Valve/Tube

Valve in Britain, tube in America. A device for controlling the flow of electrons. In a simple amplifier, a filament inside this evacuated tube heats up (like a lamp) and causes electrons to be emitted from the hot metal (cathode). If a positive charge is applied to another piece of metal (anode) inside the tube, the electrons go rushing towards it.

By inserting a metal mesh (called the grid) between the two, you can apply small voltages to it that can control the big rush of electrons in exactly the same way, and since the small voltage controls a large flow you get an amplifier. You can see why it's called a valve.

Transistors can achieve the same thing without the size and power requirements, so most tubes in the world are so yesterday. But in Russia, they kept making them because a nuclear EMP can knock out transistors but tubes will continue to function. And some die-hard audiophiles still think a tube amp sounds "warmer" than a transistor amp. Some people prefer vinyl records too, go figure.

These days some people are making steam punk jewelery from tubes. And you can see why it's called a tube.

Scale: the glass part is 5cm long (high).