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Field - (1) Qualitatively: a region in which a physical force is operative. Quantitatively: the description of the way in which energy, momentum, etc. are carried in such a field. Examples are the elctromagnetic field of Maxwell and the gravitational field of Einstein.
Field Emission Microscope - An instrument which produces an image of the surface of a metal tip, or of material adsorbed by the tip, by the projection of field emission electrons -on to a fluorescent screen. 'The resolution in some 20 or 30 A.
Field Ion Microscope - An instrument similar to the field emission microscope in which the image from the surface of a metal tip is produced by gaseous ions which originate close to that surface. By field evaporation lower and lower depths of the original tip may be examined. The resolution is good enough to permit the visulization of individual atoms.
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Field of View - The area of an object which can be "seen" by optical system. It is expressed as an angle or can also be a diameter suitable circumstances.
Field Strength - The magnitude of the force experienced in a field by unit charge, mass etc. as appropiate.
Field Theory - A thory of the way in which the field poten als, be they electromagnetic, gravitational or nuclear, account for the prop'agation of a field and the consequent transfer of energy, momentum, etc.
Filament - The resistive element in a thermionic device or electric lamp.
Fiam Balance - An instrument, incoporating a torsion balance. for measuring the two-dimensional pressure of s1,U'face films sp ead on water or other liquids.
Filter - (1) A device for eliminating or reducing certain frequencies in the transmission of electrical energy, sound. radio, light, X-rays, etc. (2) A device for separating out solid particles from a liquid or gaseous phase.
Filter, band-pass - An optical or electrical filter which transmits a single band of frequencjes. Such a filter may be claSsified as broaa or narrow according to the range of frequencies transmitted. The frequency range within which transmission takes place without appreciable attenuation is termed the pass-band. See also: Stop band.
Filter Network - A system for filtering a.c. voltages or currents involving either the interconnection of various circuit eleme,nts (inductors, capacitors or resistors) or the use of mechanically resonant elements (coaxial lines, waveguides! or piezoelectric crystals).
Filter, Wav - An electrical network whose insertion loss is low (and approximately constant)' over a band or bands of frequencies and high over other bands, and can thus act as a filter.
Fireball - A very bright met or, also known as a bolide. It is not be confused with ball lightning, which is purely electrical in origin.
First sound. In liquid helium II: ordinary sound which results when the super fluid and normal components of liquid helium II oscillate in phase with each other to produce periodic density variations.
Fissile - (l) Of a nuclide: capable of undergoing nuclear fission by any process. (2) of a material: containing one or more fissile nuclides.
Fissionable - (1) Of a nuclide: capable of undergoing nuclear fission by any process. (2) Of a material: containing one or more fissionable nuclides.
Fission Chamber: Fission Counter - An ionization' counter containing fissile material.which is used to detect and measure neutrons by the
ionization produced by fission fragments.
Fission Fragments - The nuclei resulting from nuclear fission.
Fission Products - Nuclides produced either directly by nuclear fission or by the radioactive decay of such nuclides.
Fission Spike - A displacement spike in a solid resulting from damage by fission fragments.
Fizeau Experiment - An experiment carried out to measure the etherdrag in moving water by measuring the velocity of light up and down stream, using an interferometric method. It was a predecessor of the Michelson-Morley experiment. The results were inconclusive.
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Fizeau Fringes - Interference fringes at constant optical thickness. Also known as contour fringes.
Flare Spot - A diffuse bright patch in an optical image produced, for example, by multiple reflections at lens surfaces.
Flashover - A form of surface breakdown between two electrodes on a Dielectric surface.
Flash Radiography. The production of radiographs of moving objects by the use of intense bursts of X-rays lasting for microseconds or less. A succession of such flashes will permit cineradiography.
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Flash Spectrum - The chromospheric emission spectrum of the Sun, visible for a few seconds just before and just after totality in a solar eclipse. This spectrum furnishes evidence of the great abundance of helium in the sun.
Flash Tube. A glass or quartz tube filled with inert gas which, when discharged across condenser, produces a brief flash of white light which can be used for high-speed photography.
Filcker Effect - Of a thermionic valve or tube: minute variations in the cathode current caused by random. changes in cathode activity or positive ion emission.
Fluid - Any substance that flows and which offers no permanent resistance to changes of shape Induced by pressure. Only a uniform isotropic pressure can be supported without distortion. A fluid may be compressible (a gas) or practically incompressible (a liquid).
Fluid Dynamics - The mathematical study of the motion of perfect fluids.
Fluid flow, Equations of Motion For - (1) Euler equations: describe the motion in terms of the fluid as a whole i.e. they are essentially statistical. They apply to a homogeneous, incompressible fluid. of constant density. (2) Lagrange equations: describe the motion interms of individual moving particles. The fluid is assumed to be ideally inviscid and incompressible. (3) Navier-Stokes equations: describe the motions of an incompressible Newtonian fluid.
