www.PMEL.org       Glossary of Terms


A broad term covering several types of image defects in a lens or lens system.
absolute pressure
Actual pressure on a confined gas, irrespective of the atmosphere on the outside. Absolute pressure = gage pressure + atmospheric pressure.
absolute system
A system of units in which a small number of units is chosen as fundamental and all other units are derived from this group.
absolute temperature
Temperature measured from absolute zero as in the Kelvin and Rankine scales.
absolute zero
(1) This is the temperature at which the volume of an ideal gas would become zero. The value calculated from the limited value of the coefficient of expansion of various real gases is -273.15 °C.  (2) The temperature at which all thermal (molecular) motion ceases; zero point in absolute temperature scale equal to -273.15 °C or -459.67 °F, 0 Kelvin and 0 °Rankine.
(1) The loss of energy in traveling through a medium.  ie: A yellow filter absorbs all wavelengths except yellow just as red paint will absorb all colors except red which is reflected.  (2) The internal taking up of one material by another.  (3) Transformation of radiant energy into other forms of energy when passing through a material substance.
(1) A rate of change in velocity per unit time. Positive acceleration means an increase in velocity while negative acceleration means a decrease in velocity per unit time. Avoid the use of the term "deceleration."   (2) The time rate of change of velocity in either magnitude or direction. Cgs Unit: cm/sec .
acceleration due to gravity (g)
The acceleration of a freely falling body in a vacuum, 980.665 cm/sec or 32.174 ft/sec at sea level and 45° latitude.
Changes in focus of the crystalline lens to adjust the eye for various object distances.
The term accuracy refers to how close we are to the nominal value. In the past we have used this term to indicate error in a measurement device. For instance, the accuracy of a standard cell is plus or minus 0.01 percent. Use of the word accuracy in this sense is incorrect because what we mean is the inaccuracy or error is plus or minus 0.01 percent. However, this is still a common method of describing accuracies. To remedy this practice, the National Bureau of Standards has dropped the term accuracy, when used in this respect, and uses instead the term "uncertainty."
A lens doublet, to two lenses combined to eliminate chromatic aberration.
Free from hue
activation energy
The energy necessary to start a particular reaction.
actual value (true value)
It is not possible to determine a completely true value of a quantity as there is always some error in every measurement. Theoretically we could say the "true" value of a measured quantity can be derived by taking the average of an infinite number of measurements assuming that the conditions contributing to deviations act is a completely free and random manner.
Visual acuity is the resolving power of the eye, normally taken as 1 minute arc. Vernier acuity is the ability of the eye to make coincidence settings.
The adhesion of one substance to the surface of another.
A continuation of arithmetic in which letters and symbols are used to represent definite quantities whose actual values may or may not be known.
Step-by-step procedure for the solution to a problem. First the problem is stated and the algorithm is devised for its solution.
alignment   telescope
A telescope specifically designed to be mounted and used in conjunction with an end target in order to form a fixed line of sight. Can also be used to measure linear displacement (alignment of a rail for straightness) by using the optical micrometers.
A mixture of two or more metals, such as brass (zinc and copper), bronze (copper and tin), and manganin (nickel, manganese, and copper).
An alloy consisting chiefly of aluminum, nickel, and cobalt. It has high retentivity and is used to make powerful small-size permanent magnets which hold their magnetism indefinitely.
The current amplification factor when connected in a common base configuration.
alpha particle
(1) Particle identical with a helium nucleus emitted from the nucleus of a radioactive atom.  (2) A helium nucleus, consisting of two protons and two neutrons, with a double positive charge. Its mass is 4.002764 a mu (mass units).
alpha ray
A stream of fast-moving helium nuclei; a strongly ionizing and weakly penetrating radiation.
Set of all alphabetic and numeric characters.
One half of a complete cycle, consisting of a complete rise and fall of voltage or current in one direction. There are 120 alternations per second in 60 Hz alternating current.
An aircraft instrument that indicates the elevation in respect to a reference. The aneroid altimeter is referenced to sea level, while an electronic altimeter uses the radar method. See barometer.
ambient temperature
The temperature of the air in the immediate vicinity.
The quality of having more than one meaning.
amici prism
direct vision prism, beam of light is dispersed into a spectrum without mean deviation.
An instrument used for measuring the amount of current in amperes. A meter that indicates the current value in milli-amperes is a milli-ammeter, and one that indicates values in micro-amperes is a micro-ammeter.
The extent of a vibrator movement measured from the mean position to an extreme.
As a related to detection instruments, the process (either gas, electronic, or both) by which ionization effects are magnified to a degree suitable for their measurement.
angle of incidence
The angle formed by the line of an incident ray and a perpendicular line arising from the point of incidence.
angle of lag
The angle with which one alternating electrical quantity lags behind another quantity in time,
angle of reflection
The angle formed by the line of a reflected ray and a perpendicular line arising from the point of incidence.
angle of refraction
The angle formed between the line of a refracted ray and a perpendicular line drawn through the point of refraction.
angular acceleration
The time rate of change of angular velocity either in angular speed or in the direction of the axis of rotation (precession). Cgs unit, 1 radian/sec/sec.
angular velocity
(1) The speed of a rotating object measured in radians per second and generally designated by the lower case Greek letter omega. In the case of a periodic quantity, such as alternating current, the angular velocity is equal to a 2 p f.  (2) The time rate of angular motion about an axis. Cgs unit: radians/sec. If the angle described in time t is q , the angular velocity , w = q / t , q in radians and t in seconds gives w in radians per second.
angstrom unit
10 cm, a convenient unit for measuring wavelength of light. Abbreviation: A.
Number from which the log was derived. Obtained as a result of using the inverse procedure of obtaining a log. It is often written as "antilog."
An opening or gap. In optics, the effective aperture is the portion of an objective lens that is actually used.
aplantic lens
A lens that is corrected for spherical, coma, and chromatic aberrations.
apparent power
The power value obtained in an alternating current circuit by multiplying the effective values of voltage and current. The result is expressed in volt-amperes, and must be multiplied by the power factor to secure the average or true power in watts.
The point at which an orbiting body is the greatest or least distance from the center of attraction. The greatest distance is called the higher apses and the least distance is called the lower apses.
A portion of the circumference of a circle.
Archimedes' principle
When a body is placed in a fluid, it is buoyed up by a force equal to the weight of the displaced fluid.
(1) A visual aberration caused by lack of sphericity of the cornea.  (2) A blurring of the trace of an oscilloscope.
Smallest particle of an element that can enter into combination with other elements.
atomic number
(1) The number of protons in the nucleus, hence the number of positive charges on the nucleus.  (2) The number of protons in the nucleus, hence the number of positive charges on the nucleus. It is also the number of electrons outside the nucleus of a neutral atom. Symbol: Z.
atomic weight
(1) The relative weight of the atom of an element based on an atomic weight of 16 for the oxygen atom as the usual chemical standard. The sum of protons plus neutrons is the approximate atomic weight of an atom.  (2) The weight of an atom according to a scale of atomic weight units, awu, valued as one-twelth of the carbon atom (C12 = 12.00000). Thus is identical to the mass number.
attached method(optics)
A method of measuring when all test equipment and standards are physically located on the same reference plane.
