Gilbert - The unit of magnetomotive force in the CGS system. It is equal to 4π/10 ampere-turns.

Glide Plane - A plane which can be drawn in the crystal structure so that reflection of any atomic position at the plane, followed by a movement of translation, gives an atomic position with identical environment.

Globar - A rod, consisting mainly of silicon carbide, which at white heat radiates almost like a black body.

Gloss - Of a surface: the ratio of the amount of light reflected specularly by the surface to the total amount of, reflected light. The photometer used for measuring gloss is known as a glossmeter.

Golay Cell - A detector of radiation (ultraviolet to micro-wave) which registers the rise in temperature of a gas' contained in a small transparent cell by recording it as a change in pressure.

Greinacher Circuit: Latour Circuit - A constant-potential voltage doubling circuit in which two capacitors connected in series are each charged in alternate half-cycles.

Greninger Chart - A chart for reading directly, from a back-reflection X-ray diffraction photograph, the angular relations between planes and zones.

Grenz Rays - Very soft X-rays, of 15 kV or less.

Grey Body - A body which, for a given temperature, emits radiant energy which, at every wavelength, is a constant fraction of the energy emitted by a black body at the same wavelength.

Goniometry - The measurement of angles,' usually those between flat reflecting surfaces. The term is commonly applied to. measurements on crystal faces, optical prisms, and angle gauges. It has also been used in radio direction finding.

Gouy Balance - A balance for determining para-and diamagnetic susceptibility. The specimen is mounted on one arm of a chemical balance and the apparent, change in weight is noted when a uniform magnetic field is applied to its lower end.

Gradient Wind - Wind whose motion is in the direction of the isobars, with the coriolis, pressure, and centrifugal forces, all normal to the motion, being in balance.

Grain Boundary. The boundary between two coherent crystals which differ in orientation, in lattice dimensions, in crystal structure, in composition, or in any combination of these.

Gramme Rad - The unit of integral absorbed dose. One gramme-rad is 100 ergs or l0-5J,
Grassntann Laws of Colour Vision - State that: (a) the eye can distinguish only three types of variation; luminance, dominant wavelength and purity for the stimulus; and brightness, hue, and saturation for the sensation: (b) if one component in a two component mixture is steadily changes, the other remaining constant, the colour of the mixture steadily changes: and (c) lights of the same colour produce identical effects in mixtures, regardless of their spectral composition.

Grassot Flux Meter - A type of flux meter in which the restoring couple of the suspended coil is negligible in comparison with its electromagnetic damping, the ends of the coil being connected to an outside search coil of known area.

Gravimeter - An instrument for the measurement of gravity differences between observation points. It may be dynamic (involving the measurement of time, generally vibrations of a mechanical system), stable (involving the balancing of weight against some other force-springs or gas pressure), or astatized (any gravimeter in which the deflection from equilibrium is increased by the application of an impressed force, often in the form of a spring).

Gravimetre - An instrument for the determination of the density of a liquid by the use of a total-immersion float which is attached to the bottom of the containing vessel by a spiral spring. The extension of this spring gives a measure of the density of the liquid.

Gravitatiornd Acceleration - The acceleration towards the centre of mass of the Earth experienced by a body of negligible mass (compared with that of the Earth) moving in vacuo. Its value (usually denoted as g) varies from place to place but has a mean value of about 9.81 m.s2 or 32.2 ft/s2 . Various formulae have been proposed for calculating g at any point on the Earth's surface, the results of which all differ to some extent from the actual value observed. This difference at a given point is known as the gravitational anomaly at that point.

Grid - (1) In an electron tube: a metallic electrode, located between a source of electrons and their final collecting point, whose function is to exercise control over the electron flow. (2) In geodesy: lines of a Cartesian coordinate system plotted on a map or projection. (3) In an ionization chamber: an electrode, located between the plates of a parallel-plate ionization chamber, which is used to ensure that the charge appearing on the collector is equal to the total ionization produced by the ionizing particles.

Ground State - The state of lowest energy of an atomic, nuclear, or molecular system.

Group Velocity - The velocity of propagation of a disturbance or pulse. e.g. the crest of a group or interfering waves where the component wave trains have slightly different wavelengths. It is the velocity of propagation of the associated energy and, in a dispersive medium, differs from the phase velocity.
Gravitational Constant - (1) The constant G appearing in Newton's law of gravitation. (2) The Gaussian constant of gravitation, equal to √G.

Gravitational Field. The field in which gravitational forces are operative. According to Einstein's general theory of relativity this field may be described in terms of gravitational waves and quanta which are analogous to the waves and quanta of the electromagnetic field.

Gravitational Potential - At a point in the gravitational field of an isolated spherically symmetric mass: the quantity Gmlr, where G is the gravitational constant, m is the mass cor cerned, and r is the distance of the point from the centre of mass. It has the dimensions of the square of a velocity.

Gravitation, Newtons Law Of - States that every particle of matter in the Universe attracts every other particle with a force acting along the line joining the particles. For two particles of masses ml and m², separated by a distance d. the force between the two particles, known as the gravitational force, is given by GmlGm²/d2, where G is the gravitational constant. Its value is about 6.67 x l0-8dyn cm²/g2, or 6.67 x 10-11 Nm²/kg2.
Gibbs Theorem - States that when two perfect gases, each occupying, the same: volume, are mixed isothermally so as to form a perfect gas mixture also occupying the same volume, the change in entropy is zero.

Gramme-Röntgen - The energy absorbed when 1 röntgen is delivered to 1 gramme of air, which has now been superseded by the gramme- rad as a measure of integral absorbed dose, The gramme-rönlgen was approximately equal to 84 ergs.

Granularity. Of a processed photographic emulsion: a measure of the graininess of a photographic image, based upon observations of the variation in photographic density.

Gyrocompass - A compass which relies on the principles of the rigidity of a gyroscope for its direction-finding ability. It embodies a spinning rotor or f1ywheel driven at high speed, together with some , means of controlling its position relative to the meridian plane. Once set, the gyrocompass will continue to point in the same direction irrespective of its attiude. It forms the basis of all types of automatic pilot and of various al systems.

Gyromagnetic Ratio - In an atomic or nuclear system: the ratio of the magnetic moment to the angular momentum. Thus, for an atomic nucleus the gyro magnetic ratio (usually denoted by g) is equal to µ/ I, where µ is the nuclear magnetic moment and I the nuclear angular momentum quantum number.

Gyroscope - A rapidly spinning, symmetrical, well-balanced wheel, with its mass distributed as far as possible from the axis of spin, and having bearings possessing the minimum of friction. It possesses the property of maintaining the direction of its spin axis in space (i.e. with reference to the "fixed" stars), except when a force acts on it to cause a torque around an axis other than that axis, in which case it processes about the spin axis.