Helium is an inert gaseous element.
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.
Header Bond
A particular arrangement of bricks.
The main light on the front of a vehicle.
Fabric or vinyl upholstery on the interior of the roof of a vehicle.
A term related to dynamic range, used to express in dB, the level between the typical operating level and the maximum operating level above which the signal will be clipped.
The fixed or stationary end of a lathe or similar machine tool.
The subjective human response to sound.
Hearing Damage
A person exposed to high noise levels can suffer hearing damage. The damage may be gradual or traumatic.
Hearing Level
A measured threshold of hearing at a specified frequency, expressed in decibels relative to a specified standard of normal hearing.
Hearing Loss
An impairment of auditory acuity.
Hearing Protector
Personal device worn to reduce harmful auditory or annoying subjective effects of sound.
Hearing Range in Animals
Many animals hear a much wider range of frequencies than is possible for humans.
Hearing Sensitivity
The human ear is less sensitive at low frequencies than in the midrange.
Hearing Threshold Level
The amount in decibels by which an individual′s threshold of audibility differs from a standard audiometric threshold.
A muscular organ that pumps blood through the body, a human heart is about the size of a fist.
The fully developed wood which surrounds the pith.
If a temperature difference exists between two points heat will flow from the point at the higher temperature to the point at the lower temperature. This flow will continue until equilibrium has been reached. The transmission of heat may be due to three different processes.
Heat Affected Zone
That part of the base metal which has been altered by the heat from the welding, brazing, or cutting operation.
Heat Balance
A tabulation showing the percentages of the heat developed by combustion in the engine cylinder that are (1) delivered in the form of power at the crankshaft, (2) lost in friction, (3) lost to the cooling water, and (4) lost in the exhaust gases.
Heat Capacity
The amount of heat required to raise the temperature of a body through 1K.
Heat Check
A pattern of parallel surface cracks that are formed by alternate rapid heating and cooling of the extreme surface metal.
Heat Content
The SI unit for heat content is the Joule per cubic metre.
Heat Cracking
A pattern of small, irregular cracks, that result from local heating of an object.
Heat Engine
A device for converting heat into mechanical work.
Heat Exchanger
Device built for efficient heat transfer from one medium to another.
Heat Flow Rate
The SI unit of heat flow rate or power is the watt.
Heat Index
A number representing the effect of temperature and humidity on humans by combining the two variables into an apparent temperature.
Heat Island Effect
A dome of elevated temperatures over an urban area caused by structural and pavement heat fluxes, and pollutant emissions.
Heat Load
Terms for the amount of heating (heat loss) or cooling (heat gain) needed to maintain desired temperatures and humidities in controlled air.
Heat of Combustion
The amount of energy released in burning completely an amount of substance.
Heat of Condensation
The heat liberated by a unit mass of gas at its boiling point as it condenses to a liquid. Equal to the heat of vaporization.
Heat of Formation
The enthalpy change when 1 mole of a substance is formed from the elements in their standard states.
Heat of Fusion
The enthalpy change that occurs to melt a solid at its melting point.
Heat of Hydration
The enthalpy change associated with placing gaseous molecules or ions in water.
Heat of Neutralization
The heat released by an acid-base neutralization reaction running at constant pressure.
Heat of Reaction
The heat absorbed or released by a chemical reaction running at constant pressure.
Heat of Solution
The enthalpy change associated with dissolving a solute in a solvent.
Heat of Sublimation
The change in enthalpy when one mole of solid vaporizes to form one mole of gas.
Heat of Vaporization
The energy required to vaporize one mole of a liquid at a pressure of one atmosphere.
Heat Pipe
A device that can transfer large amounts of heat with a small difference in temperature between the hot and cold interface.
Heat Pump
A reversible heat engine that acts as a furnace in winter and an air conditioner in summer.
Heat Radiation
This is the electromagnetic radiation emitted by a body as a function of its temperature.
Heat Shield
Any device that protects something from heat.
Heat Shrink Tubing
A tubing that shrinks when heated.
Heat Shunt
A device used to absorb heat and protect heat-sensitive components during soldering.
Heat Sink
A structure that is mechanically attached to a device that generates heat.
Heat Transfer
Movement of heat from one body or substance to another. Heat may be transferred by radiation, conduction, convection or a combination of these three methods.
Heat Treating Cast Iron
Cast iron has plenty of carbon available and is easy to heat treat.
Heat Treatment
Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties.
Heat Value
The heat developed by the combustion of one pound of fuel, Btu per Pound.
Heating Curve
A plot of temperature verses time for a substance where energy is added at a constant rate.
Heating Value
The amount of heat produced by combustion of a unit quantity of a fuel.
Heating Ventilation and Air Conditioning
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Heating Ventilation and Air Conditioning Books
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Heaviside Function
Also known as the unit step function.
Heaviside Oliver
An English electrical engineer who introduced Laplace transforms into electrical engineering.
Heavy Duty Engine
An engine that is designed to allow operation continuously at or close to its peak output.
Heavy Metals
Metallic elements with high atomic weights can damage living things at low concentrations and tend to accumulate in the food chain.
Heavy Water


Water that contains 2H, rather than 1H. Heavy water is about 11% denser than ordinary water.

