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


Holmium
Holmium is a member of the lanthanide group and whose properties closely resemble those of erbium and dysprosium. It is soft and malleable and is slowly attacked by oxygen and water. It is soluble in acids. Applications for holmium are limited, but it is used as a flux concentrator in magnetic fields and also as a poison in nuclear reactors where i…

Holocene
The current geological time period that started about 10,000 years ago.

Hologram
A three-dimensional record of visual information. See also: Holography.

Holography
The photographic process for producing three-dimensional images by recording the standing wave pattern caused by the interaction of two coherent beams of radiation, one having been modified by interaction with the object. See also: Hologram.

Homogeneous Catalysis
A form of catalysis in which the catalyst is in the same physical state as the reactants, for example reactants and catalyst are all gases. See also: Catalyst, Heterogeneous Catalysis.

Homogeneous Equations
A differential equation is homogeneous if of the form

Homopolymer
A polymer having a chain structure in which all mer units are of the same type. See also: Polymer.

Honing
The process of finishing ground surfaces to a high degree of accuracy and smoothness with abrasive blocks applied to the surface under a light controlled pressure and with a combination of rotary and reciprocating motions.Photograph courtesy of Draper.co.uk One surface that requires particular attention is the cylinder bore.

Hooke's Law
A Law stating that, in a linear system, the restoring force is proportional to the displacement of the body, acting in a direction as to restore equilibrium.whereF = restoring force [N]k = spring constant [Nm-1]x = elongation of material [m]The constant of that proportionality is the Young modulus of elasticity for that substance.whereE …

Horizontal Line
A line whose slope is zero.

Horsepower
Mechanical engineering unit of power.Originally defined by James Watt as 33000 ft lb of work done by a horse in 1 minute.Conversions1 horsepower (hp)=550 ft lbf s-11 hp=33000 ft lbf min-11 hp=745.7 W1 hp=1.014 metric horsepower (PS or ch)1 hp=2546 Btu h-11 hp=42.433 Btu min-11 hp=0.178 kg cal s-1
Hot Pressing
A method used to increase the density of a material, whereby heat and pressure are applied simultaneously, and the pressure is typically applied unidirectionally via rigid tooling.

Hot Working
Any metal forming operation that is performed above a metal recrystallization temperature.

Hour
A unit of time equal to 60 minutes or 3600 seconds in length. See also: Minute, Second, Time.

Huffman Coding
For a given character distribution, by assigning short codes to frequently occurring characters and longer codes to infrequently occurring characters, Huffman's minimum redundancy encoding minimizes the average number of bytes required to represent the characters in a text. Static Huffman encoding uses a fixed set of codes, based on a representativ…

Human Machine Interface
The interface between man and machine. See also: Graphical User Interface, Haptic.

Humidity
Measure of the amount of water present in the air at any given time.A comfortable working environment is with an air temperature between 20 and 24 degrees C and with a relative humidity between 40% and 60%. See also: Absolute Humidity, Dew Point, Humidity at Saturation, Hygrometer, Relative Humidity, Temperature.

Humidity at Saturation
The mass per unit volume of water vapour required to saturate the air. See also: Absolute Humidity, Dew Point, Humidity, Relative Humidity, Temperature.

Hundredweight
Abbreviated as cwt, this is an imperial unit of mass.Conversions1 hundredweight (cwt)=50.8 kg1 hundredweight (cwt)=112 pounds (lb)1 short hundredweight=100 pounds (lb)20 hundredweight (cwt)=1 tonttle='Mass';xiunt='kg';yiunt='hundredweight';mconv=50.8;cconv=0.0; See also: Mass, Quintal, Short Hundredweight.

Hunds Rule
When electrons are put into orbitals having the same energy, degenerate orbitals, one electron is put into each orbital before putting a second electron into an orbital. See also: Atom, Electron.

Huygens' Principle
Named after Christian Huygens who originally stated this principle in 1678'Every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave'. The new wavefront is constructed by the tangent to these secondary wavelets

Hybrid
Of mixed origin or composition.

Hydrate
In chemistry, a hydrate is a compound that has discrete water molecules combined with it. The water is known as water of crystallization and the number of water molecules associated with one molecule of the compound is denoted in both its name and chemical formula.Example:copper(II) sulphate pentahydrate (CuSO4·5H2O)

Hydration
In chemistry, hydration is the combination of water and another substance to produce a single product. It is the opposite of dehydration. See also: Water.

