Anode

Anode, the terminal or electrode from which electrons leave a system. In a battery or other source of direct current the anode is the negative terminal, but in a passive load it is the positive terminal. For example, in an electron tube electrons from the cathode travel across the tube toward the anode, and in an electroplating cell negative ions are deposited at the anode. Compare cathode.

Electrode

Electrode, electric conductor, usually metal, used as either of the two terminals of an electrically conducting medium; it conducts current into and out of the medium, which may be an electrolytic solution as in a storage battery, or a solid, gas, or vacuum. The electrode from which electrons emerge is called the cathode and is designated as negative; the electrode that receives electrons is called the anode and is designated as positive. In an electron tube, the anode is called the plate, and conducting elements that regulate the electron flow inside the tube are also called electrodes.

Electrode, electric conductor, usually metal, used as either of the two terminals of an electrically conducting medium; it conducts current into and out of the medium, which may be an electrolytic solution as in a storage battery, or a solid, gas, or vacuum. The electrode from which electrons emerge is called the cathode and is designated as negative; the electrode that receives electrons is called the anode and is designated as positive. In an electron tube, the anode is called the plate, and conducting elements that regulate the electron flow inside the tube are also called electrodes.

Electron tube, also called vacuum tube, device usually consisting of a sealed glass or metal-ceramic enclosure that is used in electronic circuitry to control a flow of electrons. Among the common applications of vacuum tubes are amplification of a weak current, rectification of an alternating current (AC) to direct current (DC), generation of oscillating radio-frequency (RF) power for radio and radar, and creation of images on a television screen or computer monitor. Common types of electron tubes include magnetrons, klystrons, gyrotrons, cathode-ray tubes (such as the thyratron), photoelectric cells (also known as phototubes), and neon and fluorescent lamps.

Until the late 1950s, vacuum tubes were used in virtually every kind of electronic device—computers, radios, transmitters, components of high-fidelity sound systems, and so on. After World War II the transistor was perfected, and solid-state devices (based on semiconductors) came to be used in all applications at low power and low frequency. The common conception at first was that solid-state technology would rapidly render the electron tube obsolete. Such has not been the case, however, for each technology has come to dominate a particular frequency and power range. The higher power levels (hundreds of watts) and frequencies (above 8 gigahertz [GHz]) are dominated by electron tubes and the lower levels by solid-state devices. High power levels have always been required for radio transmitters, radar systems, and implements of electronic warfare, and microwave communications systems may require power levels of hundreds of watts. Power in these cases is frequently provided by klystrons, magnetrons, and traveling-wave tubes. Extremely high average power levels—several megawatts at frequencies above 60 GHz—are achieved by gyrotrons; these are used primarily for deep-space radars, microwave weapons, and drivers for high-energy particle accelerators.

Vacuum tube technology continues to advance, because of a combination of device innovation, enhanced understanding through improved mathematical modeling and design, and the introduction of superior materials. The bandwidth over which electron tubes operate has more than doubled since 1990. The efficiency with which DC power is converted to RF power has increased up to 75 percent in some devices. New materials, such as diamond for dielectrics, pyrolitic graphite for collectors, and new rare-earth magnets for beam control, greatly improve the power handling and efficiency of modern electron tubes.