# Ra

**Ra**- Radium is a radioactive element of the alkaline earth metals. White in colour and properties close to barium.
**Rabbet**- The portion of a doorframe into which the door fits.
**RAC Horsepower Rating**- An obsolete engine horsepower rating system that was used as a designation as to the power of an engine.
**Racemic Mixture**- A mixture that has equal amounts of left- and right-handed enantiomers of a chiral molecule.
**Racing Line**- An imaginary line around a circuit that provides the quickest lap time.
**Rack and Pinion**- A pair of gears which convert rotational motion into linear motion.
**Rack and Pinion Steering**- The most common steering system on passenger cars.
**Racking**- The distortion of a rectangular shape to a skewed parallelogram.
**Racon**- A transponder for interrogation by a primary radar.
**rad**- An abbreviation of radian the SI unit of plane angle.
**Rad**- The rad is a unit used to measure absorbed radiation dose.
**Radar**- A method, system, or technique of using beamed, reflected, and timed radio waves for detecting, locating, or tracking objects.
**Radar Altimeter**- Airborne radar that measures the distance of the aircraft above the ground.
**Radar Beam**- The space in front of a radar antenna where a target can be effectively detected or tracked.
**Radar Detector**- A detector that, in its simplest form, only needs to be capable of producing an output when RF energy is present at its input.
**Radar Mile**- Time interval, 12.36 microseconds, for RF energy to travel out from a radar to a target and back to the radar.
**Radial**- A direction perpendicular to the centreline of a shaft.
**Radial Engine**- An engine with a number of cylinder arranged in a circle around the crankshaft centre line. A design often used for aircraft engines.
**Radial Force Variation**- Radial Force Variation is a dynamic force generated at the wheel due to tyre non-uniformity.
**Radial Motion**- Motion along a radius, or a component in such a direction.
**Radial Position**- The average location, relative to the radial bearing centreline, of the shaft′s dynamic motion.
**Radial Vibration**- Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centerline.
**Radian**- The radian is the plane angle between two radii of a circle which cut off on the circumference an arc equal in length to the radius.
**Radian per Second**- A common unit of angular velocity.
**Radiancy**- The rate of radiant-energy emission for a unit area of a source in all the radial directions of the overspreading hemisphere.
**Radiant Exposure**- The total energy per unit area incident upon a given surface.
**Radiant Heating**- Heating system in which warm or hot surfaces are used to radiate heat into the space to be conditioned.
**Radiant Intensity**- Energy of radiation striking a unit area per unit time.
**Radiant Panel Test**- Used to measure and compare the surface flammability of materials when exposed to a prescribed level of radiant heat energy.
**Radiated Noise**- The dissemination of sound energy as acoustic waves from a source (e.g. a vibrating panel, running engine etc.).
**Radiation**- This is the electromagnetic radiation emitted by a body as a function of its temperature.
**Radiation Absorbed Dose**- The rad is a unit used to measure absorbed radiation dose.
**Radiation Belt**- An envelope of charged particles trapped in the magnetic field of a spatial body.
**Radiation Field**- The electromagnetic field that radiates from an antenna and travels through space.
**Radiation Intensity**- Another name for Radiant Intensity.
**Radiation Losses**- The losses that occur when magnetic lines of force about a conductor are projected into space as radiation and are not returned to the conductor as the cycle alternates.
**Radiation Pattern**- A plot of the radiated energy from an antenna.
**Radiation Resistance**- The resistance that if inserted in place of the antenna would consume the same amount of power as that radiated by the antenna.
**Radiator**- A device that cools the liquid in the cooling system by allowing it to circulate through a series of water channels, which are exposed to air ducts
**Radiator Fan**- A fan to provide cooling air toward the radiator.
**Radical Axis**- The locus of points of equal power with respect to two circle.
**Radical Centre**- The radical centre of three circles is the common point of interesection of the radical axes of each pair of circles.
**Radii**- Plural of radius.
**Radio**- Communication by electromagnetic waves, without a connecting wire.
**Radio Clock**- A clock that automatically synchronizes to a signal received by radio.
**Radio Direction Finder**- An instrument that intercepts and uses electromagnetic radiation in the radio portion of the spectrum to determine the direction of the radio transmitter.
**Radio Frequency**- Any frequency of electromagnetic energy capable of propagation into space.
**Radio Frequency Choke**- A choke coil with a high impedance at radio frequencies.
**Radio Frequency Heating**- A process for heating plasma by transferring energy to ions or electrons using waves generated by an external oscillator at an appropriate frequency, and propagated into the plasma.