Fluid flow, Equations of Motion For - (1) Euler equations: describe the motion in terms of the fluid as a whole i.e. they are essentially statistical. They apply to a homogeneous, incompressible fluid. of constant density. (2) Lagrange equations: describe the motion in terms of individual moving particles. The fluid is assumed to be ideally inviscid and incompressible. (3) Navier-Stokes equations: describe the motions of an incompressible Newtonian fluid.
Fluid Flow, Isentropic - Flow in which the entropy of each element of fluid remains constant.
Fluid Flow, Laminar - Flow in which there is no macroscopic mixing between adjacent layer. It is the same as stream line flow.
Fluid Flow, One-Dimensional - Flow whose behaviour can be approximated by assuming that the rate of change of fluid properties across the stream lines is negligible compared with that along the stream lines.
Fluid Flow, Rotational - Flow in a region of the fluid in which vorticity exists.
Fluid Flow, Secondary - A flow which is additional to the main flow and arises from local effects within the fluid.
Fluid Flow, Separation of - The detachment of the flow of the fluid from a solid surface with which it was previously in contact.
Fluid Flow, Turbulent - Flow in which small irregular fluctuations (with time) are superposed on a mean flow:
Fluid Flow, Two-Dimensional - Flow whose behaviour can be approximated by considering two-dimensional space only
Fluid Flux. The flow of fluid across a surface and normal to it. It is usually expressed as the volume of fluid flowing in unit time.
Fluidics - The technology of the use of gaseous or liquid fluids in motion for purposes such as switching, sensing, and amplifications. The fluid energy is manipulated in a manner similar to that of electricity in electronic devices, with the advantage that there are no moving parts.
Fluid, Ideal. Usually refers either to an inviscid incompressible fluid, or to a perfect gas.
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Fluidized Bed - A bed of powdered solids which is maintained in a state of non-circulating suspension by a flow of fluid through it at right angles to the bed. Its technological advantages stem from its fluidity and from the very high area of contact between the solid particles and the liquid or gas in which the suspension is maintained.
Fluid, Newtonian - A fluid in which the shear stress is proportional to the shear rate.
Fluorescence - The excitation of a substance so as to excite radiation of a wavelength, or wavelengths, characteristic of that substance. If the emitted radiation is in the visible or near-visible regions of the spectrum the process is optical fluorescence, as distinct from X-ray fluorescence which occurs at much shorter wavelengths. When the emitted radiation is of the same wavelength as that of the incident radiation it is termed resonance radiation. Substances which continue to emit characteristic radiation after the excitation has ceased are termed phosphorescent
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Fluorescence Microscopy - The microscopic examination of fluorescent material to establish its position (e.g. in biological tissue) and to reveal fine structure which might not otherwise be visible. Fluorescent dyes may also be used as "stains" in such work.
Fluorescence X-Rays - Characteristic X-rays arising from the absorption of X- or γ-rays. .
Fluorescent Screen - A suitably mounted layer of material which fluoresces in the visible region of the spectrum when irradiated by X-rays or other ionizing radiation.
Fluorimetry - (1) The measurement of the fluorescent properties of materials in general. (2) An analaytical technique involving the photometric measurement of fluorescence to detect, identify, and estimate fluorescent material. Also known as fluorometry and fluorophotometry.
Fluorography - The photography of a fluoroscopic image.
Fluoroscopy - The production of a visual image on a fluorescent screen by X-rays or other ionizing radiation and its use for medical diagnosis, the examination of materials, or for other purposes.
Flux - Refers to the flow of physical entities such as energy, charge, radiation, or atomic particles across a given area or in a given direction, The precise units of flux employed depend on the types of physical quantity involved.
Flux Density - The flux per unit area. Its precise definition depends on the type of physical quantity involved.
Fluxgate Magnetometer - An electronic saturable-core magnetometer in which the degree of saturation by an external magnetic field is used as a measure of the strength of the field. It was developed during World War II and used as an airborne detector of submarines.
Flux Meter - An instrument used for measuring magnetic flux. It is essentially a moving-coil galvanometer with its coil mounted in such a manner that the suspension exerts negligible restoring electromagnetic. It is sometimes known as a gauss meter.
Flux of displacement - Another term for electric flux.
F-Number: Stop number - Of a lens or lens system: the ratio of the focal length to the diameter of the entrance pupil. Since the amount of light passing through the system decreases as the square of this diameter the conventional f-numbers used in photography increase by steps of -Y2 (1, 1.4, 2, 2.8, 4, 5.6, ete.) so that each rise in the series requires a doubling of the exposure time, other things being equal.
Focometer - An instrument for measuring the focal length of an optical system.
Focus - The position of convergence and divergence of a beam of radiation (particulate, electromagnetic or acoustic). The focus is usually a point or a line, but may take other forms.
Fog - A term usually taken to mean a cloud of small water drops near ground level, which is sufficiently dense to reduce horizontal visibility to below 1 km (1100 yards). It may, however, refer to clouds of other composition (e.g. smoke fog, ice fog), cloud at other levels (e.g. high fog), or cloud of other densities (e.g. thin fog, dense fog).
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