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DiracDelta - Science & Engineering encyclopedia Category: Sciences > Science & Engineering Date & country: 01/12/2007, UK Words: 4470

Half Wave Rectifier
A tube or solid-state diode which passes current in only one directionand thus converts ac to pulsating dc by rectification of one alternation of each cycle.

Halide
In chemistry, a halide is a compound composed of two elements, one of which is a halogen (flourine, chlorine, bromine, iodine and astatine). See also: Astatine, Bromine, Chlorine, Fluorine, Iodine.

Hall Effect
The phenomenon whereby a force is brought to bear on a moving electron or hole by a magnetic field that is applied perpendicular to the direction of motion. The force direction is perpendicular to both the magnetic field and the particle motion directions.This effect is exploited in such devices as the mass spectrometer and in the Thompson experime…

Hall Mobility
A measure of the flow of charged particles perpendicular to both a magnetic and an electric field. See also: Hall Effect.

Halo
A ring of light that appears around the Sun or Moon. It is produced by refraction in ice crystals. See also: Moon, Sun.

Halogen
Halogen is a particular group of elements with similar bonding properties, consisting of flourine, chlorine, bromine and iodine.HHeLiBeBCNOFNeNaMgAlSiPSClArKCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKrRbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXeCsBaLaHfTaWReOsIrPtAuHgTlPbBiPoAtRnFrRaAcLanthanidesCePrNdPmSmEuGdTbDyHoErTmYbLuActinidesThPaUNpPuAmCmBkCfEsFmMdNoLr See also:…

Halon
Halons are organic chemical compounds containing one or two carbon atoms, together with bromine and other halogens. The most commonly used are halon 1211 (bromochlorodifluoromethane) and halon 1301 (bromotrifluoromethane). The halons are gases and were widely used in fire extinguishers until there use was banned in 1994 because of the damage they c…

Hammer
A hand tool with a metal head and a handle. It is used to force one item against or through another.Air hammerA hammer that is powered by compressed air.Ball pien hammer (ball peen hammer)Photograph courtesy of Draper.co.ukA hammer with two ends on the head, one that is round and the other flat. They are best used for hammering and shaping metal. O…

Hammer Welding
A forge welding process. See also: Hammer, Welding.

Hamming Window
This is the sum of a rectangle and a Hanning window. An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum. This window has similar properties to the Hanning window with the added feature that it suppresses the fi…

Hand
Unit of measure. Used for measuring the height of a horse.Conversions1 hand=10.16 cm1 hand=4 inchesttle='Hand';xiunt='m';yiunt='hand';mconv=0.1016;cconv=0.0;

Handshaking
The initial exchange between two communications systems prior to and during transmission to ensure proper data transfer.

Handwheel
Any adjusting or feeding mechanism shaped like a wheel and operated by hand.

Hankel Matrix
A matrix in which all the elements are the same along any diagonal that slopes from northeast to southwest. See also: Diagonal Matrix, Matrix.

Hanning Window
An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.amplitude correction = 2.0, energy correction = 1.633The Hanning window is a general purpose window for the analysis of continuous signals and should be used i…

Hanning-Poisson Window
A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis. This is constructed as the product of the Hanning and the Poisson windows and is given by:wherethe sidelobes falloff like

Haptic
Pertaining to the sense of touch, from the Greek word haptein, to grasp. There are four types of sensory neurons (mechanoreceptors) involved in the haptic modality. The haptic, or tactile, sensory modality is the only active sense that can be used to explore our environment; vision and hearing are passive senses since they cannot act upon the envir…

Hard Facing
A particular form of surfacing in which a coating or cladding is applied to a surface for the main purpose of reducing wear or loss of material by abrasion, impact, erosion, galling, and cavitations. See also: Hard Surfacing.

Hard Room
A room in which the surfaces have very low values of sound absorption and are therefore highly reflective. See also: Anechoic Chamber, Reverberant Sound Field, Semi-Anechoic Chamber.

Hard Surfacing
The application of a hard, wear-resistant alloy to the surface of a softer metal. See also: Hard Facing.

Hardenability
In a ferrous alloy, the property that determines the depth and distribution of hardness induced byquenching. See also: Hardening, Jominy Test.

Hardening
Increasing the hardness by suitable treatment, usually involving heating and cooling. When applicable,the following more specific terms should be used: age hardening, case hardening, flame hardening, inductionhardening, precipitation hardening,, quench hardening and through hardening. See also: Hardenability, Jominy Test, Oil Hardening, Vicker's Ha…

Hardness
Resistance of metal to plastic deformation usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting.Indentation hardness may be measured by various hardness tests, such as Brinnell, Rockwell, and Vickers.Many ceramics, and even glasses, are ductile under small indents, all…

Harmonic
If a signal (representing acceleration, displacement, sound pressure etc.) is composed of a number of components of frequencies which are all integer multiples of one (fundamental) frequency, these components are said to form a harmonic train. The component at twice the fundamental frequency would be known as the second harmonic, the component at t…

Harmonic Distortion
Distortion of a signal by adding content that is harmonically related to the original signal. Clipping overload of an amplifier adds odd-order harmonics to the signal.

