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Hodoscope - An array of radiation counters used in studying the paths of cosmic-ray particles.
Hohlraum - A uniform temperature cavity which acts as a black body.
Hole Theory - A theory leading to the prediction of the existence of the positron and, by extension, to that of other antiparticles. According to this theory the states of negative energy required by the Dirac equation are normally all occupied so that no transition to them can occur. If sufficient energy is applied, however, a particle may be excited from a negative to a positive energy state and will appear as an electron. The hole left behind in an otherwise occupied negative sea, i.e. the absence of an electron with certain values of energy, momentum and spin, will then appear as the presence of a particle with the same mass and spin, but opposite charge-the positron.
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Humidity Relative - The ratio of the partial pressure of the water vapour in the atomosphere to the saturation vapour pressure at the same temperature. The ratio of the amount of water vapour present to the amount present in saturated air at the same temperature is known as the saturation ratio.
Humidity, Specific - The mass of water vapour per unit mass of air.
Humphrey Series - A series in the line spectrum of the hydrogen atom.
Hum tone - The lowest note emitted by a bell.
Huygens' Principle. States that every element of an advancingwavefront acts as a source of secondary disturbances, i.e. as a source of a new train of waves. A modification, the FresnalHuygens' principle, states that the secondary disturbances interfere with one another in accordance with the usual principle of superposition.
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Hydraulics - The science and engineering of the flow of fluids. It embraces the flow of fluids in closed boundaries, e.g. flow in pipes, pumps, and turbines, and the flow of liquids with a free surface within rigid or loose boundaries, e.g. flow in rivers and estuaries, and wave motion. It may also include the flow of water in saturated and partly saturated soils.
Hydrodynamics - The study of the motion of incompressible fluids.
Hydrofoil - A lifting wing, analogous to an aerofoil, 'used in a liquid,
use. water.
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Hydrogen Atom, Line Spectrum of. Consists of several series of spectra, named after their discoversrs, whose wave numbers, v, are given by the following expressions: Lyman series:
v = R [1/12-1/n2 ], n= 2, 3, 4 ...
Balmer series:
v = R [1/22-1/n2 ], n= 3, 4, 5 ...
Paxchen series:
v = R [1/32-1/n2 ], n= 4, 5, 6 ...
Brackett series:
v = R– [1/42-1/n2 ], n= 5, 6, 7 ...
Pfund series:
v = R– [1/52-1/n2 ], n= 6, 7, 8 ...
Humphreys series:
v = R– [1/62-1/n2 ], n= 7, 8, 9 ...
In these expressions R is the Rydberg constant.
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Hypermetropia: Hyperopia - A condition of the eye in which parallel rays incident upon the lens are focused at a point behind the retina. Commonly known as long sight.
Hyperon - A term formerly denoting one of a group of unstable elementary particles which are heavier than a neutron, but now extended to cover any baryon with non-zero strangeness. Such particles, which (in order of increasing mass) are lambda (Λ), sigma (Σ0, Σ+ and Σˉ), cascade or xi Ξº and Ξˉ and omega (Ωˉ particles, eventually decay to a proton or a neutron.
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Hypsochromic Shift - The shift of an absorption band to a region of shorter wavelength. The opposite of bathochromic shift. See also. Bathochromic shift.
Hypsometer - An instrument used for determining the boiling point of water, either to determine altitude or correct the upper fixed point of a thermometer.
Hysteresis - The phenomenon exhibited by a system whose properties depend on its previous history. The term is commonly applied to magnetic properties, but hysteresis may, for example, be mechanical (dynamic or static according as the number of cycles is greater than or equal to unity) or dielectric. The term is also used to denote the difference between the response of a physical instrument or system to an increasing signal and the response to a decreasing signal.
Hysteresis Heating - A method of supplying a controlled amount of heat to a thermally isolated sample at a temperature below 1 K, involving taking the sample through a magnetic hysteresis loop. The sample must be paramagnetic and near or below its Curie point.
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Hypersonic - Refers to flow fields, phenomena and problems appearing at speeds for greater than that sound, usually taken as five times that speed or greater.
Hydrologic Cycle - The circulation of water from the oceans. through the atmosphere, and back from the oceans, through the atmosphere, and back to the land. and back to the oceans by overland or subterranean routes.
Hydrology - The study of the movement and storage of ground water.
Hydromechanics - The science of the mechanics of fluids. It comprises
hydrostatics and hydrodynamics.
Hydrometer - An instrument for measuring the density of a liquid. A common form the constant-mass type, consists of a floating bulb, weighted at the lower end so that it floats in the vertical position, the depth of immersion being inversely proportional to the density.
Hydrostatic Pressure - Describes a system of forces acting on a body to compress it equally in all directions. It was originally applied to the pressure imposed on a body in a liquid due to the weight of the liquid.
Hydrostatics - The study of the equilibrium of fluids at rest.
Hygrograph - An instrument for continuously recording the relative humidity of the air.
Hygrometer - Any instrument for measuring the humidity of the atmosphere. The commonest is the wet and dry bulb thermometer or psychrometer.
Hygrometer, Hair - A hygrometer in which is measured the extension of grease-free human hair under tension. The humidity-extension relationship is, however, somewhat variable.
Hypercharge - Of a high-energy particle: the sum of the strangeness
and the baryon number.
Hyperfragment Hypernucleus - A short-lived nucleus, emitted in nuclear disintegration, which contains a bound hyperon, usually the lambda hyperon Å.
Holography - The technique of recording an optical image of an object; which preserves the amplitude, wavelength, and phase of the light reflected, diffracted or transmitted by that object; and the use of this record to produce a three-dimensional image of the object itself.
Holography, Acoustic - Holography in, which sound waves are used instead of light waves.
Holohedral Crystal - One exhibiting the maximum symmetry of the crystal system to which it belongs.
Homopolar induction - The generation of an induced e.m.f of constant
polarity by the motion of a circuit element through a magnetic field
of constant strength.
Hooke Law - For an elastic solid: states that the strain is proportional to the stress within the elastic limit.
Hope Apparatus - An apparatus for showing the variation of the density of water with temperature in the neighbourhood of the freezing point, and for determining approximately the temperature of maximum density.
Horn - (l) Acoustic: a tube intended for the transmission and radiation of sound, of which the cross-sectional area increases progressively from the throat to the mouth. (2) Radio: an elementry aerialconsisting of a waveguide in which one or more transverse dimensions increase towards the aperture.
Horse power - (1) A British unit of power defined as a rate of work of 33000 ft-lb/min. It is equal to 745.700 W. (2) A metric unit defined as a rate of work of 75 m-kg/s. It is equal 735.5 W.
Horse Power, Brake - The power actually delivered and measurable by a ,brake or dynamometer, as distinct from the indicated horse power, i.e. the theoretical horse power in the absence of friction and other losses in the mechanism.
Hospital Physics - The branch of medical physics that deals mainly with problems arising from the application of ionizing radiations and radioactive tracers in hospitals.
Hovercraf - A jet-driven vessel which "flies" over land or water on a cushion of air. The air is drawn in vertically by a fan and discharged circumferentially downwards and inwards below the flat undersurface of the craft.
Hue - Of a colour: The property by which a colour is identified as tending to resemble the appearance of some particular wavelength of light in the visible spectrum.
Humidity - The presence of water vapour in the atmosphere. Various ways of specifying the degree of humidity are given in the following entries.
Humidity Absolute - The mass of water vapour per unit volume of moist air, usually expressed in grammes per cubic metre.
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