Sealed container, usually glass, with all the air taken out (creating a vacuum) and two electrodes placed at opposite ends of the container from which electrons can flow (electrons flow freely in the vacuum environment with no air present). One of the electrodes, the cathode, is heated so that it may release electrons. The other electrode, the anode or plate, attracts and accepts electrons when it is positive in respect to the cathode but does not attract electrons when it is negative (electrons are negative so a positive polarity attracts them but a negative polarity repels them – two items of the same polarity either both negative or both positive repel each other). In this way, signals can be turned on and off by controlling the polarity, positive or negative, of the anode. acuum tubes used for amplification feature an additional electrode, called a grid, placed between the anode (the emitter of electrons) and cathode (plate – the acceptor). The additional electrodes (grids) either attract or (in most cases) repel electrons flowing through a vacuum tube. The grid needs only a small voltage change to create a large change in the vacuum tube between the anode and cathode. This small change resulting in a large change creates amplification. Vacuum tubes using a grid (using three electrodes – the grid, the cathode and the anode) are called triodes. Those using only a cathode and anode are called diodes. acuum tubes have been mostly supplanted by semiconducting transistors, which are easier to manufacture, take up much less space, are more efficient and reliable. Vacuum tubes are still used in some high-end amplifiers and audio components for their mellow sound. Cathode ray tubes used in many video displays are also a form of vacuum tube and are the most widely used today.