1-2
reached where some of the free electrons become so agitated that they actually escape from the conductor.
They "boil" from the conductor’s surface. The process is similar to steam leaving the surface of boiling
water.
Heating a conductor to a temperature sufficiently high causing the conductor to give off electrons is
called THERMIONIC EMISSION. The idea of electrons leaving the surface is shown in figure 1-1.
Figure 1-1.—Thermionic emission.
Thomas Edison discovered the principle of thermionic emission as he looked for ways to keep soot
from clouding his incandescent light bulb. Edison placed a metal plate inside his bulb along with the
normal filament. He left a gap, a space, between the filament and the plate. He then placed a battery in
series between the plate and the filament, with the positive side toward the plate and the negative side
toward the filament. This circuit is shown in figure 1-2.
