vacuum tube

A vacuum tube is an obsolete piece of electronic equipment, used in several electronic devices. It was largely replaced by the more durable Transistor. Microwave ovens still use them, however, since they are more resistant to overload.

A Vacuum Tube consists of a glass jar containing a heated metal pin at one end and a metal plate on the other. The space between the anode and cathode has nothing in between; as the name "vacuum tube" suggests. This causes current to go one way through the circut.

It also refers to a male-specific sex toy that simulates fellatio.