A vacuum tube is typically used in electronics to somehow modify a signal. By controlling the movement of electrons, vacuum tubes are able to amplify, switch, or in some other way modify the signal in an evacuated space. For the most part, the vacuum tube has been replaced by the transistor, which is much smaller and less expensive. However, the vacuum tube is still in use in various applications such as radio systems and high power RF transmitters. Vacuum tubes are also in use in television sets and computer monitors (in the form of cathode-ray tubes) and in microwaves and microwave ovens (in the form of magnetrons).
Also known as “thermionic valves”, vacuum tubes are voltage-controlled devices, and therefore the relationship between the input and output circuits is determined by a transconductance function. The tubes are a collection of electrodes organized inside a vacuum. These electrodes are encased in an insulating and temperature-resistant envelope. Classically glass, envelopes currently used in power tubes are often made of ceramic or metal. The electrodes are connected to leads which, through an air-tight seal, pass through the envelope and generally are designed to plug in to a tube socket to facilitate replacement.
The solid-state device most similar to the vacuum tube is the junction gate field-effect transistor, though the vacuum tube tends to operate at a higher voltage and power and in its simplest form, a vacuum tube resembles an incandescent light bulb: it has a filament sealed inside of a glass envelope that has been evacuated of all air.