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

Thermal Equilibrium
Two objects are in thermal equilibrium if there is no energy flow from one to the otherand both are at the same temperature. See also: Zeroth Law of Thermodynamics.

Thermal Gradient
The rate at which the temperature changes with position. See also: Temperature.

Thermal Insulation
The materials and methods used to reduce heat transfer. See also: Heat Flow Rate, Thermal Conduction.

Thermal Resistance
The property of a material, device or system that impedes the flow of heat. Thermal impedance is defined as the temperature difference between two specified points or regions divided by the power dissipation under conditions of thermal equilibrium.

Thermal Shock
Stresses induced in a material because of a rapid temperature change or a thermal gradient. See also: Stress, Thermal Cycling.

Thermie
Unit of heat energy. Equal to the amount of energy required to raise the temperature of 1 tonne of water by 1°C.Conversions1 thermie=4.1868x10⁶ joules1 thermie=3968.3 Btuttle='Thermie';xiunt='joule';yiunt='thermie';mconv=4.1868e6;cconv=0.0;

Thermion
A thermion is an electrically charged particle emitted from a heated body.

Thermionic Emission
The production of electron emission by thermal means. The first thermionic observation, though not understood at the time, was made by Edison, and is known as the Edison effect.

Thermistor
A resistor that varies in value according to its temperature. It is a semiconductor that exhibits a large change in resistance as a function of temperature. Most thermistors exhibit a negative temperature coefficient, where resistance decreases as temperature rises. See also: Resistor, Temperature.

Thermite Process
The Thermite Process or Goldschmidt Process is the method of obtaining liquid metal by reduction of the oxide with aluminium powder, when ignited with magnesium ribbon it reacts by producing iron and aluminium oxide at an intensely high temperature approaching 3000 degrees Celsius.It was developed for making welding repairs in situ, such as rail tr…

Thermochemistry
The study of heat absorbed or released during chemical changes. See also: Thermodynamics.

Thermocline
A vertical temperature gradient in some layer of a body of water, which is appreciably greater than the gradients above and below it.

Thermocouple
A device for measuring temperatures, consisting of two dissimilar metals which produce anelectromotive force roughly proportional to the temperature difference between their hot and cold junction ends.Cold JunctionThe reference junction of a thermocouple which is kept at a constant temperature.

Thermodynamic Equilibrium
A system is at thermodynamic equilibrium if the energy it gains from its surroundings is exactly balanced by the energy it loses, no matter how much time is allowed to pass. See also: Thermodynamics.

Thermodynamics
The study of energy transfers and transformations. See also: First Law of Thermodynamics, Partition Function, Second Law of Thermodynamics, Thermodynamic Equilibrium, Third Law of Thermodynamics, Zeroth Law of Thermodynamics.

Thermoelectron
An electron emitted by a very hot object.

Thermohygrograph
An instrument for measuring humidity and temperature.Used extensively in museums, libraries and where the temperature and humidity need to be monitored to ensure that items are kept in the optimum conditions. See also: Humidity, Temperature.

Thermometer
An instrument for measuring temperature.Historical Notes1592 Thermometer was invented by Galileo. He built a crude thermometer using the contraction of air to draw water up a tube.1612 Graduation and inclusion of fixed points was added by Santorre Santorio who used snow and the heat of a candle, dividing the range obtained into degrees. He was the …

Thermometry
The science of temperature measurement. See also: Celsius Temperature Scale, J-Type Thermocouple, K-Type Thermocouple, N-Type Thermocouple, Ttype Thermocouple.

Thermoplastic
A polymer that softens or melts on heating, and becomes rigid again on cooling. Thermoplastic polymer chains are not cross-linked. Polystyrene and polythene are thermoplastics. See also: Plastic, Polymer.

Thermostat
A thermostat is a device which automatically maintains temperature at a constant value or gives notice of an undue change in temperature.

Thevenin's Theorem
Theorem that replaces any complex network with a single voltage source in series with a single resistance.

Thin Rod
The moments of inertia of a thin rod of length, L and mass, m

Thiokol
The trade name of a synthetic rubber, derived from sodium tetrasulphide and organic dichlorides.

Third Law of Thermodynamics
For changes involving only perfect crystalline solids at absolute zero, the change of the total entropy is zero. See also: Entropy, First Law of Thermodynamics, Second Law of Thermodynamics, Thermodynamics, Zeroth Law of Thermodynamics.

Thixotropic Fluid
A liquid that becomes less viscous when stirred. Paint and printing inks are thixotropic fluids; they are formulated so that they flow more freely when brushed or rolled.