Harmonic Mean
The harmonic mean of two numbers a and b is 2ab/(a + b). See also: Geometric Mean, Mean.

Harshness
Harshness is a typical transient interdisciplinary phenomenon of noise and vibration. The major frequency range for the generation of harshness is from 7 to 60 Hz for vibration, and for shock noise is from 40 Hz to 240 Hz. These ranges can be divided into two major bands respectively. For vibration one band is that under 20 Hz, mostly due to unspru…

Hawking Radiation
A theory first proposed by British physicist Stephen Hawking, that due to a combination of properties of quantum mechanics and gravity, under certain conditions black holes can seem to emit radiation. See also: Black Hole, Hawking Temperature.

Hawking Temperature
The temperature inferred for a black hole based on the Hawking radiation detected from it. See also: Black Hole, Hawking Radiation.

Hazard
The potential that the use of a product will result in an adverse effect on a person or the environment.

Head Related Transfer Function
The frequency response between the point in space where a sound source is located, and the ear, due to anatomical features of the head, upper torso and pinnae. These features shape the response in such a way as to allow the ear to localize a sound source in space.

Headroom
A term related to dynamic range, used to express in dB,the level between the typical operating level and the maximum operatinglevel above which the signal will be clipped. See also: Clipping, Dynamic Headroom.

Headstock
The fixed or stationary end of a lathe or similar machine tool.

Hearing
The subjective human response to sound.The ability to hold a conversation depends on the background noise level and the distance between the people.

Hearing Damage
A person exposed to high noise levels can suffer hearing damage. The damage may be gradual or traumatic. Gradual Hearing DamageSustained exposure to moderately high noise levels over a period of time can cause gradual hearing loss. It starts out as a temporary hearing loss, such as immediately after a loud rock concert. The hearing usually restores…

Hearing Protector
Personal device worn to reduce harmful auditory or annoying subjective effects of sound.Equal-Energy HypothesisA hypothesis stating that equal amounts of sound energy will produce equal amounts of hearing impairment, regardless of how the sound energy is distributed in time.

Hearing Range in Animals
Many animals hear a much wider range of frequencies than is possible for humans. For example, dog whistles vibrate at a higher frequency than the human ear can detect, while evidence suggests that dolphins and whales communicate at frequencies beyond human hearing (ultrasound).

Heat Capacity
The amount of heat required to raise the temperature of a body through 1K.Units of heat capacity are JK^-1Constant volume:Constant pressure:Difference in heat capacities:Ratio of heat capacities:whereC_V = Heat capacity at constant volumeC_P = Heat capacity at constant pressureQ = HeatT = TemperatureV = VolumeU = Inter…

Heat Check
A pattern of parallel surface cracks that are formed by alternate rapid heating and cooling of theextreme surface metal, sometimes found on forging dies and piercing punches. There may be two sets of parallelcracks, one set perpendicular to the other.

Heat Content
The SI unit for heat content is the Joule per cubic metre.1 J/m³ = 26.8392x10^-6 Btu/ft³ = 4.30886x10^-11 therm/UK galttle='Heat Content';xiunt='J/m³';yiunt='Btu/ft³';mconv=1/0.0000268392;cconv=0.0;

Heat Engine
A device for converting heat into mechanical work. Heat engines have the following characteristics:They receive heat from a high-temperature source.They convert part of this heat to work.They reject the remaining waste heat to a lowtemperature sink such as the atmosphere or a body of water.They operate on a cycle.They usually involve a fluid used i…

Heat Flow Rate
The SI unit of heat flow rate or power is the watt.1W = 3.41214Btu h^-1Btu = British thermal units

Heat of Hydration
The enthalpy change associated with placing gaseous molecules or ions in water. See also: Enthalpy.

Heat of Solution
The enthalpy change associated with dissolving a solute in a solvent. See also: Enthalpy, Solute.

Heat of Vaporization
The energy required to vaporize one mole of a liquid at a pressure of one atmosphere.The kinetic energy of the molecules remains the same, the latent heat is used to overcome the potential energy of the intermolecular forces. See also: Mole, Standard Atmosphere.

Heat Pump
A reversible heat engine that acts as a furnace in winter and an air conditioner in summer. See also: Heat Engine.

