A vacuum tube is an electronic device
that controls an electron current through a
vacuum from its cathode to its anode. A vacuum
is normally a good insulator for current but
many mechanisms exist which allow electrons to
leave a surface and flow through empty space.
The main method that vacuum tubes use to conduct
electricity in empty space is Thermionic
Emission and that is why they were sometimes
referred to as Thermionic Valves. The vacuum
tube uses an electric or magnetic field or
sometimes both to control the flow of electrons
through the envelope.
Simple Diagram of a vacuum tube. This is of
the triode type.
Image credit:
http://www.privateline.com/tube/triode.jpg