Hydraulic Set Cement
A cement that sets through reaction with water. See also: Cement.

Hydride
In chemistry, a hydride is a compound containing a negatively charged hydrogen, as in sodium hydride(Na-H). See also: Hydrogen.

Hydrocarbon
A hydrocarbon is a chemical containing only hydrogen and carbon. See also: Carbon, Hydrogen.

Hydrochloric Acid
A solution of hydrogen chloride in water. Concentrated acid is a colourless, fuming, corrosive liquid. Symbol HCl Also known as Chlorohydric acid

Hydrodynamic Bearing
A bearing which supports the shaft on a thin film of oil, the fluid-film layer is generated by journal rotation. See also: Bearing, Fluid Film Bearing, Hydrostatic Bearing.

Hydrofluoric Acid
Aqueous solution of hydrogen fluoride. Dissolves most metals and etches glass. Symbol HF

Hydrogen
The least dense element. It has metallic and non-metallic properties. The most abundant element in the universe. Hydrogen occurs as H2 at ambient temperature and pressure, a colourless, odourless, and extremely flammable gas. In its free gaseous state it is only found in nature in small quantities issuing from crevices in volcanic areas …

Hydrogen Arsenide
Intensely poisonous colourless gas. Symbol AsH3 Also known as Arsine

Hydrogen Bonding
Strong type of intermolecular dipole-dipole attraction. A strong secondary interatomic bond which exists between a bound hydrogen atom (its unscreened proton) and the electrons of adjacent atoms. Occurs between hydrogen and fluorine, oxygen or nitrogen. See also: Fluorine, Hydrogen, Nitrogen, Oxygen.

Hydrogen Burning
Hydrogen burning is the fusion of four hydrogen nuclei (protons) into a single helium nucleus (two protons and neutrons.) The process is a series of reactions. The type of reactions depend on the mass of a star and its core temperature and density. In our Sun, the process is a proton-proton chain. In more massive stars, the C-N-O cycle (Carbon-Nitr…

Hydrogen Chloride
Dissolves in water to produce hydrochloric acid. Symbol HCl Also known as Chlorane, Hydrochloride, Acid air.

Hydrogen Cyanide
Colorless, very poisonous and highly volatile liquid. Symbol HCN Also known as Hydrocyanic acid, prussic acid, formonitrile, formic anammonide, carbon hydride nitride, cyclon

Hydrogen I Selenide
Colourless gas. Symbol H2Se

Hydrogen I Telluride
Colourless gas. Symbol H2Te

Hydrogen Iodide
Colourless gas. Symbol HI

Hydrogen Peroxide
A colourless, viscous, unstable liquid compound of hydrogen and water with oxidising and reducing properties. Used for bleaching wool, silk, linens, furs and hair products since the 19th century. During the Second World War hydrogen peroxide was used as a propellant in V1 rockets and was subsequently also used as a propellant for submarines. Hydrog…

Hydrogen Persulphide
Colourless liquid. Symbol H2S2

Hydrogen Sulphide
Poisonous and smells of rotten eggs. Symbol H2S

Hydrogenation
In chemistry, hydrogenation is a chemical reaction in which hydrogen is added to a compound. See also: Hydrogen.

Hydrolysis
When water reacts with another substance and as a result the oxygen in water makes a bond with the substance.

Hydrometer
An instrument used for measuring the specific gravity of a liquid.A hydrometer consists of a glass volume with a weight at the bottom and a graduated scale on the stem.The hydrometer is gently lowered into a volume of liquid and allowed to settle. The reading is taken at the top surface of the liquid.This liquid is denser than the liquid in the nex…

Hydrophilic
A polar molecule or group that can form strong hydrogen bonds with water. See also: Hydrogen, Water.

Hydrophobic
A nonpolar molecule or group that has little affinity for water. Hydrophobic groups on molecules in solution tend to turn in on themselves or clump together with other hydrophobic groups because they are unable to disrupt the network of strong hydrogen bonds in the water around them. See also: Water.

Hydrophone
A hydrophone converts acoustic energy into electrical energy and is used in underwater passive systems for listening only. Hydrophones are usually used below their resonance frequency over a much wider frequency band where they provide uniform output levels.The beam width of a hydrophone determines its directivity. A narrow beam will give it greate…

Hydroquinone
A reducing agent used in photographic developers.