A process in which collimated rays of light emanating from an instrument, and carrying the image of a reticule, are aimed at a reflective surface. The reticule image is reflected back into the focal place of the telescope for comparison with the actual reticule as a measure of relative tilt, between the optical axis and the reflective surface. An instrument used for this purpose is called an Autocollimator.
A telescope provided with both an eyepiece and a focal plane target. The basic operating principle is to align a plane with a reflective capability at right angles to the autocollimator axis and/or measure the out-of-squareness of a plane to the autocollimator axis.
A process in which the reflected image of a target surrounding the front end of a telescope is compared with the telescope reticle as a measure of relative tilt. (The focal length is twice the dimension from the instrument to reflective surface.)
autumn equinox
First day of autumn in the northern hemisphere. It usually falls on September 21st in the northern hemisphere. There are about 12 hours of light and 12 hours of darkness every place on the Earth during an equinox.
average value
(1) The value obtained by dividing the sum of a number of quantities by the number of quantities represented.  (2) The average of many instantaneous amplitude values taken at equal intervals of time during an alternation (half-cycle). The average value of an alternation of a pure sine wave is 0.637 times its maximum or peak amplitude value.

Avogadro's law (principle)
The hypothesis that equal volumes of all gases at the same pressure and temperature contain equal numbers of molecules. Hence the number of molecules contained in 1 cm3 of any gas under standard conditions is a universal constant.

Avogadro's number
The number of molecules in a gram-molecular weight of any chemical substance (6.0221367 x 1023 molecules in a gram mole); also, the number of atoms in a gram-atomic weight of any element.

A straight line, real or imaginary, passing through a body, on which the body revolves.
axis, optical
A line formed by the coinciding principal axis of a series of optical elements.
axis, principal
A line through the centers of curvature of a refracting lens.
Horizontal direction or bearing of one object with respect to another, expressed as an angle measured in a horizontal plane and in a clockwise direction from the north (true north, unless otherwise indicated).
B+ (B plus)
The positive terminal of a B battery or other plate-voltage source for a vacuum tube, or the plate-circuit terminal to which the positive source terminal should be connected.
B- (B minus)
Symbol used to designate the point in a circuit to which the negative terminal of the plate supply is to be connected.
B-H curve
A characteristic curve showing the relation between magnetic induction (B) and magnetizing force (H) for a magnetic material. It shows the manner in which the permeability of a material varies with flux density. Also called "magnetization curve."
A form of mechanical hysterysis (lag) in which there is a lag between the application of a driving force and the response of the driven object.
The unit expressing the probability of a specific nuclear reaction taking place in terms of cross-sectional area. It is 10-24 cm2 . (See Cross Section.)
An instrument for measuring atmospheric pressure. There is a direct relationship between atmospheric pressure and altitude and many barometers are equipped with an altitude scale. Two types of barometers are "mercury" and "aneroid." The aneroid barometer with an altitude scale is an altimeter.
A beam of light can be regarded as the path traced by a small section of an advancing wave front, which is comprised of an infinite number of light rays.
Bernoulli's principle
With a fluid in motion, if the velocity is low, the pressure is high and vice versa.
The current amplification factor of a transistor when connected in a common-emitter configuration.
beta particle
(1) Particle identical to an electron emitted from the nucleus of a radioactive atom.  (2) A charged particle emitted from the nucleus and having a mass and charge equal in magnitude to those of the electron.
beta ray
A stream of beta particles, more penetrating but less ionizing than alpha rays; a stream of high-speed electrons.
bifilar winding
A method of winding transformers in which the wires are placed side by side, and wound together.
Having, or arranged upon, two sides.
binding energy
The energy represented by the difference in mass between the sum of the component parts and the actual mass of the nucleus.
bimetallic element
Two strips of dissimilar metal bonded together so that a change in temperature will be reflected in the bending of the element, as a result of differential expansion. Used in thermostats, dial thermometers, and temperature compensating devices in the better pressure gages.
Rapid vaporization which disturbs a liquid, and which occurs when the vapor pressure within a liquid is equal to the pressure on its surface.
bonded strain gage
A thin metallic resistance element, usually of wire or foil, chemically cemented to a device being subject to loading or stress. As the load (stress) changes, the electrical resistance of the strain gage changes. Thus, for a fixed value of applied voltage, the output voltage from the strain gage varies in proportion to the strain and provides an indication proportional to the load causing the stress and resultant strain.
bourdon element
A curved, hollow tube sealed at one end. When fluid under pressure is forced in the tube it has a tendency to straighten out. With a pointer attached to the sealed end and allowed to move across a scale it becomes a bourdon gage.
Boyle's Law
If the temperature of a gas is kept constant, then the volume of the gas will be inversely proportional to the pressure.
bridge circuit
An electrical network that is basically composed of four branches connected in the form of a square. One pair of diagonally opposite junctions is connected to the input, and the other pair is connected to the output circuit which contains an indicating device.
bridge rectifier
A full-wave rectifier with four elements connected as in a bridge circuit. Alternating voltage is applied to one pair of junctions.
British Thermal Unit (BTU)
The amount of heat that will raise the temperature of 1 pound of water 1 °F at maximum density (39.1 °F).
To place an instrument so that its line of sight passes through two given points or fulfills two requirements simultaneously. Usually the first operation in setting up control is to establish a width plane.
The power to float or rise in a fluid.
buoyant force
The upward force which any fluid exerts on a body placed in it.
To determine by measurement or comparison the correct value of each scale reading on a meter or other device being calibrated. To determine the settings of a control that corresponds to particular values of voltage, current, frequency, or some other characteristic.
The amount of heat required to raise the temperature of 1 gram of water 1 °C at 15 °C.
Unit of luminous intensity. It is of such a value that the luminous intensity of a full radiator at the freezing temperature of platinum (1773 °C) is 60 candela per centimeter squared. Candela was formerly termed candlepower, or simply candle.
The characteristic of a liquid to be raised or depressed in a tube or small bore. This action is caused by a combination of cohesive, adhesive, and surface tension forces.
Of the sky or the heavens. A celestial telescope is one in which the image appears inverted, as in astronomical telescopes with no erector.
Celsius temperature scale
A temperature scale based on mercury in glass thermometer with the freezing point of water defined at 0 °C and the boiling point of water defined at 100 °C, both under conditions of normal atmospheric pressure.
center of instrument
In optics, the intersect point of the vertical, horizontal, and optical axis of a transit or similar instrument when perfectly calibrated.
To attest a being true or as represented, or to meet a certain standard.
centripetal force
The force required to keep moving mass traveling in a circular path. The force is directed toward the axis of the circular path.
