Vacuum tube

A glass tube that has had all gas has been removed creating a vacuum. Vacuum tubes contain electrodes for controlling electron flow and were commonly used in early computers as a switch or an amplifier. To the right is a collection of different types of vacuum tubes to give you a better understanding of what these look like. Today, vacuum tubes are no longer used and have been replaced by transistors in computers.
 
Also see: Electronics definitions, Nixie tube