Alternatively referred to as an electron tube or valve and first developed by John Ambrose Fleming in 1904. The vacuum tube is a glass tube with its gas removed, creating a vacuum. Vacuum tubes contain electrodes for controlling electron flow and were used in early computers as a switch or an amplifier. The picture shows a collection of different vacuum tubes used with different devices.
By using vacuum tubes instead of a mechanical relays computers could move away from mechanical switching and speed up the switching on and off the flow of electrons. Vacuum tubes were also used in radios, televisions, radar equipment, and telephone systems during the first half of the 1900s.
In the 1950s, the transistor started to replace the vacuum tube as vacuum tubes were larger in size, fragile like a light bulb, and expensive. As computing devices started to become smaller in size, transistors were more ideal to use due to their smaller size as well.
Today, vacuum tubes are primarily used in high-end audio equipment, preferred by some audiophiles as an alternative to digital systems.