Active Electronics

The Vacuum Tube Triode

In 1907, Lee De Forest patented the vacuum tube triode. This "thermionic valve" (as it is called in England) is essentially a light bulb with a metal cylinder (called the plate) around the filament. The tube is wired to a battery with the negative terminal connected to the filament. The positive terminal connected to the plate.

An early vacuum tube on the left with a schematic of a tube circuit on the right.

The heat of the filament frees electrons that then stream through the vacuum of the tube to the plate. This constitutes an electric current just like the electrical current that flows in wires. The third element of the triode is a grid between the filament and the plate. If the grid has a weak negative charge, it will block the flow of electrons. This makes the triode an electrically-controlled switch with no moving parts. The triode can switch much faster than electromechanical relays (electromagnetic switches), doesn't create sparks, and it has no contacts to bounce^[1] and wear down.