The Vacuum Tube 
A vacuum tube is just that: a glass tube surrounding a vacuum (an area
from which all gases have been removed). What makes it interesting is
that when electrical contacts are put on the ends, you can get a current
to flow though that vacuum. Thomas Edison noticed this first in 1883.
While fiddling with lightbulbs he saw that he could get current to jump
from the hot filament to a metal plate at the bottom. What Edison discovered
(and it was promptly dubbed the "Edison effect") was that electrical current
doesn't need a wire to move through. It can travel right through a gas
or even a vacuum. The Edison effect, incidentally, is the only piece of
scientific work Edison ever did. He was not a scientist but an inventor,
a tinkerer. This kind of thinking would be as important as science for
the invention of the transistor.
Edison's discovery that current can travel through a vacuum didn't
turn out to be very useful information until 1904. That's when a British
scientist named John A. Fleming made a vacuum tube known today as a
diode. Then the diode was known as a "valve," because it forced
current in the tube to travel exclusively in one direction. Getting
that single directional flow was critical for radio sets which needed
to turn alternating current into direct current.
The vacuum tube didn't reach its full maturity until Lee De Forest came along a decade later. De Forest invented something he
called the "audion." Not only did it force current to move in a single
direction, but it could be used to increase the current along the way.
De Forest put a metal grid in the middle of the vacuum tube. By using
a small input current to change the voltage on the grid, De Forest could
control the flow of a second, more powerful current, through the tube.
The strength of two currents was not necessarily related -- a weak current
might be applied to the tube's grid, but a much stronger current could
come out the main electrodes of the tube.
Turning weak currents into strong currents was crucial for a number
of new technologies at that time. Bell Labs made use of it for its coast to coast phone system and vacuum tubes soon found their way into everything
from hearing aids to radios to televisions.
Sources
-Crystal Fire
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