Cgs system
The common metric system of units (centimeter-gram-second).
chain reaction
Any chemical or nuclear process in which some of the products of the process are instrumental in the continuation of magnification of the process.
Charles Law
The volume of a gas is directly proportional to its absolute temperature, providing the pressure is constant.
chemical compound
A pure substance composed of two or more elements combined in a fixed and definite proportion by weight.
chromatic aberration
A property of lenses that causes the various colors in a beam of light to be focused at various points, this causing a spectrum to appear.
The clinometer is, in principle, a level mounted on a rotatable member, whose angle of inclination relative to its base can be measured by a circular drum scale.
coefficient of linear expansion
The change in unit length in a solid when its temperature is changed 1 °.
coefficient of volume expansion
The change in unit volume of a solid when its temperature is changed 1 °.
The force that causes molecules which are brought close together, as in liquids and solids, to stick together. This force is especially strong in solids when the distance between molecules is very small.
Exact correspondence; aligning two lines; placing one beside the other. In optics, a coincidence bubble is equipped with a prismatic or mirror arrangement for simultaneously viewing both ends of the bubble for more precise adjustment.
To render parallel.
The process of making light rays parallel. Also; The process of aligning the optical axis of optical systems to the reference mechanical axes or surfaces of an instrument, or the adjustment of two or more optical axes with respect to each other.
An instrument designed to produce collimated (parallel) rays of light, usually equipped with displacement and tilt graticules.
Lying on or passing through the same straight line
complex number
The expression resulting when a real number is united with an imaginary number by a plus or minus sign.
complex vibration
The combination of two or more sinusoidal vibrations existing simultaneously.
Two or more substances combined in definite proportions by weight and united chemically.
A lens that is thicker at the ends than the middle. A concave lens diverges (spreads) rays of light.
Having a common center, as circles or spheres one within another.
The change of state from a gas or vapor to a liquid.
conservation of energy
The principle that energy can neither be created nor destroyed, and therefore the total amount of energy in the universe is constant. This law of classical physics is modified for certain nuclear reactions. (See Conservation-of-Mass-Energy.)
conservation of mass-energy
The principle that energy and mass are interchangeable in accordance with the equation E = mc2 ; where E is energy, m is mass, and c is velocity of light.
The correction is the value in proportional parts, that must be algebraically added to the nominal value to obtain the certified value. The correction is equal in absolute magnitude but opposite in sign to the error. Correction is what must be done to the nominal to reach the actual.
cosmic rays
Rays of higher frequency than radioactive gamma rays; highly penetrating, of unknown origin, traversing interplanetary space.
Unit of quantity of electricity. The quantity of electricity transported in 1 second by a current of 1 ampere, or a movement of 6.28 X 1018 electrons past a given point in 1 second.
Coulomb's law of electrostatic charges
The force of attraction or repulsion exerted between two electrostatic charges, q and q', a distance r in a vacuum, the force between them is: 
Tend to meet at a point.
convex lens
A lens that is thicker in the middle than the ends. A convex lens converges rays of light.
The long term change in dimensional characteristics of a body under load, in an elastic force measurement device. This term refers to the change in reading which occurs when a constant load is applied for a period of time.
critical angle
The angle between and at which there is neither refraction or internal reflection.
critical size
For fissionable material, the minimum amount of a material which will support a chain reaction.
cross section (Nuclear)
The area subtended by an atom or molecule for the probability of a reaction; that is, the reaction probability measured in units of area.
The science of refrigeration pertaining to the methods for producing and measuring very low temperatures.
(1) The complete sequence of instantaneous values of a periodic event that occurs during one period.  (2) In electricity, one complete positive alternation and one complete negative alternation of an alternating current.  (3) 1 cycle expressed in degrees = 360° or expressed in radians = 2 p radians.
(1) The prevention of free swinging or vibration by some means, usually friction or resistance.  (2) The dissipation of energy with motion or time.
decade box
In measurement work, a special device containing two or more sections. Each section is divided into 10 equal parts and has a value of 10 times the value of the preceding section. Switching arrangements permit selection of any desired value in its range.
The disintegration of the nucleus of an unstable element by the spontaneous emission of charged particles and/or photons.
decay time
The time required for the trailing edge of a pulse to decrease from 90 percent to 10 percent of its maximum amplitude. Also referred to as fall time.
deionization potential
The potential at which the ionization of the gas within a gas-filled tube ceases and conduction stops.
The mass per unit volume. Cgs unit: gm/cm .
detached method
A very flexible method of optical tooling. The instruments are mounted on stands or on optical tooling bars which are free of the actual work.
A heavy isotope of hydrogen having 1 proton and 1 neutron in the nucleus. Symbol: D or d.
The nucleus of a deuterium atom or the ion of deuterium. Its structure contains one proton and one neutron.
dew point
The temperature at which the water vapor in the air begins to condense. At this temperature the relative humidity is 100 percent.
dial indicator
This is a mechanical lever system used for amplifying small displacements and measuring it be means of a pointer which transverses a graduated dial.
differential voltmeter
A voltmeter that operates on the potentiometric principle. The unknown voltage is compared to an adjustable calibrated voltage developed within the differential voltmeter.
differentiating circuit
A circuit in which the output voltage is proportional to the rate of change of the input voltage. In an RC circuit the output is taken across the resistor, and in an RL circuit it is taken across the inductor.
The bending of waves, light, sound, or radio, as they pass an obstruction or pass through a small aperture.
(1) The penetration of one type of particle into a mass consisting of a second type of particle.  (2) To spread out in all directions.
Sign or symbol used to convey a specific quantity of information either by itself or with other numbers of its set; 2, 3, 4, and 5 are digits. The base or radix must be specified and each digit's value assigned.
digital voltmeter
An automatic electronic measuring instrument which displays its measurements directly in the decimal system. It is an automatic potentiometric measurement.
dimensional analysis
A process whereby the metrologist separates a quantity into its constituent parts to facilitate the solution to a problem.
The unit of lens power, is usually denoted by D and is the power of a lens of 1 meter focal length.
(1) The amount of change in position from a reference.  (2) Misalignment from a line of sight, usually measured vertically and horizontally.
displacement graticule
A graduated reticle used in Collimators measuring vertical and horizontal displacement. Generally in terms of linear displacement.
Any deviation from the desired waveform.
To spread out, as in the effect of a concave or negative lens. Diverges away from the focal point.
A prism which inverts the image without displacement. Also called a rotating prism.
That unit of force which, when acting upon a mass of 1 gm, will produce an acceleration of 1 cm/sec/sec.
Edison effect
The emission of electrons from hot bodies. The rate of emission increases rapidly with temperature. Also known as thermionic emission.
effective mass
The mass of a body which is being acted upon by the buoyant forces of air. The effective mass of a weight is its true mass minus the buoyant force of air displaced by the weight.
effective value (RMS)
The alternating current value that will produce the same amount of heat in a resistance as the corresponding direct current value. All alternating current meters, unless otherwise marked, indicate effective values of voltage or current. The effective value is also called RMS (root-mean-square) value.
