gage block (precision tool). A precision-ground block of hardened and polished steel used as the standard for precise linear measurements in most manufacturing processes, and to check the accuracy of micrometer calipers. The dimensional accuracy of a gage block is normally measured in millionths of an inch.
gage pressure. Pressure referenced from the existing atmospheric pressure. Engine oil pressure and hydraulic pressure are normally measured as gage pressure. If gage pressure is measured in pounds per square inch, it is spoken of as so many psig.
Two other commonly used types of pressure are differential pressure (psid), which is the difference between two pressures, and absolute pressure (psia), which is referenced from zero pressure (a vacuum).
gain (electrical). An increase in the power of an electrical signal produced by an amplifier.
galena. A bluish-gray crystal that is primarily lead sulfide (PbS). It is the main ore from which lead is obtained. Galena is one of the crystals used in the early days of radio experimentation. When a crystal of galena is in contact with a fine-pointed wire called a cat whisker, it acts as a rectifier and can be used as a detector for radio signals picked up on an antenna.
galling. A form of damage to the mating surfaces of moving metal parts. When localized high spots rub against each other, they become heated by friction enough to weld together, and as they continue to move, the welded areas pull apart and destroy some of the surface.
gallium. A rare, metallic chemical element that melts at approximately 30°C. Gallium’s symbol is Ga, its atomic number is 31, and its atomic weight is 69.72. Gallium is used as a doping agent in the manufacture of semiconductors.