulator. A positively charged
electrode situated near the source of these "secondary" electrons will collect them.
In actual tubes the secondary electrons may be attracted back to the electrode they
come from, as from the plate, or they may be collected by another electrode which
is positively charged. In many tubes these secondary electrons give rise to
undesirable effects, design steps being taken to reduce their number and to control
their movements. In a few tubes, such as electron multipliers, the desired operation
is based on the principle of secondary emission.
Photoelectric Emission. When light of proper wavelength is allowed to fall upon
certain metals, electrons are released from the surface of the metal as a result of the
energy imparted by the light. Here, then, is another electron source. Such sources
are used in phototubes and in certain types of television camera tubes.
Diodes
The modern diode consists of a glass envelope in which are erected a small metal
plate and a fine wire called the filament. The filament in a modern tube is referred
to as the cathode or emitter, its material being chosen so that it will heat when an
electric current flows through it. The cathode may be a sleeve or a cylinder of