The ratio of useful output energy to input energy, usually expressed as a percentage. A perfect electrical device would have an efficiency of 100 percent.
The property of material to return to its original shape after stress is removed.
elastic limit
The maximum unit stress which can be obtained in a structural material without causing permanent deformation.
E Layer
An ionized layer in the E region of the ionosphere. This layer occurs during daylight hours; its ionization depends on the angles of the sun.
electric field intensity
The magnitude of the intensity of an electric field at a particular point, equal to the force which would be exerted upon a unit positive charge placed in the field at that point. The direction of the electric field is the direction of this force.
(1) A subatomic particle possessing a unit negative charge.  (2) A negatively charge particle which is a constituent of every atom. A unit of negative electricity equal to 4.80 x 10-10 esu. Its mass is 0.00548 mu.
That branch of physics which relates to the emission behavior and effects of electron conduction through a vacuum, gaseous media or semiconductors.
electronic switch
An electronic circuit designed to cause a start and stop action or a switching action.
electrostatic field
The region surrounding an electric charge in which another electric charge experiences a force.
electrostatic unit of charge (Statcoulomb)
That quantity of electric charge which, when placed in a vacuum 1 cm distant from an equal and like charge, will repel it with a force of 1 dyne. Abbreviation: esu.
(1) In chemistry, one of the 100-odd primary substances that cannot be divided into simpler substances by chemical means.  (2) A pure substance consisting of atoms of the same atomic number, which cannot be subdivided by ordinary chemical means.
The vertical distance above a reference level, usually sea level, to a point or object on the surface of the Earth, as distinguished from altitude, which refers to points above the Earth's surface.
Based on actual measurement, observation, or experience without regard to science and theory.
endoergic reaction
A reaction which absorbs energy.
Capacity for performing work. Energy due to the motion of a piece of matter is called kinetic energy. Energy due to the position of a piece of matter is called potential energy.
a condition in which all forces processes or tendencies present are exactly counterbalanced by equal and opposite forces, processes, or tendencies.
Not inverted, the normal position.
erector lens
Additional optics fitted to the eyepiece lens system enabling the image to be viewed in the normal (erect) position.
The unit of work done by a force of 1 dyne acting through a distance of 1 cm. The unit of energy which can exert a force of 1 dyne through a distance of 1 cm. Cgs units: dyne-cm, or gm-cm2/sec2.
The error is the difference between an observed value or calculated value and the true or actual value.
The change of state from a liquid to a gas.
exoergic reaction
The reaction which liberates energy.
Power of ten by which a number is multiplied, used in floating point representation. For example, the exponent in the decimal number 0.9873 X 107 is 7.
Pertaining to varying exponents or to an expression having varying exponents. Any constant base affected with an exponent is exponential.
An essential component of a telescope which receives a real image in its focal plane and forms a magnified virtual image.
Fahrenheit scale
A thermometric scale on which the freezing point of water is 32 ° and boiling point 212 °, at 760 mm Hg (standard pressure).
field of view
Expressed as an angle and representing the arc through which observations are possible through a telescope. The field angle is controlled by the aperture of the eye lens and decreases as magnification increases.
Also known as; cross hair, reticle. In optics a superimposed reference line. For two parallel lines called; bifilar  See also; reticle .
fission products
The elements and/or particles produced by fission.
fixed point
The point where all heat energy applies or removed is used to change the state of a substance.
(1) A material used to promote fusion or joining of metals in soldering, welding, or smelting. Rosin is widely used as a flux in electric soldering.  (2) A general term used to designate collectively all the electric or magnetic lines of force in a region.
focal length
The distance from the optical center of a lens to the point where light rays converge.
focal plane
A plane that is perpendicular to the optical axis at the focal point. All light coming from infinity will focus somewhere on the focal plane.
focal point
The point at which light rays converge after passing through a convex (positive) lens.
Correct adjustment of a lens to produce a clear image.
A push or pull. That which produces or prevents motion or has a tendency to do so.
force measurement device
Refers to any device by which a quantitative determination of an applied force can be made.
forced vibration
Motion caused by some mechanical excitation.
The amount of illumination which a standard source of 1 candle (candlepower) will throw upon a surface placed 1 foot away and at right angles to the rays of light.
free vibration
Vibration that occurs without forcing, as after a tuning fork is struck.
The number of recurrences of a periodic phenomenon in a unit of time. In specifying electrical frequency, the unit of time is the second.
frequency meter
An instrument for measuring the frequency of an AC signal.
fundamental mode of vibration
The lowest natural frequency.
fusion (heat)
The change of state from a solid to a liquid.
An instrument for measuring or testing; a device for determining whether specific dimensions are within specified limits.
gage block
A block of alloy steel, usually rectangular, with two gaging surfaces. The standard length as nominally represented on the side is in inches between the two gaging surfaces with an uncertainty in the neighborhood of 6 microinches.
Galilean telescope
Devised and constructed by Gailieo in 1609. The device consists of a positive objective lens and a negative eyepiece with their focal points in coincidence. The system is suitable for two or three power magnification and produces an erect image.
A D'Arsonval laboratory instrument usually of the suspension type capable of measuring very small electrical currents. It is usually used to indicate a null. Since the galvanometer is used in this application, to indicate whether or not a current is present, and not necessarily the actual magnitude of the current, the primary requirement of the galvanometer is to show a readable deflection for the smallest current that is significant for a particular measurement.
gamma ray
Radiant energy of extremely short wavelength emitted spontaneously by a radioactive substance.
The state of matter that has no definite shape or volume. The molecules of a gas have almost no cohesive forces, hence the expansion of a gas in free space is almost unlimited.
Unit of magnetic induction (also called magnetic flux). One gauss represents one line of flux per square centimeter.
Study of the properties, measurement, and relations between lines, angles, surfaces, and solids.
Go and No-go gages
These are gages that do not measure actual size but merely determine whether parts are within specified limits.
A measure of mass in the English gravitational system equal to one seven-thousandth (1/7000th) pound.
Metric unit of mass or weight. One pound is equal to 453.59 grams.
gram-atomic weight
The relative atomic weight of an element, expressed in grams.
gram-molecular weight (Gram-Mole)
The relative molecular weight of a compound, expressed in grams.
A pictorial presentation of the relation between two or more variable quantities.
A scale on a transparent material in the focal plane of an optical instrument for the location and measurement of objects.
Any two bodies in the universe attract each other with a force that is directly proportionate to the product of their mass and inversely proportionate to the square of their distance apart.
gravitational acceleration
The acceleration due to the force of gravity.
gravitational units or "G" units
The usual way of expressing acceleration intensity, in terms of gravitational constant, is equal to the acceleration in inches/sec/sec divided by 386.087 inches/sec/sec.
gross error
A gross error is simply a mistake.