**Radio Frequency Interference**- This is unwanted Electromagnetic Emissions in the radio reception bands.
**Radio Frequency Telemetry**- A telemetry system that transmits data over a radio frequency.
**Radio Horizon**- The boundary beyond the natural horizon in which radio waves cannot be propagated over the earth′s surface.
**Radio Interference**- The interference with radio reception caused by a system.
**Radio Interference Suppression**- The methods employed to minimize radio interference.
**Radio Interferometer**- An interferometer operating at radiofrequencies.
**Radio Waves**- Electromagnetic radiation in the region of the spectrum smaller than about 3 x 10
^{8}Hz and wavelengths greater than about 1 m. **Radioactive Decay**- The spontaneous change of an atomic nucleus accompanied by the emission of alpha, beta particles or gamma radiation.
**Radioactivity**- Spontaneous emission of particles or high-energy electromagnetic radiation from the nuclei of unstable atoms.
**Radioisotope**- A radioactive isotope.
**Radiometer**- Instrument for detecting and measuring radiant energy.
**Radiometry**- A branch of science which deals with the measurement of radiation.
**Radiosity Equation**- Used in the construction of realistically lit scenes in computer graphics.
**Radiosonde**- A small, expendable, balloon-borne instrument package with a radio transmitter used to measure the air temperature, pressure and humidity of the atmosphere above the earth′s surface.
**Radium**- A radioactive element of the alkaline earth metals. White in colour and properties close to barium.
**Radius**- In a circle, the radius is the distance from the centre of the circle to a point on the circle.
**Radius Gauge**- Any one of a number of small flat, standard-shaped metal leafs or blades used for checking the accuracy of regular concave and convex surfaces.
**Radix**- The number of distinct symbols used in a number system.
**Radix Point**- The generalization of decimal point to bases of numeration other than base 10.
**Radome**- Dielectric housing for an antenna.
**Radon**- A radioactive element and the heaviest of the nodle gases.
**Rafter**- A framing member that runs up and down the slope of a pitched roof.
**Ragone Plot**- A chart used for performance comparison of various energy storing devices.
**Rain Bucket**- Device for measuring rain fall.
**Rain Gauge**- Instrument used to measure the total amount of precipitation that falls upon a given unit area during a specified time interval.
**Rainflow Analysis**- A method of representing a random signal by various numbers of sinusoids that would produce the same amount of fatigue damage.
**RAM**- Abbreviation of Random Access Memory, a data storage device for which the order of access to different locations does not affect the speed of access, except for bursts.
**Ram Air**- Air entering an air inlet as a result of the high-speed forward movement.
**Ramjet**- A jet engine with no mechanical compressor, consisting of specially shaped tubes or ducts open at both ends.
**Ramp Voltage**- A steadily rising voltage.
**Ram-Rocket**- A rocket engine in which an air-breathing inlet and duct system are added, permitting atmospheric air to be introduced at the exit of the rocket combustion chamber.
**Ramsay, William**- Discovered Argon, Krypton and Xenon and independently discovered Helium on earth.
**Random Access Memory**- A data storage device for which the order of access to different locations does not affect the speed of access, except for bursts.
**Random Copolymer**- A polymer in which two different mer units are randomly distributed along the molecular chain.
**Random Errors**- Errors that remain after blunders and systematic errors have been removed.
**Random Experiment**- An experiment or trial whose outcome is not perfectly predictable, but for which the long-run relative frequency of outcomes of different types in repeated trials is predictable.
**Random Noise**- A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform spectral distribution of energy.
**Random Number Generators**- A device used to produce a selection of numbers in a fair manner, in no particular order and with no favour being given to any numbers.
**Random Process**- A random process is a collection of time-history records that can be described by appropriate statistical parameters, such as averaged properties of these records at a number of fixed times.
**Random Sampling**- A sampling method in which all elements in the population have an equal chance of being selected.
**Random Vibration**- A vibration whose instantaneous magnitude is not specified for any given instant of time.
**Random Wound Stator**- The stator windings are wound onto a bobbin without a defined structure to the windings.
**Randomised Controlled Experiment**- A controlled experiment in which the assignment of subjects to the treatment group or control group is done at random, eg by drawing straws.
**Range**- The maximum and minimum allowable full-scale signal.
**Range Extender**- A device that extends the range of an electric vehicle by converting an energy dense fuel into electricity.
