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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.
gallon, Imperial. A unit of liquid measurement used in the United Kingdom. One Imperial gallon is the
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volume occupied by ten pounds of pure water under specified conditions of temperature and pressure. One
imperial gallon is equal to 277.4 cubic inches, 4.55 liters, or 1.201 U.S. gallons.
gallon, U.S. A unit of liquid measurement used in the United States of America. One U.S. gallon is equal to
231 cubic inches, 128 fluid ounces, 3.785 liters, or 0.832 imperial gallon.
galvanic action. Electrical pressure within a substance which causes electron flow because of the
difference of electrode potential within a material.
galvanic corrosion. A form of corrosion that occurs between two metals having different locations in the
electrolytic, or galvanic, series. See galvanic series.
Corrosion can take place any time two metals are covered with an electrolyte. The amount of
corrosion is determined by the difference in the location of the two metals in the galvanic series. The more
active metal (the metal highest in the galvanic series) acts as the anode in the electrolytic process, and is
eaten away. The more noble metal is not affected.
galvanic couple. A pair of unlike metals that produce an electrical potential when they are covered with an
electrolyte.
Copper and zinc form a commonly used galvanic couple. When a piece of copper and a piece of
zinc are covered with the same electrolyte, a voltage is produced between them. The zinc, being the less
noble of the two metals, forms the anode. Electrons leave the zinc and travel to the copper. The copper is
the positive terminal of the galvanic couple, and the zinc is the negative terminal.
galvanic generation of electricity. The production of electricity by chemical action. Electricity produced
by dry-cell batteries and storage batteries are examples of galvanic production of electricity.
galvanic grouping. The arrangement of metals in a series according to their electrode potential difference.
galvanic series. A list of metals arranged in the order of their chemical activity. The metals are listed in
their order of activity with the more active (least noble) metal being the anode in an electrolytic action.
galvanizing. A form of corrosion protection for steel parts in which the surface of the steel is covered with
a coating of zinc. The zinc may be applied by hot dipping or by electroplating.
galvanometer. An instrument used to measure electrical current. A galvanometer measures the amount of
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current flowing in a circuit by measuring the interaction between two magnetic fields. One field is
produced by a permanent magnet, and its strength is fixed. The other field, an electromagnetic field, is
produced by current flowing through a moving coil, and its strength is determined by the amount of current
being measured. The moving coil is restrained by a calibrated hairspring in such a way that its deflection is
proportional to the amount of current flowing through it.
gamma iron. The molecular arrangement of iron at temperatures between 911°C and 1,392°C.
gamma rays. A form of electromagnetic radiation that results from nuclear fission. Gamma rays have the
ability to penetrate solid material, and they are used in the radiographic method of nondestructive
　
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