Hydrosphere
All the water on the surface of a planet. See also: Atmosphere.

Hydrostatic Bearing
A bearing which supports the shaft on a thin film of oil, the fluid-film layer is generated by externally applied pressure. See also: Bearing, Fluid Film Bearing, Hydrodynamic Bearing.

Hydroxy Group
An -OH group attached to an organic molecule.

Hygrometer
A tool for measuring the humidity of the air. See also: Humidity.

Hygroscopic
Able to absorb moisture from air. For example, sodium hydroxide pellets are so hygroscopic that they dissolve in the water they absorb from the air. See also: Air, Water.

Hyperbarism
Disturbances in the body resulting from an excess of the ambient pressure over that within the body fluids, tissues, and cavities.

Hyperbola
An open curve with two branches, all points of which have a constant difference in distance from two fixed points called focuses.

Hyperbolic Sine
Hyperbolic sine of x.DefinitionRelationshipsNegative ArgumentsAddition FormulaDouble angle formulaHalf Angle Formulafor x>0andfor x<0Multiple Angle FormulaePowersSum and DifferenceProductInverse HyperbolicHyperbolic and Trigonometric RelationshipsInverse Hyperbolic and Inverse Trigonometric Relationships

Hypereutectic Alloy
Any binary alloy whose composition lies to the right of the eutectic on an equilibrium diagram,and which contains some eutectic structure. The concentration of solute is greater than the eutectoid composition. See also: Eutectic, Hypoeutectic Alloy.

Hyperopia
Farsightedness. Images of distant objects are formed beyond the retina.

Hyperoxia
A condition in which the total oxygen content of the body is increased above that normally existing at sea level. See also: Hypobarism.

Hypersonic
A range of speed that is five times or more the speed of sound in air. See also: Hypervelocity, Subsonic, Supersonic.

Hypervelocity
A range of speed that is about 12 times or more the speed of sound in air. See also: Hypersonic, Subsonic, Supersonic, Velocity.

Hypobaric
Pertaining to low atmospheric pressure, particularly the low atmospheric pressure of high altitudes.

Hypobarism
Disturbances resulting from a decrease of ambient pressure to less than that within the body fluids, tissues, and cavities. See also: Hyperoxia.

Hypoeutectic Alloy
Any binary alloy whose composition lies to the left of the eutectic on an equilibrium diagram,and which contains some eutectic structure. An alloy for which the concentration of solute is less than the eutectoid composition See also: Eutectic, Hypereutectic Alloy.

Hypotenuse
The longest side of a right triangle. See also: Right Triangle.

Hypothesis
A hypothesis is a proposed answer to a problem, or an explanation that accounts for a set of facts and that can be tested by further experimentation and observation. The results of experimentation provide evidence that may or may not support the hypothesis.

Hypothesis Tests
Tests based on a sample of data to determine which of two different states of nature is true. The two states of nature are commonly called the null hypothesis, which gets the benefit of the doubt, and the alternative hypothesis.

Hysteresis
If the input varies from an initial point to a final point and then retraces its value back to the initial point, then the two paths of the output should be identical. If there is hysteresis in the system then the output paths will not coincide.Magnetic HysteresisThe irreversible magnetic flux density versus magnetic field strength (B-versus-H) beh…

Hysteresis Damping
Hysteresis damping produces a force which is in phase with the velocity and proportional to the displacement, resulting in a linear equation of motion. Transmissibility at resonance is equal to 1/n where n is the loss factor of the material. Response characteristics are similar to viscous damping except that increasing frequency has no effect on a …

Ice
Formed when water is cooled below its freezing point.

Ice Point
The temperature at which pure ice can exist in equilibrium with water at standard atmospheric pressure. See also: Kelvin, Steam Point, Triple Point.

Icosahedron
A 20 sided solid.

Ideal Gas
An enormous number of very tiny particles separated by relatively large distances. The particles have no internal structure, are indestructible, do not interact with each other except when they collide, and all collisions are elastic. See also: Gas, Ideal Gas Law, Isothermal Expansion.

Ideal Gas Law
Gases obey the gas laws at low pressures and at temperatures above those at which they liquefy.An ideal gas will obey:wherep = PressureV = Volumen = Number of molesR = Molar gas constantT = TemperatureAn ideal gas will also obey the following laws:Joule's lawBoyle's lawAdiabatic equationsInternal energyReversible isothermal expansionJoule expansion…

Idle
Internal combustion engine operating at no load and minimum engine speed.Idle StabilityThe variation in engine rotational speed when operating under no load and minimum engine speed. The idle stability will be affected by a number of parameters:Engine speed control strategyNumber of cylindersRotational component moment of inertia

Ignition Timing
The time of occurrence of ignition measured in degrees of crankshaft rotation relative to TDC (Top Dead Centre).