Heat Sink
A structure that is mechanically attached to a device that generates heat, in order to lower the overall thermal impedance between the point source of the heat within the device and its cooler surroundings.Number of FinsMaking the fins thicker will increase the heat transfer through the heatsink and to the fins, but will reduce the number of fins t…

Heat Treatment
Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions orproperties. Heating for the sole purpose of hot working is excluded from the meaning of this definition.Colour MethodA technique of heat treating metal by observing the colour changes that occur to determine the proper operation to perform to achieve the desi…

Heaviside Function
Also known as the unit step function. It is defined as:

Heaviside, Oliver (1850-1925)
An English electrical engineer who introduced Laplace transforms into electrical engineering.

Heavy Water
Water that contains ²H, rather than ¹H. Heavy water is about 11% denser than ordinary water. The neutron in the deuterium nucleus allows this type of water to slow, or moderate, neutrons from fissioning uranium, permitting a sustained chain-reaction in reactors using natural uranium as fuel. Symbol D₂O

Hectare
A derived SI unit of area that is mainly used in agriculture to describe the area offields.Conversions1 hectare=2.47105 acre1 hectare=10000 m²ttle='Area';xiunt='acre';yiunt='hectare';mconv=2.47105;cconv=0.0; See also: Acre, Are, Area.

Height
Vertical distance, the distance above some reference point or plane, as, height above sea level.The vertical dimension of anything, the distance which something extends above its foot or root.

Height Gauge
Photograph courtesy of Draper.co.ukA device used to accurately measure height above a ground flat surface. See also: Dial Indicator Gauge, Vernier, Vernier Caliper.

Heisenberg Uncertainty Principle
This principle states that it is not possible to know two complementary parameters precisely (such as position and momentum, energy and time, or angular momentum and angular displacement), the more you know about one, the less you know about the other. This is central to quantum mechanics. First explained by Werner Heisenberg in 1927. See also: Hei…

Heisenberg, Werner Karl (1901-1976)
German physicist; published the first theory of quantum mechanics in 1925. He postulated the 'uncertainty principle in 1927 and received Nobel Prize for physics in 1932. See also: Heisenberg Uncertainty Principle.

Helicopter
An aircraft that produces lift using airfoils that are driven and rotate about a vertical axis. See also: Aircraft, Autogyro.

Heliocentric Model
A model of the Universe with the Sun at its centre.Polish astronomer, Nicolaus Copernicus (1473-1543) advanced the theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the centre of the universe, and all objects, including the sun, cir…

Heliopause
The boundary between the solar wind and the interstellar wind, where the pressure of both are in balance. See also: Bow Shock, Heliosheath, Solar System, Termination Shock.

Heliosheath
A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matter.A vast expanse where the Sun's influence ends and particles blown off its surface crash into the thin gas that drifts between the stars.

Helium
An inert gaseous element. The atom is characterised by possessing two protons in its nucleus. Usually there two neutrons as well. This abundant form of helium is about four times as heavy as hydrogen. Helium accounts for just under one quarter of the atomic matter in the universe and was principally formed during the first few minutes after the big…

Helium Burning
When temperature in the core of a star reaches 100 million degrees, three colliding helium nuclei fuse to form a carbon nucleus. This process occurs when the star is a red giant. See also: Fusion, Helium, Hydrogen Burning.

Helix
A path formed as a point advances uniformly around a cylinder, as the thread on a screw or the flutes on a drill.

Helmholtz Hermann
German physicist, anatomist and physiologist.

Helmholtz Number
The cut-off frequency of a duct is generally described in terms of the Helmholtz Number.whereH_n = helmholtz numberk = wavenumber [m^-1]a = duct radius [m]The plane wave cut-off frequency is when the Helmholtz Number is of the order of 1.84

Helmholtz Resonator
The Helmholtz resonator is an acoustic filter element. It is effectively a mass on a spring (single degree of freedom system). The large volume is the spring and the volume of air in the neck is the mass.The acoustic length of the neck is longer than the physical length. An additional amount must be added at each end. This amount is related to whet…

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Hemisphere
Half of a sphere.Drag CoefficientC_d = 0.38 R_e = 1e3 A = pd²/4

henry
The henry is the SI unit of inductance. A circuit has an inductance of 1 henry when an electromotive force of 1 volt is induced when there is a current variation of 1 ampere per second. Symbol HBasic Unit Derivationkgm²s^-2A^-2

Heptadecagon
A 17 sided polygon. See also: Polygon.

Heptagon
A polygon with 7 sides. Each internal angle is 128.6° and the sum of the internal angles is 900°. See also: Hexagon, Octagon, Polygon.

Heptane
An alkane hydrocarbon. A constituent of petroleum. Symbol C₇H₁₆

Heptode
A heptode is a high-vacuum thermionic valve having seven electrodes, namely an anode, a cathode and five grids. See also: Valve.