A reference point in an electrical circuit which is usually a connection between an electrical circuit and the Earth or some conducting body serving in place of the Earth.
group velocity
The axial velocity at which a signal travels through a waveguide. Group velocity is always less than the velocity of a signal in open air.
half life
The length of time during which half of a given number of atoms of a radioactive element will disintegrate.
half thickness
The thickness of absorbing material necessary to reduce the intensity of radiation by one-half.
The internal resistance of an object to having its molecules forced further apart or closer together.
A sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency. Thus, a component whose frequency is twice the fundamental frequency is called the second harmonic.
The energy of molecular motion measured in terms of the effect on some material substance.
heat of fusion
The amount of heat needed to melt a unit mass or weight of a substance at its normal melting point.
heat of vaporization
Heat required to vaporize a unit mass or weight of a liquid at its normal boiling point.
heavy water
The popular name for water which is composed of 2 atoms of deuterium and 1 atom of oxygen.
A unit of frequency equal to 1 cycle per second.
Hooke's Law
Within the limits of perfect elasticity, stress is directly proportional to strain.
Refers to a tendency of a mechanical system to oscillate about a normal condition, or about the point of alignment.
See relative humidity.
hydrogen atom
The atom of lightest mass and simplest atomic and nuclear structure, consisting of 1 proton with 1 orbital electron. Its mass is 1.008123 mu.
An instrument used to determine the specific gravity of liquids.
The study of liquids in motion.
The study of liquids at rest.
Any of several instruments for measuring the humidity of the atmosphere.
Readily absorbing and retaining moisture, often reflecting this absorption by changing physical appearance and shape.
(1) The word hysteresis means "lag." One example is the lagging of the magnetic flux, in a magnetic material, behind the magnetizing force which is producing it. Another example is the lag of a standard cell in returning to its initial voltage following a change in temperature.  (2) In force measurement, hysteresis may refer to the difference in indication for two identical loads, one obtained by reducing from a larger load and the other built up from a lesser value.
ice point
0.00 °C,  0.01 °C below the triple point of water.
To supply or brighten with light.
(1) A virtual image is the impression of an object as viewed by an observer. Rays do not pass through, but only appear to come from the image.  (2) A real image is one through which rays actually pass and can be projected onto a screen.
incident ray
A ray of light entering into a lens or mirror.
Refers to a difference between the slope of the line or place in question and some other reference line or plane.
Adding the value one to the contents of a register or memory location.
index of refraction
The ratio of the speed of light in a vacuum to its speed in a given substance.
That property of mass which resists a change in motion.
Subject to no limitation or external determination, extending indefinitely.
infinite line
A transmission line having characteristics corresponding to those which would be obtained with an ordinary line that is infinitely long.
infinity (optical)
An infinite distance from which collimated or parallel light rays are assumed to emanate (approximately 2000 yards).
Setting a system to a known state.
An undesired change over a period of time, which change is unrelated to input, operating conditions, or load.
intensity of radiation
The amount of radiant energy emitted in a specific direction per unit time and per unit surface area.
In optics, a boundary between two media in which light travels with different velocities.
In optics, when two sets of light waves of equal wave length and amplitude from the same source meet, so that the crests of one coincide with the troughs of another, they cancel out. Similarly, if two sets of light waves meet when the crests of one coincide with the crests of the other they reinforce each other.
An instrument that is used to measure minute linear displacement through the phenomena of light interference.
The use of light interference patterns for measurements with apparatuses such as the optical flat.
The process of estimating in a transmission line due to power dissipation.
The condition that exists when both axes of an image are reversed.
Any mechanical or electrical device for converting direct current into alternating current.
An atomic particle, atom, or chemical radical (group of chemically combined atoms) bearing an electrical charge, either positive or negative, caused by an excess or deficiency of electrons.
The process by which molecules of a gas are converted into positive ions by loss of electrons, or into negative ions by gain of electrons. Ionization can be produced in a number of ways, by collisions of ions with electrons, by the action of ultraviolet light or other radiations.
ionization potential
The potential necessary to separate 1 electron from an atom.
ionizing event
An event in which an ion is produced.
Elements having the same mass number but different atomic numbers.
One of two or more forms of an element having the same atomic number (nuclear charge) and hence occupying the same position in the periodic table. All isotopes are identical in chemical behavior, but are distinguishable by small differences in atomic weight. The nuclei of all isotopes of a given element have the same number of protons but have different numbers of neutrons.
That region of the atmosphere, 70 to 250 miles above the surface of the Earth, containing layers of highly ionized air that are capable of bending or reflecting radio waves back to Earth. Reflection from the ionosphere makes possible long distance reception of radio waves.
Small, rapid variations in a waveform due to mechanical disturbances.
Unit of energy. The work done when the point of application of 1 newton is displaced a distance of 1 meter in the direction of the force.
Symbol for 1000 (103). When referring to bits or words, K=1024 (210).
Kelvin temperature scale
The absolute temperature scale in the Cgs system. Kelvin is equal to degrees Celsius plus 273.15.
Unit of mass. The mass of a particular cylinder of platinum-iridium alloy, called the International Prototype Kilogram, which is preserved in a vault at Sevres, France, by the International Bureau of Weights and Measures.
kinetic energy
Energy due to motion.
A smoothing or polishing operation.
An optical cavity capable of oscillating in the visible and nonvisible light spectrum. The laser is a true light amplifier because light energy is used for excitation.
From the side. Usually refers to movement of a given reference made from left to right to left.
A body of glass or similar material ground to fine limits, used to either converge or diverge rays of light by refraction.
Perpendicular to the force of gravity. Also, a device for determining true level by means of a gravity seeking level.
A narrow band of radiation which is the visual section of the electromagnetic spectrum. It consists of wavelengths of 15.7 to 27.5 microinches.
line of sight
A straight line that passes through the cross hairs and the principal point of lens is called the line of sight or the line of collimation; it always strikes the object where the cross hairs appear to fall. Accordingly, the cross hairs and the principal point of the lens are said to define the line of sight.
A relation such that any change in one of two related quantities is accompanied by an exactly proportional change in the other.
The state of matter which has definite volume but no definite shape.
load cell
A type of force transducer designed primarily for the measurement of load or weight. Electric load cells usually employ bonded strain gage resistance elements to provide an electrical output signal proportional to the load. Hydraulic and pneumatic load cells generally make use of a bourdon-type device, such as a Heise gage.
loading effects
An error of measurement resulting in a change of the system under test caused by insertion of the test instrument.
The logarithm of a number is the power to which a second number, called the base, must be raised in order to yield the original number. Bases in common use are 10 and 2.718.
Unit of luminous flux. It is the luminous flux emitted in a solid angle, 1 steradian, by a uniform point source having an intensity of 1 candela.