Illuminance
This is defined as the luminous flux arriving at a surface that is perpendicular to it perunit area. The metric unit is the lumen m-2 or abbreviated to lux or lx.In the British system this was defined as the lumen ft-2 or known as the footcandle or fcExamplesSunny day10000 luxOvercast day1000 luxPrecision mechanical workshops,…

Imaginary Axis
The y-axis of an Argand diagram. See also: Complex Numbers, Real Axis.

Imaginary Number
i denotes the imaginary number formed by square rooting negative one.

Imaginary Part
The imaginary part of a complex number x+iy where x and y are real is y. See also: Complex Numbers.

Imbalance
Unequal radial weight distribution on a rotor system; a shaft condition such that the mass and shaft geometric centerlines do not coincide.

Immiscible
In chemistry, immiscible means incapable of being mixed, as with oil and water.

Impact
A collision of a mass in motion with a second mass. See also: Conservation of Momentum.

Impact Energy
A measure of the energy absorbed during the fracture of a specimen of standard dimensions and geometry when subjected to very rapid loading See also: Charpy Test, Izod Test.

Impact Harshness
Impact harshness characterizes the interior sound and vibration resulting from tyre interactions with discrete road disturbances. Impact Harshness has both audible and tactile components. It is triggered by low and medium-speed vehicle contact with road surface irregularities such as: drop-offs, expansion joints, frost heaves, potholes, railroad cr…

Impact Isolation Class
A measure or specification of isolation effectiveness of building structures from impact noises such as slammed doors, dropped objects, footfalls, shuffled furniture, etc. The higher the IIC rating, the better such isolation. Impact noises can be transmitted through walls, floors, and ceilings throughout a building and re-radiated at distant locati…

Impact Test
A test to determine the behavior of materials when subjected to high rates of loading, usually inbending, tension or torsion. The quantity measured is the energy absorbed in breaking the specimen by a singleblow, as in the Charpy or Izod tests.

Impedance
Acoustic ImpedanceThe total reaction of a medium to the transmission of sound through it, expressed as the ratio of sound pressure to particle velocity at a given point in the medium. The acoustic impedance Z is useful in describing the acoustic radiation from sources such as surfaces and ducts.Mechanical ImpedanceMechanical impedance is the ratio …

Impedance of Free Space
Z0 = 376.730313461 Ohmttle='Impedance of Free Space';consttxt='b';constval=376.730313461;constunt='Ohm'; See also: Physical Constants.

Impeller
A device that imparts motion to a fluid. In a centrifugal compressor, a rotary disk faced on one or both sides with radial vanes, accelerates the incoming fluid outward into a diffuser.

Implosion
A violent inward collapse. An inward explosion. See also: Explosion.

Improper Fractions
A fraction whose numerator is of greater absolute value than it's denominator. See also: Denominator, Fraction.

Impulse
The product of the force and the time during which it acts. This vector quantity is equal to the change in momentum. See also: Force, Pulse Range, Time.

Impulsive Noise
A noise level that fluctuates over a range greater than 10 dB during observation is classified as impulsive.Impulsive sounds, such as gun shots, hammer blows, explosions of fireworks or other blasts, are sounds that significantly exceed the background sound pressure level for a very short duration.Measurements with the meter set to ‘Fast` response …

Impurity
A foreign atom in a crystal. See also: Crystal, Donor.

In Phase
Two periodic waves reaching peaks and going through zero at the same instant are said to be 'in phase.' See also: Phase.

Incandescence
Incandescence is the emission of light due to heat.

Inches
A secondary unit in the British system of units, now replaced by the SI systemand the metre. Often abbreviated with ' e.g. 12 inches = 12'Conversions1 inch=25.4 mm1 foot=12 inches1 yard=36 inches6 feet=1 fathomttle='Length';xiunt='m';yiunt='inches';mconv=25.4e-3;cconv=0.0; See also: Foot, Length, mil, SI Units, Yard.

Inclinometer
A gravity device that measures angular position in degrees. See also: Angles, Standard Acceleration due to Gravity.