Hermetic
Sealing of an object so it is airtight.

Heronian Triangle
A triangle with integer sides and integer area. See also: Integer, Triangle.

Hertz
The SI unit of frequency indicating the number of cycles per second (symbol Hz). The name is in honour of Heinrich Hertz, an early German investigator of radio wave transmission. See also: Frequency, Hertz Heinrich.

Hertz, Heinrich (1857 - 1894)
A German physics professor who did the first experiments with generating and receiving electromagnetic waves, in particular radio waves. In his honor, the units associated with measuring the cycles per second of the waves are called hertz. See also: Hertz.

Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram relates the brightness of a star to its temperature.Zero-age Main SequenceThe position, on the main sequence of a Hertzsprung-Russell diagram, reached by stars after they have passed through their T Tauri phase. It corresponds to a star with 75% hydrogen, 23% helium and 2% metals in its core. Nuclear fusion then grad…

Hess's Law
In going from a particular set of reactants to a particular set of products, the enthalpy change is the same whether the reaction takes place in one step or a series of steps; in other words, enthalpy is a state function. See also: Enthalpy.

Heterocyclic Compounds
In chemistry, heterocyclic compounds are cyclic compounds in which the ring system of the molecule contains other elements than carbon.

Heterodyne
A Heterodyne is generated when two frequencies which are close together (and of similar level) interact to give a 'beating' effect i.e. a noise which gets quieter and louder alternatively. An example of this is a twin propeller aeroplane whose engines (running at slightly different speeds) give a throbbing type of noise. See also: Beats.

Heterogeneous Catalysis
A form of catalysis in which the catalyst is in a different physical state than the reactants. See also: Catalyst, Homogeneous Catalysis.

Hexadecagon
A 16 sided polygon. See also: Polygon.

Hexadecimal
Counting system based on 16. Has 0 through 9 and A through F as the digits. In spoken language, pronounced Hex. Usually used as a condensed notation for binary numbers.function init(){}This will convert decimal numbers up to 65535 and hexadecimal up to FFFF multicalc_form(0)

Hexagon
A polygon with 6 sides. Each internal angle is 120° and the sum of the internal angles is 720°. See also: Heptagon, Octagon, Polygon.

Hexagonal Close Packed
A crystal structure found for some metals. The HCP unit cell is of hexagonal geometry and is generated by the stacking of close-packed planes of atoms. See also: Body Centred Cubic, Crystal, Face Centred Cubic.

Hexahedron
A polyhedron having 6 faces. The cube is a regular hexahedron. See also: Polyhedral.

Hidden Line
Broken line used to signify a line that normally would not be seen in a drawing.

High Impact Characteristic
Product and/or process characteristics that, when outside of thespecification tolerance, severely affect subsequent manufacturing operations or customer satisfaction; the productor process is not, however, unsafe.

High Level Language
A computer language with commands that do not directly representthe machine instructions. Usually the commands resemble English statements.

High Tension
High Tension is a comparative term used in electronics to denote high voltages. See also: Volt.

Hipparchus (160-125 BC)
A Greek astronomer who carried out his observations at Rhodes. He discovered the precession of the equinoxes and the eccentricity of the sun's path, determined the length of the solar year, estimated the distances of the sun and moon from the earth, drew up a catalogue of 1080 stars, fixed the geographical position of places by latitude and longitu…

Hippocrates (460-377BC)
The most celebrated physician of antiquity. He believed that 'the four fluids or humours of the body (blood, phlegm, yellow bile and black bile) are the primary seats of disease.' But, it is the ethics of Hippocrates to which, for many years, the medical doctor subscribed by the taking of the Hippocrates Oath. Famous Quotes:'There are in fact two t…

Histogram
A graphical display showing the distribution of data values in a sample by dividing the range of the data into non-overlapping intervals and counting the number of values which fall into each interval. These counts are called frequencies. Bars are plotted with height proportional to the frequencies. See also: Cumulative Frequency, Relative Frequenc…

HMI
Abbreviation of Human Machine Interface.

Hob
A cylindrical cutting tool shaped like a worm thread and used in industry to cut gears. See also: Gear Design, Hobbing.

Hobbing
The operation of cutting gears with a hob. See also: Gear Design, Hob.

Hohlraum
In radiation thermodynamics, a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity.This idealized cavity can be approximated in practice by making a small perforation in the walls of a hollow container of any opaque material. The radiation escaping through such a perforation will be a good approximation to blac…

Hole
Particle associated with an empty electron level in an almost filled band.For semi-conductors and insulators, a vacant electron state in the valence band that behaves as a positive charge carrier in an electric field.

Hole Flow
Current carriers (in transistors) which have a flow in a direction opposite to electron movement. See also: Hole, Transistor.