Any object which has the property of attracting iron, nickel, or cobalt objects with forces which are much greater than those of gravitation and which do not depend on the presence of electric charges on either body.
magnetic deflection
Method of bending electrons in a CRT by means of the magnetic field produced by coils placed outside the tube.
The value of magnification is the apparent size of an object viewed through a telescope divided by the size it appears to the unaided eye from the same distance.
The property of a metal which allows it to be hammered or rolled into sheets.
Fractional value used as part of a floating point number. For example, the mantissa in the number 0.9873 X 107 is 0.9873.
The measure of the quantity of matter that a body contains.
mass density (r )
Mass per unit volume.
mass number
The number of protons and nutrons in the atomic nucleus of an element. (equal to the atomic number)
mass unit
A unit of mass based upon 1/18 the atomic weight of an oxygen atom taken as 18.00000. Abbreviation: mu, or atomic mass unit, amu.
master flat
A surface plate, usually round rather than square with a high degree of surface flatness.
Anything which has weight and occupies space.
Mcleod gage
A primary instrument for the measurement of pressure in a vacuum system. The gage consists of a glass bulb with a vertical capillary tube at the top.
mean free path
The average distance a particle moves between collisions. Abbreviation: mfp, symbol, I.
mean solar day
The average of all apparent solar days in a given year.
A particular quantity subject to measurement.
The overall process that a person goes through in reaching a decision as to the magnitude of some quantity.
mechanical axis
The true centerline of the mechanical components within the telescope. For a perfectly calibrated instrument the mechanical axis would be coincident with the optical axis.
The curved upper surface of a column of liquid which is concave when the walls of the container are wet and convex when the walls of the container are dry.
A heavy, silver-colored metal which is liquid at ordinary room temperatures.
A short-lived particle carrying a positive, negative, or zero charge, and having a variable mass in multiples of the mass of the electron. Also called mesotron.
metastable state
An excited state of nucleus which returns to the ground state by the emission of a gamma ray over a measurable half life.
(1) Unit of length. The length of exactly 1,650,763.73 wavelengths of the radiation in vacuum corresponding to the unperturbed transition between the levels 2p10 and 5d of the atom of Krypton 86, the orange-red line.  (2) The length of  the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.(1984 GCWM)
The science of measurement.
The abbreviation for million electron volts. See Electron-Volt.
A unit of length equal to one-millionth of a meter.
A minute is 1/60th of a degree. This is more correctly described as a "minute of arc."
MKS system
The meter-kilogram-second system.
The smallest particle of any substance which can exist free and still exhibit all properties of the substance.
molecular weight
The sum of the atomic weights of all the atoms in a molecule.
moment arm
The length of a torque wrench from the center of pivot to the point where force is applied.
The product of the mass of a body and its velocity. Cgs unit: gm-cm/sec.
monochromatic light
Light of only one wavelength or color.
The point of the celestial sphere that is directly opposite the zenith and vertically downward from the observer.
National Institute of Science and Technology (NIST)
Formerly the National Bureau of Standard (NBS). An independent agency of the U.S. Department of Commerce charged with the improvement and maintenance of all kinds of standards. The bureau operates radio stations WWV, WWVH, WWVB, and WWVL which broadcast accurate frequency and time standards.
Negative lens
A concave lens, thicker at the edges than the center, which diverges or spreads rays of light through refraction.
Negative mirror
A convex mirror curved out. Produces reflected diverging light rays away from the focal point.
An inert element which is a gas at room temperature. When ionized by current flow it produces a bright orange-red glow.
An elementary nuclear particle with a mass approximately the same as that of a hydrogen atom and electrically neutral; a constituent of the atomic nucleus. Its mass is 1.00894 mu.
A particle with zero rest mass and zero charge, emitted to preserve spin, momentum, and energy in decay and other processes.
Unit of force. That force which gives to a mass of 1 kilogram an acceleration of 1 meter per second. One newton equals 100,000 dynes.
Newtonian fluid
A fluid whose absolute viscosity is the same for all values of shear stress.
nominal value
This is normally the value indicated by the manufacturer. Also the indicated value of an instrument under test.
A chart or diagram with which equations can be solved graphically by placing a straightedge on the two known values and reading the answer where the straightedge crosses the scale of the unknown values.
nonsinusodial wave
Any waveform that differs from that of a sine wave.
Perpendicular to a tangent at a point of tangency.
nuclear fission
A special type of nuclear transformation characterized by the splitting of a nucleus into at least two other nuclei and the release of a relatively large amount of energy.
nuclear fusion
The act of coalescing two or more nuclei.
The common name for the constituent parts of the nucleus. At present applied to protons and neutrons, but will include any other particle that is found to exist in the nucleus.
The heavy central part of an atom in which most of the mass and the total positive electric charge are concentrated. The charge of the nucleus, an integral multiple Z of the charge of the proton, is the essential factor which distinguishes one element from another. Z is the atomic number.
A general term referring to all nuclear species--both stable (about 270) and unstable (about 500)-- of the chemical elements, as distinguished from the two or more nuclear species of a single chemical element which are called isotopes.
null method
Any method of measurement in which the reading is taken at zero. Galvanometers, sensitive voltmeters, oscilloscopes, and earphones are used as null detectors.
objective lens
The objective lens of a telescope optical system causes a real image to be formed which, when adjusted to lie within the focal plane of the eyepiece lens can be magnified as a virtual image.
Unit of electrical resistance. The electric resistance between two points of a conductor when a constant difference of potential of 1 volt, applied between these two points, produces in this conductor a current of 1 ampere, this conductor not being the source of any electromotive force.
An instrument for measuring resistance.
Ohm's Law
A fundamental electrical law which expresses the relationship between voltage, current, and resistance in a DC circuit, or the relationship between voltage, current, and impedance in an AC circuit.
Neither reflecting nor emitting light.
optical axis
Centers of curvature of a lens define a line called the axis of the lens. When several lenses combine to form an optical system, the line defined as these axis' is called the optical axis.
optical flat
A piece of glass or quartz which is accurately flat to within one-tenth of a wave length on one or both surfaces, used as a reference (proof plane) for comparison of flatness.
optical infinity
A section of a wave front which has advanced a great distance from its source and assumed essentially a zero curvature. In optics approximately 2000 yards
optical pyrometer
An instrument designed to estimate the temperature of glowing surfaces.
optical tooling
The geometric method of optically establishing a precise line and/or reference plane.
The branch of physics which deals with the phenomena of light.
The most favorable degree or condition.
out of phase
Having waveforms that are of the same frequency but not passing through corresponding values at the same instants.
The high and low spots in a true circle. It is also the ovality or lobing effect which causes a change of true roundness of cylindrical objects.
A load that is greater than the device is designed to handle.
The initial transient response to an unidirectional change in input which exceeds the steady state response.