Thomson Cross Section
s_e = 0.665245854x10^-28m²ttle='Thomson cross section';consttxt='s_e';constval=0.665245854e-28;constunt='m²';wheres_e = Thomson cross sectione₀ = permittivity of free spacem_e = electron rest mass [kg]c = speed of light [ms^-1]r_e = classical elec…

Thorium
Thorium is a dark grey, radioactive metal of which the principal source is the ore, monazite, a complex phosphate of thorium, uranium, cerium and lanthanides. The metal is soft and ductile and is extracted by precipitation as the hydroxide, along with cerium and uranium; separation is achieved by further extraction with tributyl phosphate from an a…

Thou
British abbreviation of thousandths of an inch, the American abbreviation is mil. A unit of length used by engineers.ttle='thou';xiunt='mm';yiunt='thou';mconv=25.4e-3;cconv=0.0; See also: Length, mil.

Thousand
A number equal to 10³. See also: Million.

Thread
A helical projection of uniform section on the internal or external surface of cylinder or cone. Also, the operation of cutting a screw thread.Left Hand Thread A screw thread that is screwed in by rotating counterclockwise. Right Hand Thread A screw thread that is screwed in by rotating clockwise. Most threads are right handed. Symmetrical ThreadBo…

Threshold of Feeling
The threshold of feeling for a specified signal is the minimum effective sound pressure of that signal which, in a specified fraction of the trials, will stimulate the ear to a point at which there is the sensation of discomfort.

Threshold of Hearing
The threshold of hearing is defined by the equation:whereT_q = threshold of hearing [dB]f = frequency [Hz]Human hearing is most sensitive just below 4kHz. Above 10kHz the sensitivity of the ear rapidly deacreases.Threshold of hearing versus frequency based on above equation.

Threshold of Pain
The minimum sound pressure level of a sound outside the ear that will produce a transition from discomfort to definite pain.Roughly located 120 dB above the threshold of hearing. See also: Auditory Area.

Threshold Shift
Temporary threshold shiftOccurs when a person has been exposed for a few hours to noise levels of about 80 dB and above. These often leave a ringing the ears for some time afterwards. The greater part of the hearing loss occurs soon after exposure and, simply, recovery occurs largely in the 30 minutes following removal from the noise. Persons expos…

Thrust
The force exerted in any direction by a fluid jet. See also: Force, Propulsion, Rocket, Thrusters.

Thrust Bearing
A bearing designed to only take axial or thrust load. See also: Bearing.

Thrusters
Small reaction engines on a spacecraft that can provide thrust used to control its orbit, orientation and attitude. See also: Propulsion, Rocket, Thrust.

Thulium
A metallic element that is a member of the rare earth group. Symbol Tm

Thyratron
A gas-tube rectifier with a grid electrode that has the ability to initiate current flow when the anode is positive. Once conduction occurs, the grid loses control until the plate current is interrupted.

Thyristor
A component rather like a diode but will not conduct until a voltage is applied to its third terminal known as the gate. See also: Transistor, Triac.

Tidal Force
The force of the tides upon anything in their path. See also: Tidal Heating, Tide.

Tidal Heating
The frictional heating of the interior of a satellite due to flexing caused by the gravitational pull of the parent planet. See also: Tidal Force, Tide.

Tide
The dynamic deformation of the spherical shape of a rotating celestial object brought about by the gravitational attraction of another, nearby, body. On Earth we experience highly visible changes in sea level twice a day because our planet and satellite Moon revolve around their mutual centre of gravity. Because the moon is 81 times lighter than th…

Timbre
The quality of a sound that distinguishes it from other sounds of the same pitch and volume. The distinctive tone of an instrument or a singing voice. It is this quality which allows you to distinguish between a flute and an oboe playing the same pitch at the same volume. See also: Sound.

Time
The SI unit of TimeThe SI unit of time is the second and is the duration of 9192631770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of a caesium 133 atom.

Time Dilation
Time in the moving reference frame always flows more slowly. Time is slowed by about 40 nanoseconds in a flight from London to New York but the effect is more easily observed at speeds approaching that of light. Time will also pass more slowly in a strong gravitational field than in a weak one. See also: Length Contraction, Proper Time.

Time History
A continuous record of the variation of a physical quantity (e.g. displacement, acceleration, force, etc.) with time. The record may be stored in a chart recorder, magnetic tape, computer disc or any other storage medium. See also: Data.

Time Invariant
Describes a system whose output will experience no change other than an equal shift in time when the input is shifted in time.

Time of Flight
AcousticsA method of locating a source in space based on the arrival times of the signal at a number of receivers. Often used to locate structural and acoustic noise sources.ElectronicsThe time taken for a signal to propagate from one logic gate or opto-electronic component to another.

