1 gram is equal to 15.43234874 grains. (Prof. W. H. Miller.)

Gram-atom.

The number of grams of an element equal numerically to the atomic weight, as 16 grams of oxygen, 1 gram of hydrogen, 35.5 grams of chlorine; all which might be expressed as gram-atoms of oxygen, hydrogen and chlorine respectively.

The gram-atom approximately expresses the number of gram-calories required to heat one gram of the substance 1? C. (1.8? F.). This is in virtue of Dulong and Pet.i.t's discovery that the atomic weight of an element multiplied by its specific heat gives approximately a constant for all elements.

[Transcriber's note: A gram-atom is the ma.s.s, in grams, of one mole of atoms in a monatomic element. A mole consists of Avogadro's number of atoms, approximately 6.02214E23.]

Gram-molecule.

The number of grams of a substance equal numerically to its molecular weight.

Graphite.

Carbon; one of three allotropic modifications of this element. It occurs in nature as a mineral.

It is used as a lubricant for machinery; for commutator brushes; for making surfaces to be plated conductive, and for mixing with manganese binoxide in Leclanch? cells.

Gravitation.

A natural force which causes all ma.s.ses of matter to attract each other.

Its cause is unknown; it is often supposed to be due to the luminiferous ether.

[Transcriber's note: Einstein's explanation of gravity, General Relativity and the curvature of s.p.a.ce-time, came 23 years later, 1915.]

281 STANDARD ELECTRICAL DICTIONARY.

Gravity, Acceleration of.

The velocity imparted to a body in one second by the action of gravitation at any standard point upon the earth's surface in a vacuum.

This will vary at different places, owing princ.i.p.ally to the variation in centrifugal force due to the earth's rotation. For standard valuation it must be reduced to sea level. The following are examples of its variation:

Equator, 978.1028 centimeters per second Paris, 980.94 "

Greenwich 981.I7 "

Edinburgh, 981.54 "

Pole (N. or S.), 983.1084 (theoretical) "

As round numbers for approximate calculations 981 centimeters or 32.2 feet may be employed.

[Transcriber's note: The acceleration of gravity at the equator is also reduced by the increased distance from the center of the earth (equatorial bulge). Increased alt.i.tude reduces gravity. Reduced air density at alt.i.tude reduces buoyancy and increases apparent weight.

Local variations of rock density affects gravity.]

Gravity, Control.

Control by weight. In some ammeters and voltmeters gravity is the controlling force.

Grid.

A lead plate perforated or ridged for use in a storage battery as the supporter of the active materials and in part as contributing thereto from its own substance.

Ground.

The contact of a conductor of an electric circuit with the earth, permitting the escape of current if another ground exists.

Ground-wire.

A metaphorical term applied to the earth when used as a return circuit.

Fig. 192. GROVE'S GAS BATTERY.

Grove's Gas Battery.

A voltaic battery depending for its action on the oxidation of hydrogen instead of the oxidation of zinc. Its action is more particularly described under Battery, Gas. In the cut B, B1 * * * are the terminals of the positive or hydrogen electrodes, marked H, and A, Al * * * are the terminals of the negative or oxygen electrodes marked O, while M, M1 * * * is dilute sulphuric acid.

282 STANDARD ELECTRICAL DICTIONARY.

Guard Ring.

An annular horizontal surface surrounding the balanced disc in the absolute electrometer. (See Electrometer, Absolute.)

Guard Tube.

A metal tube surrounding a dry pile used with a quadrant electrometer, or other electrometers of that type. It prevents the capacity of the lower bra.s.s end of the pile (which bra.s.s end closes the gla.s.s tube containing the discs) from momentary change by approach of some conductor connected to the earth. There are other guard tubes also.

Gun, Electro-magnetic.

An electro-magnet with tubular core. If, when it is excited a piece of an iron rod is pushed into the central aperture of the core and is released, the magnetic circle will try to complete itself by pushing the rod out so that it can thus be discharged, as if from a popgun.

Synonym--Electric Popgun.

Fig. 193. "ELECTRIC POPGUN."

Gutta Percha.

The hardened milky juice of a tree, the Isonandra gutta, growing in Malacca and other parts of the Eastern Archipelago. It is much used as an insulator or const.i.tuent of insulators.

Resistance after several minutes electrification per 1 centimeter cube at 54? C. (75? F.), 4.50E14 ohms.

The specific resistance varies--from 2.5E13 to 5.0E14 ohms. A usual specification is 2.0E14 ohms. The influence of temperature on its resistance is given in Clark & Bright's empirical formula, R = R0 at, in which R is the resistance at temperature t? C--Ro the resistance at 0? C (32? F), a is the coefficient .8944.

The resistance increases with the time of pa.s.sage of the current, the variation being less the higher the temperature.

283 STANDARD ELECTRICAL DICTIONARY.