An element combined with oxygen. Rust is an oxide of iron.
packing fraction
The difference between the atomic weight in mass units and the mass number of an element divided by the mass number and multiplied by 10,000. It indicates nuclear stability. The smaller the packing fraction, the more stable the element.
pair production
The description of an electron leaving the valence band to enter the conduction band due to absorption of energy (usually heat). This provides a free electron carrier and a free hole carrier at the same time.
The apparent displacement of the position of an object caused by a shift in the point of observation. Thus, the pointer of a meter will appear to be at different positions on the scale depending on the angle from which the meter is read. To eliminate errors in meter reading due to parallax, the line of sight should be perpendicular to the pointer.
parallel (optical)
A piece of glass with one side parallel to the other side. An optical parallel gives linear displacement.
(1) In mathematics, one of the constants entering into a functional equation and corresponding to some characteristic property, or dimension.  (2) In an electronic circuit, a characteristic element or constant factor, such as: resistance, capacitance, or inductance values.
Pascal's Law
The pressure applied on a confined fluid is transmitted undiminished in every direction.
peak-to-peak amplitude
The amplitude of an alternating quantity measured from positive to negative peak. This is the value indicated on an oscilloscope.
peak-to-peak value
The algebraic difference between extreme values (as DA or double amplitude is twice the single amplitude).
A five-sided prism which deviates rays of light by 90° without reversing or inverting the image.
pentavalent impurity
Any impure atom that has five electrons in its valence band.
The time corresponding to one cycle of a periodic phenomenon. The period of a galvanometer is the elapsed time between consecutive passages of the pointer in the same direction through its zero point.
Being at right angles to a given line or plane.
photoelectric effect
The electrical effect of light or other radiation. This effect can be emission of electrodes, penetration of voltage, or a change in electrical resistance upon exposure to light.
The measurement of luminous intensity from a light source by comparison to a known standard.
Small particles of light energy according to the quantum theory of light.
photon generator
A light source.
The physical science which deals with matter and energy and with the transformations of energy.
Physi-optical practices combine the use of specific physical measuring standards with optical instruments and physical indicating apparatus.
Planck's constant
A natural constant of proportionality h relating the frequency of a quantum of energy to the total energy of the quantum;  
To rotate the telescope of a Theodolite 180° about the horizontal axis of the instrument.
The needle-shaped rod that moves over the scale of a meter or dial.
polarized light
Light in which vibrations occur in a single plane perpendicular to the ray.
A tough, flexible, plastic compound that has excellent insulating properties, even at the ultra high frequencies. It is widely used as the insulating material in coaxial cable.
A clear thermoplastic material having very desirable dielectric properties. Many standard capacitors use polystyrene as dielectric.
Small openings or spaces between particles of matter.
porro prism
A prism which causes an image to be rotated 180°, or reflected. The image is reversed in the plane in which the reflection takes place.
positive lens
A convex lens, thicker at the center than at the edges, which converges rays of light through refraction.
positive mirror
A concave mirror that is curved toward the middle, which converges rays of light through refraction.
A nuclear particle equal in mass to the electron and having an equal but opposite charge. Its mass is 0.000548 mu.
The amount of voltage or charge between a point and a zero reference point. Bodies with an excess of electrons have a negative potential. Bodies with a deficiency of electrons have a positive potential. The electric potential at any point in an electric field is equal to the work done on a unit charge to bring the charge to that point from a place where the potential is zero.
potential difference
The difference in potential between any two points in a circuit; the work required to carry a unit positive charge from one point to another.
potential energy
Energy due to position.
potentiometer (pot)
A variable resistance unit having a rotating contact arm that can be set at any desired point along a resistance element. The voltage source is connected to the end terminals of the resistance element, and the output circuit is connected between one end terminal and the moveable contact to give a voltage dividing action.
potentiometric measurement
DC voltage can be most accurately measured using the potentiometric method. It consists of comparing the unknown voltage with a known voltage from a calibrated potentiometer.
The term precision can best be defined as repeatability. If a measurement is made a number of times and nearly the same value is read each time, it is a precise measurement, the readings may be all wrong. Care should be taken not to confuse precision with accuracy.
(1) Force per unit area (closed system).  (2) Height times density (open system).
primary colors
Colors in terms of which all colors may be described or from which all colors may be evolved by mixtures.
primary electron
The electron ejected from an atom by an initial ionizing event, as caused by an photon or beta particle.
primary standard
A unit established by some authority or developed through practical exact application of a formula. Secondary standards are calibrated against the primary standard.
principal focus
A point to which rays parallel to the principal axis converge, or from which they diverge after reflection.
principal quantum number
The number, n = 1, 2, 3, . . . . which describes the basic state of atomic system in quantum theory.
A transparent body bounded in part by two plane faces that are not parallel, used to deviate or disperse a beam of light.
The likelihood of the occurrence of any particular form of an event, figured as the ratio of the number of ways in which that form might occur to the whole number of ways in which the event might occur in any form.
proving ring
An elastic ring in which the deflection of the ring, when loaded along a diameter, is measured by means of a micrometer screw and a vibrating reed. Note that all ring-type elastic force measuring devices are not proving rings, and such devices which do not make use of a micrometer screw and vibrating reed should not be called proving rings.
proving ring deflection
The difference between the reading for a given load and the reading for no load.
A positively charged particle occupying the nucleus of an atom that has a charge equal to that of an electron.
An instrument for measuring relative humidity.
A device for measuring high temperatures.
One of the four sections in which a plane is divided by two perpendicular lines.
One of the very small parts into which many forms of energy are subdivided.
quantum level
An energy level of an electron or of any atomic system, distinct from any other of its energy levels by discrete quantities dependent upon Planck's constant.
quantum mechanics
The science of description of atomic systems in terms of discrete quantum states.
quantum number
One of a set of integral or half-integral numbers, one for each degree of freedom, which determines the state of an atomic system in terms of the constants of nature.
quantum state
A term defining the way in which an atomic system exists at any specific time. This state is often described by means of a complex mathematical function called quanta.
quantum theory
The transfer of light and matter occurs only in discrete quantities proportional to the frequency of the energy transferred.
The angle for which the arc length is equal to the radius. There are 2 p radians in 1 revolution (360°). A radian represents an angle of approximately 57.3°.
radiant energy
Energy in the form of electromagnetic radiation such as radio waves, heat waves, light waves, ultra violet rays or X-rays.
A method of transmission of energy.  Specifically: (1) Any electromagnetic wave (quantum). (2) Any moving electron or nuclear particle, charged or uncharged, emitted by a radioactive substance.
The process whereby certain nuclides undergo spontaneous atomic disintegration in which energy is liberated, generally resulting in the formation of new nuclides. The process is accompanied by the emission of one or more types of radiation, such as alpha particles, beta particles, and gamma radiation.
The shortest distance from the center of a circle or arc, to a point on the circumference.
random error
Random errors are sometimes called "accidental" errors because they are as likely to occur in one direction as the other. They are the error left when all gross errors and systematic errors have been corrected.