Time Zones
The Earth is divided into 24 time zones so that everyone in the world can be on roughly similar schedules.The idea to divide the Earth into time zones was proposed by the Canadian railway planner and engineer Sir Sandford Fleming in the late 1870s.

Tin
Tin is a silvery white metal which is soft and pliable, and which emits the characteristic sound of 'tin cry' when bent. It is a relatively common element, its abundance being 2.2 ppm in the earth`s crust. Its principal ore is cassiterite, SnO2 , from which the metal is obtained by reduction. Tin forms a stable oxide coating on its surface which ma…

Tin II Fluoride
White crystalline solid. Symbol SnF₂ Also known as Stannous Fluoride

Tin II Oxide
Blue-black crystalline solid. Symbol SnO Also known as Stannous Oxide

Tin IV Fluoride
White crystalline solid. Symbol SnF₄ Also known as Stannic Fluoride

Tin IV Hydride
Colourless gas. Symbol SnH₄ Also known as Stannane

Tin IV Oxide
White or grey crystalline solid. The fully oxidized state of tin metal. A white insoluble powder, no odour. Also known as Stannic Oxide. Tin oxide has been used primarily as an opacifier in all types of glazes for many centuries. The mineral Cassiterite or Tinstone is Tin IV Oxide and is an important ore of Tin. Symbol SnO₂ Also known …

Tinnitus
Persistent sensation of ringing noises in the ear of which there is no observable cause.Tinnitus aurium refers a subjective sensation of noises in the ears.Objective tinnitus refers to abnormal or pathological sounds originating within the body, in the region of the ear, which are audible to others than the subject.

Titanium
Titanium is a hard, lustrous, silvery metal which is obtained by magnesium or calcium reduction of the tetrachloride. It is a relatively abundant element, there being 5600 ppm in the earth`s crust. It forms a protective oxide coating and, hence, resists corrosion, although powdered metal burns in air. Titanium tends to be inert at low temperatures …

Titanium Dioxide
Predominantly used as a white pigment. Symbol TiO₂

Tithe
A tenth part of something.

Titration
Reacting a solution of unknown concentration with a solution of a known concentration for the purpose of finding out more about the unknown solution. See also: Back Titration, Concentration.

Toe In
AcousticsThe degree to which the inside front edges of a pair of speakers are angled toward each other.AutomotiveIdeally the front wheels would be held in a striaght-ahead position under all driving conditions by the suspension and steering linkages. However, on rear wheel drive vehicles the drag of the front wheels will result in them toeing out. …

Toe Out
Ideally the front wheels would be held in a striaght-ahead position under all driving conditions by the suspension and steering linkages. However, on front wheel drive vehicles the torque applied to the wheels when accelerating or cruising will make them toe in. Therefore, initially setting the front wheels to toe out will mean that when driven the…

Toeplitz Matrix
A matrix in which all the elements are the same along any diagonal that slopes from northwest to southeast. See also: Matrix.

Toggle
A word meaning to reverse or complement the state of something.

Tokamak
The leading magnetic confinement fusion concept named after a Russian word for toroidally (or doughnut) shaped magnetic field. The field the long way around the torus is the toroidal field and is the main confining field for the particles. The toroidal field is produced from a set of poloidally constructed electromagnets. See also: Fusion, Magnetic…

Tolerance
The maximum permissible deviation from the specified quantity, normally expressed as a percentage.

Toluene
Used as a solvent and as an intermediate for its derivatives. Symbol C₇H₈ Also known as Methylbenzene, Toluol

Tombac
A brass alloy 88% copper and 12% zinc.

Tomography
Mathematical techniques for reconstructing three dimensional volumes from the integrated projections along rays. In medical imaging tomography, x-rays are used to determine the x-ray attenuation coefficient inside of the body. This can then be related to anatomy. See also: Time of Flight, Triangulation.

Ton
A derived unit of mass in the British system of units.Conversions1 ton (UK long)=1016.047 kg1 ton (US short)=907.1847 kg1 ton (UK long)=2240 lb1 kg=2.20462 pound (lb)ttle='Mass';xiunt='kg';yiunt='ton';mconv=1.016047e3;cconv=0.0; See also: Mass, Tonne.

Ton Force
1 ton-force = 9964.02 Nttle='Force';xiunt='N';yiunt='ton force';mconv=9964.02;cconv=0.0; See also: Force.

Tone
A sound of definite pitch. A pure tone has a sinusoidal wave form. See also: Pure Tone, Tone Burst.

Tone Burst
A short signal used in acoustical measurements to make possible differentiating desired signals from spurious reflections. See also: Chirp, Tone.

