annulus. A ring or groove cut around the outside of a circular body or shaft. An annulus gets its name from its resemblance to the circular shape of the annual rings (the growth rings) which form in the trunk of a tree.
annunciator panel. A panel of warning lights in plain view of the pilot of an airplane or the operator of other types of equipment. The lights are identified by the name of the system they represent and are usually covered with a colored lens to show the meaning of the condition they announce. Red lights are used to indicate a dangerous condition, amber lights show that some system is armed, or active, and green lights show that a condition is safe.
By having all the lights together in an annunciator panel, the pilot or operator can determine the overall operating condition of the equipment at a glance.
anode. The electrode in a vacuum tube or semiconductor diode to which the electrons travel after they leave the cathode. The anode in a vacuum tube is called the plate.
The anode of a semiconductor diode is the end made of P-type material. It is not marked — the cathode has the mark. In the diode symbol, the anode is shown by the arrowhead.

anode current. Current that flows in the anode of an electrical device. In an electron tube, anode current is commonly called plate current.
anode of a chemical cell. The area in a chemical cell from which electrons have migrated. When electrons leave an area, it becomes less negative, or takes on a positive charge. Electrons then try to move from an area having an excess of electrons, the cathode of the cell, to this anodic area.
anodic area. An area inside a piece of metal that has lost some of its electrons.
This leaves a net positive charge which attracts negative ions from the electrolyte. Positive metallic ions and negative ions from the electrolyte unite to form the salts of corrosion.
anodizing. A method of preventing corrosion of aluminum alloy parts. A hard oxide film is formed on the surface of the metal by an electrolytic process. This film prevents oxygen reaching the surface of the metal.
The aluminum alloy is the anode of the electrolytic cell, and chromic acid is used as the electrolyte.
anomalous propagation (radar meteorology). The greater-than-normal bending of a radar beam which causes echoes to be received from ground targets at greater distances than normal ground clutter.
anoxia. A severe case of hypoxia, or the serious effect on the human body caused by a lack of oxygen.
A-N radio range (radio navigation aid). The low-frequency radio range which was one of the first practical radio navigational aids.