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Haze Factor - For adjacent black and white objects as seen through a diffusing medium: the quantity LB/(Lw -LB), where LB and Lw are the apparent luminances of the black and white bodies respectively.
Head Amplifier: Pre-Amplifier - A unit containing the input stage of an amplifying system and designed to be mounted integrally with, or close to, the detector.
Head, Total - The head (i.e. the height of a liquid column necessary to develop a given pressure at the base) measured by a pitot tube, i.e. the sum of the hydrostatic pressure and the kinetic head.
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Head Wave - The shock wave emanating from the front of a body in supersonic motion.
Health Physics - The branch of medical physics that is concerned with radiation protection as applied to man.
Hearing Loss - For a given individual at a specified frequency, the difference, in decibels, between the threshold of hearing of that individual and the normal (standard) threshold. The hearing loss is also known as the sensation level for the relevant frequency.
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Height, Barometric - The height of a point above sea-level as determined by barometric measurements.
Heitler-London Theory - Of covalent bonding: a theory based on consideration of the exchange forces between two atoms in which the two interacting electrons are assumed to be in atomic orbitals about each of the nuclei, the spins of the electrons being paired, i.e. anti-parallel. The theory explains the origin of a two-electron (or electron-pair) bond.
Helicity - Of an elementary particle: the component of its spin along the direction of its motion. It may be positive (right-handed) or negative (left-handed).
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Heliostat - An arrangement of mirrors driven automatically and used to reflect a beam of sunlight in a constant direction. Originally used for signalling, it is now used in some solar furnaces to direct the Sun's rays continuously on to a fixed parabolic mirror. A similar instrument used as a beacon by surveyors is known as a surveyor's heliotrope.
Helium, Liquid - (1) He-4 in the form of helium I: the form of liquid helium stable above the lambda-point (about 1.19 K). (2) He-4 in the form of helium II: the form of liquid helium stable between the lambda-point (about 2.19 K) and absolute zero. It exhibits superfluidity, and has an extremely high thermal conductivity and extremely low viscosity. (3) He-3, present in natural helium in minute proportions. It has no lambda-point transition and no superfluid properties.
Henry - The unit of inductance in MKSA and SI units, equal to 109CGS units. It is the inductance in which an induced e.m.f. of 1 volt is produced when the inducing current changes at the rate of 1 ampere per second.
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Henry Law - States that the mass of gas dissolved by a given volume of liquid at a given temperature is proportional to the pressure of gas with which it is in equilibrium. It is strictly true only for ideal solutions.
Hertz - The unit of frequency in the International System of units (SI). It is equal to 1 cycle per second.
Hertzsprung-Russell (H-R) Diagram. A diagram in which the luminosity of the stars is plotted against their spectral type, and which shows that they fall mostly into two categories: the main sequence stars (for which the luminosity is directly related to the spectral type) and the giant stars. Other categories represented are the super-giant, sub-giants, sub-dwarfs, and white dwarfs.
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Hess Law - States that the net change of energy in a chemical reaction or series of reactions depends only on the initial and final states, and is independent of the intermediate steps.
Heterodyne - Denotes the phenomenon of beats at radio frequency. The heterodyne principle is used in radio reception to bring the frequency of the incoming signals into the audio range. An intermediate stage, often employed, in which the new frequency is supersonic is described as supersonic heterodyne or superheterodyne.
Hilbert Space - An abstract space which is important in many branches of mathematics and theoretical physics, e.g. in quantum mechanics. It is a special type of Banach space .
Helium II, Mechanocaloric Effect In - The increase in temperature of helium II in a tube when the helium II flows through a porous plug. Conversely, the cooling of helium II when some of it flows into a tube, closed by a porous plug, which is plunged into the liquid. The temperature change in each case is about 0.01 degrees.
Helium II, Thermomechanical Effect In - The flow of helium II towards a region of higer temperature.
Helium II, Two-Fluid Model For A model in -which it is assumed that helium II consists of two freely interpenetrating fluids, one a normal fluid and the other a superfluid, the latter having zero entropy, zero viscosity, and zero thermal conductivity.
Helix - A curve drawn on a cylindrical or conical surface so that all the generators of the surface are cut at the same angle.
Hellman-Feynmann Theorem - A theorem used in calculating the forces on the nuclei in molecules or solids which are described by the Born-Oppenheimer approximation. It states that these forces are those which would arise electrostatically if the electron probability density were treated as a static distribution of negative electricity.
Helmholtz Coils - Two circular parallel coils of the same radius separated by a distance equal to that radius. They produce a region of constant magnetic field on either side of the midpoint between the two coils.
Helmholtz Resonator - An acoustic resonator in which an enclosed cavity is open to the air only via a small opening or neck. It resonates at a single frequency which depends on the shape and size of the resonator. Such a resonator may form part of an acoustic filter (as in a sound-absorbing panel), on may appear as the body of a musical instrument such as a cello.
Heat - A form of energy associated with the motion of individual atoms or molecules of a body. It is to be distinguished from temperature, which is a measure of the degree of hotness.
Heat Capacity - Of a system, entity, or substance at a given temperature: the amount of heat required to raise the temperature by one degree. It may be defined as the limit, as ΔT approaches zero, of the ratio ΔQ/ΔT, where ΔQ is the amount of heat that must be added to raise the temperature from T to T + ΔT. The heat capacity per unit mass of a substance is the specific heat of that substance.
Heat Engine - A device for obtaining work from a cycle of operations involving the expansion and compression of a fluid (the"working" fluid). This fluid may be gas or vapour, which, inexpanding, exerts pressure either on a piston moving in a cylinder, or on a curved, rotating turbine blade. In ideal cycles it is constantly being replaced.
Heat Engine, Efficiency Of - The ratio of the work obtained to the heat absorbed.
Heat Exchanger - Any device .in which heat is transferred from one fluid to another. If both fluids flow continuoulsy through separate channels, divided by a wall, it is termed recuperative. If the two.
fluids flow alternately through the same channels it is t termed regenerative. And if both fluids flow continuously through the same channels the heat exchanger is said to be of the direct-contact variety.
Heat Flow, Steady State - Refers to the situation in which the quantity
of heat flowing into a body is equal to that flowing out of the body.
Although the temperature may differ at different points within the
body, the temperature at a given point will remain constant.
Heat Pump - A device which absorbs heat from an external low temperature source, and rejects the whole as heat at a higher temperature. It is essentially a reversed heat engine, of which a refrigerator is an example.
Heavy Particle. An elementary particle with a mass greater than that of the meson. The term includes K-mesons, nucleons, and hyperons.
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