For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit.
and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space.
In it's simplest form a vacuum tube can be created to operate as a diode: a device that conducts current only in one direction.
The result is a current of electrons flowing from filament to plate.
This cannot work in the reverse direction because the plate is not heated and cannot emit electrons.
The electrodes are attached to leads which pass through the envelope via an air tight seal.On most tubes, the leads are designed to plug into a tube socket for easy replacement.