An SI derived unit of area.
Prefix representing a multiplication of 102.
The vertical dimension of anything.
Height Gauge
A device used to accurately measure height above a ground flat surface.
Heisenberg Uncertainty Principle
This principle states that it is not possible to know a particle's location and momentum precisely at any time.
Heisenberg Werner
Postulated the uncertainty principle in 1927 and received Nobel Prize for physics in 1932.
Having the shape of a helix, or screw.
Helical Antenna
An antenna constructed in the form of a helix.
Helical Gear
The helical gear gives a much smoother operation than the spur gear and reduces the noise levels. However, it is more difficult to manufacture.
Helical Spring
A coiled spring.
An enzyme that catalyzes the unwinding of strands in a DNA molecule before replication.
An aircraft that produces lift using airfoils that are driven and rotate about a vertical axis.
Heliocentric Model
A model of the Universe with the Sun at its centre.
The boundary between the solar wind and the interstellar wind, where the pressure of both are in balance.
A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matter.
An inert gaseous element.
Helium Burning
When temperature in the core of a star reaches 100 million degrees, three colliding helium nuclei fuse to form a carbon nucleus.
Helium-Neon Laser
A laser in which the active medium is a mixture of helium and neon.
A path formed as a point advances uniformly around a cylinder, as the thread on a screw or the flutes on a drill.
Helmholtz Free Energy
Energy released or absorbed in a reaction occurring reversibly at constant volume and temperature.
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.
Helmholtz Resonator
An acoustic filter element.
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The iron-porphyrin prosthetic group of heme proteins.
A protein containing a heme as a prosthetic group.
Half of a sphere.
The end of the highly luminous path of a visual meteor.
A haeme protein of erythrocytes which functions in oxygen transport.
Another name for Undecane.
HeNe Laser
Abbreviation of Helium-Neon Laser.
The SI unit of inductance.
Henry′s Law
At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
Hepatic Air
Old name for hydrogen sulphide.
A 17 sided polygon.
A polygon with 7 sides.


An alkane hydrocarbon. A constituent of petroleum.

A high-vacuum thermionic valve having seven electrodes.
An animal that feeds on plants.
A property of a space is hereditary if every of its subspaces possesses this property.
Sealing of an object so it is airtight.
Heronian Triangle
A triangle with integer sides and integer area.
Hero′s Engine
A device which propels itself by shooting steam from one or more orifices. Also known as an Eolipile.
Herring Bone Diagram
Also known as Fish Bone Diagram.
The SI unit of frequency indicating the number of cycles per second (symbol Hz).
Hertz Antenna
A half-wave antenna that is installed some distance above ground and positioned either vertically or horizontally.
Hertz Heinrich
A German physics professor who did the first experiments with generating and receiving electromagnetic waves, in particular radio waves.
Hertzian Waves
Now generally called radio waves.
Hertzsprung-Russell Diagram
The Hertzsprung-Russell diagram relates the brightness of a star to its temperature.
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.
Heterocyclic Compounds
In chemistry, heterocyclic compounds are cyclic compounds in which the ring system of the molecule contains other elements than carbon.
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.
The process of mixing the incoming signal with the local oscillator frequency. This produces the two fundamentals and the sum and difference frequencies.
Heterogeneous Catalysis
A form of catalysis in which the catalyst is in a different physical state than the reactants.
Heterogeneous Database
Database that contains different kinds of data, e.g, text and numerical data.
Heterogeneous Mixture
A sample of matter consisting of more than one pure substance and more than one phase.
A region of electrical contact between two different materials.
Heterotrophic Respiration
The conversion of organic matter to CO2 by organisms other than plants.
Having two different alleles of the same gene.
Abbreviation of Hybrid Electric Vehicle.
Alternative name for Molybdenum Hexacarbonyl.
A 16 sided polygon.
Counting system based on 16.
A polygon with 6 sides.
Hexagonal Close Packed
A crystal structure found for some metals.
Hexagonal Number
A number of the form n(2n-1).
Hexagonal Nut
A nut with a plain internal thread.
Hexagonal Prism
A prism with a hexagonal base.
A polyhedron having 6 faces.


Hexane isomers are largely unreactive, and are frequently used as an inert solvent in organic reactions because they are very non-polar.

A polygon in the plane made of 6 equal-sized squares connected edge-to-edge.