Tonne
A derived SI Unit of mass. Through common usage 1000kg has been named the Tonne.1 Tonne (t) = 1000kg

Tons per Square Inch
Old imperial unit of pressure, abbreviated as Tsi or tonf in^-2.ttle='Pressure';xiunt='Nm^-2';yiunt='tonf in^-2';mconv=15.44e6;cconv=0.0; See also: Pressure.

Top
The flavor of the sixth quark. See also: Bottom, Charm, Quark, Strange.

Top Dead Centre
The position of the crank when the piston is in its farthest position from the crankshaft, in its nearest position to the cylinder head. Abbreviated tdc. See also: Bottom Dead Centre, Crankshaft, Internal Combustion Engine.

Torque
The torque or turning moment acting on a body is defined as the product ofthe magnitude of the force and the perpendicular distance of the line of action ofthe force from the axis about which the body is being rotated.T = FdwhereT = torque NmF = magnitude of the force Nd = perpendicular distance from the line of action of the force to the axis of r…

Torque Steer
A tendency for a car to turn in a particular direction when power is applied. Torque steer is common in front-drive cars because reaction forces created in the half-shafts can generate uneven steering forces in the front tyres. See also: Steering Offset.

torr
A unit of pressure.Conversions1 torr=1 mmHg1 mmHg=133.3224 Pattle='Pressure';xiunt='Pa';yiunt='torr';mconv=133.3224;cconv=0.0; See also: Pressure.

Torsional Vibration
Vibration involving twisting or rotation of a component or structure about one of its axes. In machinery vibration, the term usually refers to twisting of a shaft about its own axis.

Torus
A geometric solid in the shape of a doughnut.

Total Harmonic Distortion
The RMS value of the distortion appearing at multiples (harmonics) of the input frequency to the RMS value of the input (or output) sine wave. Only harmonics within the Nyquist limit are included in the measurement. It is typically expressed in decibels.Example showing 2nd, 3rd, 4th, 5th, 6th and 7th harmonics. In this case the THD is more than 40d…

Total Internal Reflection
When light is traveling from a material with a higher index of refraction into one with a lower index of refraction, if the angle of incidence exceeds the critical angle, the light is totally reflected. See also: Refractive Index.

Touchstone
A touchstone is a device for roughly ascertaining the purity of gold alloys. It consists of a smooth strip of hard black stone, on which a corner of the alloy is rubbed so as to leave a streak, which is then moistened with an acid composed of 78.4 per cent nitric acid, 1.6 per cent hydrochloric acid and 20 per cent water. By comparing the effect wi…

Toughness
Ability of a metal to absorb energy and deform plastically before fracturing. It is usually measured bythe energy absorbed in a notch impact test, but the area under the stress-strain curve in tensile testing is also ameasure of toughness.

Toxicology
The study of poisons, including identification, isolation, biological effects, mechanism of action, and development of antidotes.

Trace
The trace of a matrix is the sum of the terms along the principal diagonal.

Tracking Filter
A low-pass or band-pass filter that tracks the input signal based on some externally supplied signal e.g. engine speed signal. A tracking filter is usually required for aliasing protection when data sampling is controlled externally. See also: Filter.

Trammel
A metal rod having pointed ends, used to mark off a span equal to its length.

Transceiver
A device that both transmits and receives data.

Transcendental
Not algebraic.

Transducer
A device that receives energy in one form and supplies an ouput in another form. Also known as a sensor or detector.

Transfer Function
In a linear elastic structure, the displacement at one point caused by the application of a dynamic force at another point depends on the transfer function between the two points. The transfer function can be obtained at any given frequency by evaluating the response of the second point when a sinusoidal force of unit amplitude is applied at the fi…

Transform
A transform is a mathematical operation that converts a function from one domain to another domain with no loss of information. For example, the Fourier transform converts a function of time into a function of frequency. See also: Fourier Transform, Laplace Transform.

Transformation Temperature Ranges
Those ranges of temperature within which austenite forms during heating and transforms during cooling. The two ranges are distinct, sometimes overlappingbut never coinciding. The limiting temperatures of the ranges depend on the composition of the alloy and on therate of change of temperature, particularly during cooling.

Transformer
A transformer is a device used for converting an alternating electric current from one voltage to another, depending upon electro-magnetic induction. The first transformer was made by Faraday in 1831.whereN_p = number of turns in primary coil.N_s = number of turns in secondary coil.V_p = voltage across primary coil [V]…

Transient Analysis
Analysis of the transient vibration which is generated in response to an input which is only applied for a short duration. Transient vibration may consist of free or forced vibration or both, and may be either deterministic or random. See also: Attack.