The Vacuum Tube
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.
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