Vacuum tube

A vacuum tube is a glass bulb with the air removed and metal parts inside that controls the flow of electric current. A heated part gives off electrons, and a small voltage on a wire grid controls how many reach a plate, which allows a small signal to control a larger one. Tubes were used in radios and amplifiers before transistors and are still common in guitar amplifiers.

Example: The glowing glass bottles at the back of a vintage guitar amp are vacuum tubes.