**Range of Linear Transformation**- The range of a linear transformation T is the set of all vectors T(v), where v is any vector in its domain.
**Rankine Cycle**- A mathematical model that is used to predict the performance of steam engines. The Rankine cycle is an idealised thermodynamic cycle of a heat engine that converts heat into mechanical work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid.
**Rankine Temperature Scale**- An absolute scale of temperature based on degrees Farenheit.
**Rankine Vortex**- An idealized vortex in unbounded fluid with uniform vorticity inside a circular patch and zero vorticity outside.
**Rankine-Hugoniot Shock Relations**- These are solutions for a steady, normal shock in the frame moving with the shock front.
**Raoult′s Law**- The vapour pressure of a solvent in an ideal solution equals the mole fraction of the solvent times the vapour pressure of the pure solvent.
**Rapid Solidification Technology**- Process in which molten metals such as titanium and aluminum are transformed into a very fine powder, which is then solidified.
**Rapid Speech Transmission Index**- Abbreviated as RASTI and expressed in a decimal range of 0.2 for "bad" to 1.00 for "Excellent".
**Rare Earth**- An oxide of a rare earth element.
**Rare Earth Element**- A metallic element that belongs to Group 3B or to the lanthanide series.
**Rarefaction**- A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
**Rarefied Wave**- A longitudinal wave that has been expanded or rarefied as it moves away from the source.
**Rasp**- A sort of blowing noise emitted often from the exhaust pipes of internal combustion engines, usually, it is dominated by a mixture of 4E, 6E and 8E engine orders on 4 cylinder engines.
**RASTI**- Rapid Speech Transmission Index, expressed in a decimal range of 0.2 for "bad" to 1.00 for "Excellent".
**Ratchet**- A mechanism that only allows rotary motion in one direction.
**Ratchet Brace**- A brace that includes a ratchet for ease of use in tight spaces, and also to use just the most powerful or easiest arc of turn.
**Rate Gyroscope**- A gyroscope used to detect and measure angular rates of change.
**Rate Law**- A rate law or rate equation relates reaction rate with the concentrations of reactants, catalysts, and inhibitors.
**Rate of Change**- The speed at which a function is changing.
**Rate of Reaction**- The rate at which the concentration of a product increases or the concentration of a reactant decreases with time.
**Rate of Strain Hardening**- Alternative name for Modulus of Strain Hardening.
**Rated Flow**- The maximum flow that the power supply system is capable of maintaining at a specific operating pressure.
**Rated Pressure**- The qualified operating pressure which is recommended for a component or a system by the manufacturer.
**Ratio**- The quotient of two numbers.
**Ratio Detector**- A detector that uses a double-tuned transformer to convert the instantaneous frequency variations of the FM input signal to instantaneous amplitude variations.
**Ratio of Specific Heats**- The ratio of molar heat capacity at constant pressure to molar heat capacity at constant volume.
**Rational B-Spline**- equation that permits the modeling of free-form curves between two endpoints as a function of a set of intermediate points whose values influence the shape of the curve.
**Rational Function**- Any function which can be defined by a rational fraction, i.e. an algebraic fraction such that both the numerator and the denominator are polynomials.
**Rational Number**- Any number that can be expressed as the ratio of two integers.
**RATO**- Utilising solid or liquid fuel rockets to augment the take-off.
**Rattle**- A sound such as that made when shaking a tin full of coins.
**Rawlplug**- The inventer of the original wall plug was John Rawlings and he marketed the Rawlplug.
**Ray**- At higher audio frequencies, sound may be considered to travel in straight lines, in a direction normal to the wave front.
**Ray Tracing**- The calculation of the propagation, reflection and refraction of rays from source to receiver. This technique is used extensively within the optics and acoustics industry.
**Rayl**- The Rayl is the SI derived unit of specific acoustic impedance.
**Rayleigh Criterion**- A criteria that gives an approximation for how finely a set of optics may be able to distinguish.
**Rayleigh Distribution**- An example is the variation of wave height in a sea where swell is the main component.
**Rayleigh Scattering**- Scattering of radiation in the course of its passage through a medium containing particles, the sizes of which are small compared with the wavelength of the radiation.
**Rayleigh Waves**- These are also known as surface waves.
**Rayleigh-Jeans Law**- A law describing blackbody radiation.
**Rayleigh′s Method**- A method used for calculating approximate natural frequencies for a vibrating system assuming a deflected shape and balancing kinetic and strain energies.