SIGMET (significant meteorological information). A weather advisory issued concerning weather significant to the safety of all aircraft. SIGMET advisories cover severe and extreme turbulence, severe icing, and widespread dust or sandstorms that reduce visibility to less than three miles.
signal generator (electronic test equipment). A piece of electronic test equipment that produces an electrical signal having a specified voltage, frequency, and waveform. Signal generators are used for troubleshooting and calibrating electrical and electronic equipment.
signal-strength meter (radio receiver component). A meter connected into the automatic volume control (AVC) circuit in a radio receiver. A signal-strength meter is calibrated in decibels (dB) or in “S” units and is used to provide an indication of the strength of the signal being picked up by the receiver.
signal tracer (electronic test equipment). A piece of electronic test equipment used to trace a signal through each stage of a piece of electronic equipment. Troubleshooting of malfunctioning equipment is simplified by comparing the waveform of the signal at each stage with the waveform that should be at that stage.
significant digits. The digits of a decimal number, beginning at the first nonzero digit and extending to the right to include all of the digits needed to give the required accuracy. One-half expressed as a decimal fraction with one significant digit is 0.5. If greater accuracy is required, this same value can be expressed with three significant digits as 0.500.
significant meteorological information. See SIGMET.
silica. Silicon dioxide (SiO2), as it occurs in nature.
silica gel. A form of extremely porous silica used as a desiccant. Silica gel, packaged in porous paper bags, is stored inside delicate instruments and electronic equipment that could be damaged by moisture. Moisture in the air is absorbed and held by the silica gel.
silicon. A dark brown, tetravalent (having four valence electrons), nonmetallic crystalline chemical element. Silicon’s symbol is Si, its atomic number is 14, and its atomic weight is 28.086. Silicon is the second most abundant chemical element in nature (oxygen is the most abundant). Because of the ease with which silicon unites with oxygen to form silicon dioxide (sand and quartz), it is never found in nature in its pure state.
Silicon is used to make semiconductor diodes, transistors, and all forms of integrated circuit devices.
silicon carbide. An abrasive that is often ground into small particles and bonded to sheets of paper or cloth or formed into grinding wheels for use on power grinders.