self-extinguishing. A combustible material is said to be self-extinguishing if it burns when it is exposed to a flame or to a high temperature, but the burning process automatically stops as soon as the flame or source of high temperature is removed.
self-healing capacitor. An electrolytic capacitor with the ability to automatically restore the oxide film dielectric to the plate if the film is ever punctured by too high a voltage.
self-induction. The characteristic of an electrical conductor that causes a counter electromotive force (CEMF, or back voltage) to be built up in it by alternating current flowing through it. The expanding and collapsing magnetic field caused by the alternating current cuts across the conductor and induces the CEMF in it. The polarity of the self-induced voltage is always opposite that of the voltage in the original circuit.

self-locking nut. A type of nut with a built-in locking device that makes the nut tightly grip the threads of the bolt. This locking device may be a fiber collar, with a hole slightly smaller than the major diameter of the bolt threads, or it may be a portion of the threads in the nut that is slightly out of phase with its main threads.
A self-locking nut must be turned onto a bolt with a wrench and grip the bolt threads tightly enough that vibration will not cause it to loosen or back off.
self-tapping screw. A type of screw with sharp threads that cut their own matching threads when screwed into soft metal, wood, or plastic. Self-tapping screws are especially suited for holding together sheets of thin metal, and for this reason, they are often called sheet-metal screws, or PK screws, after Parker-Kalon, one of their major manufacturers.

Selsyn system. The registered trade name for a DC synchro system. Mechanical movement of the transmitter shaft moves a two-arm wiper across a tapped, circular, variable resistor. Direct current enters the transmitter through the arms of the wiper and flows through the resistor out to the indicator. Here, the current flows into three sections of a coil wound around a ring-shaped core. A small permanent magnet on the pointer shaft is free to rotate inside the indicator coil.