(1) Extent of coverage of effectiveness.  (2) Measure of distance.
Rankine temperature scale
A temperature scale which corresponds to the Kelvin scale, but is based on the absolute zero of  the Fahrenheit system, so that 0 °Fahrenheit = 459.67 °Rankine.
ratio bridge
A bridge circuit that uses a calibrated resistive or calibrated inductive voltage divider for one side of the bridge. Precision resistors, inductors, and capacitors are measured with ratio bridge circuits.
ratio transformer
A precisely wound auto transformer used as an AC voltage divider.
ray of light
Can be considered as the path traced by a point on an advancing wave front.
Any process involving a chemical or nuclear change.
real image
A real image is one through which light rays actually pass and can be projected onto a screen.
reference line
A line from which all other measurements are taken.
reference plane
A reference line that has been rotated through 360°.
The change in direction of waves after striking a surface.
The bending of a ray of light, heat, sound, or a radio wave passing obliquely from one medium into another in which the velocity of propagation is different from the first medium.
relative humidity
The ratio of the amount of water vapor in the air at a given temperature to the maximum water vapor (capacity of the air) at the same temperature.
A force tending to separate objects or particles having like electrical charges or magnetic polarities.
The resilience of a body measures the extent to which energy may be stored in it by elastic deformation.
(1) The term resolution pertains to the scale of an instrument. It is the smallest readout at calibrated points. Resolution is sometimes referred to as "least count."  (2) When uncalibrated adjustments are made, resolution is the smallest change which can be obtained by manipulation of the instrument controls. Resolution can be increased by use of vernier scales.
The frequency whereby any system responds with maximum amplitude to an applied force having a frequency equal or nearly equal to its own.
An entity or quantity obtained by means of, or as a result of, a given process.
restoring force
The constant mechanical force provided.
rest point
The equilibrium point or the point at which the pointer of the balance would come to rest once it has been set into oscillation.
Cross lines found in the telescope of sight levels, transits, and theodolites. Initially in the form of a fine hair. They are now produced by engraving glass with a diamond point to achieve a line of 2.5 to 3 seconds thickness. Also known as; cross hair, filar, (For two parallel lines called); bifilar
In optics, to rotate a Theodolite 180° about the vertical axis.
The magnitude of the reflection coefficient.,
rhomboid prism
A prism which displaces the axis of a beam without introducing and without reverting the image.
right angle prism
A simple prism used when deviations of 90° are required. Reversion of the image takes place.
The quantity of X or radiation which produces 1 esu of positive or negative electricity/cm3 of air at standard temperature and pressure or 2.083 x 109 ion pairs/cm of dry air.
rosin-core solder
Solder made up in tubular form with the inner space containing rosin flux for effective soldering.
rotary motion
Motion in which every particle of a body moves in a circle and all the circles have their centers on the same straight line.
(1) A rotating member such as the armature of a motor, generator, or synchro.  (2) The rotating plates of a variable capacitor.
The point in operation where an increase in a given quantity will have a negligible effect on the output or end result.
(1) Something graduated when used as a measure or rule. A series of spaces marked by lines to indicate the magnitude of some quantity.  (2) A weighing device.
schematic diagram
A diagram which shows all of the electronic parts by means of symbols.
scintillation counter
A device used for the detection of radioactivity.
second (ephemeris second)
Unit of time. Exactly 1/31,556,925.9747 of the tropical year of 1900, January, 0 days and 12 hours ephemeris time.
secondary emission
Electron emission that is the direct result of the impact of electrons against a surface.
Seeback effect
The EMF produced in a circuit containing two contacting conductors of different metals having two junctions at different temperatures.
(1) The degree of response of a circuit to signals of the frequency to which it is tuned.  (2) An indication of the gain of a receiver.  (3) A measure of the minimum signal to which a device shows a measurable response.  (4) The ratio of a small change in instrument reading to the change in the measured quantity required to produce it.  (5) Ratio between electrical output to mechanical output.
servo system
An electromechanical system which is used for positioning one element of a system in relation to another, for example, a PPI sweep in relation to the antenna. The change in position of one element of the system results in the reproduction of an error voltage that is used indirectly to cause a motor to drive the other element of the system to the point where the error voltage no longer exists.
An action or stress from applied forces that causes two contacting parts of a body, to slide relative to each other, in a direction parallel to their place of contact.
One of a series of concentric spheres, called signals, which are designated in the order of increasing distance from the nucleus of an atom, as K, L, M, N, O, P, and Q shells. The number of electrons contained in each shell is limited.
sinusoidal vibration
A simplified back and forth motion of a constrained object which varies sinusoidally with time.
Snell's Law
(Index of refraction) x (sine of incident angle) = (index of refraction) x (sine of refracted angle).
An alloy of lead and tin which melts at a fairly low temperature (about 500° F) and is used for making permanent electrical connections in electrical circuits.
solder bridge
Glob of excess solder that shorts two conductors. A common problem on production PC boards.
The state of matter which has a definite shape and definite volume.
solid state physics
That branch of physics which deals with the structure and properties of solids. In electronics, solid state refers to those devices which can control current without the use of moving parts, heated filaments or vacuum gaps.
Sound navigation and ranging. Electronic equipment used for underwater detection of objects and determination of their range.
A vibration of a body which can be heard by human ears. The extreme limits of human hearing is 20 Hz to 20 kHz. Sound can travel through any medium which possesses the ability to vibrate; the vibrations are called sound waves.
space charge
The negative charge produced by the cloud of electrons existing in the space between the cathode and plate of a thermionic vacuum tube; formed by electrons emitted from the cathode in excess of those immediately attracted to the plate.
specific gravity
The ratio of the density of a substance to the density of a standard (distilled water).
specific heat
The ratio of the heat capacity of a body to its mass or weight.
(1) The entire range of wavelengths within which electromagnetic radiations occur.  (2) A segment of wavelengths which has a special function or possesses special properties.
spherical aberration
The failure of parallel rays to meet at a single point after reflection, causing a blurred image.
The inherent, intrinsic angular momentum of an atomic particle; a quantum number in modern atomic theory.
spindle axis
An axis found on theodolites and transits that goes directly through the center of the instrument.
spring equinox
First day of spring in the northern hemisphere. It usually falls on March 21st in the northern hemisphere.There are about 12 hours of light and 12 hours of darkness every place on the Earth during an equinox. 
stadia lines
Lines or marks on a reticle used to determine distances to objects of a known height or width by using trigonometric principles.
Anything taken as a basis of comparison. An authorized weight or measure having recognized excellence. It is desirable that the standard have an uncertainty that is one-tenth or less than the equipment being calibrated. A standard is a physical embodiment of a unit. In general it is not independent of physical condition, and it is a true embodiment of the unit only under specified conditions, for example, a yard standard has a length of one yard when at some definite temperature and supported in a certain manner.
standard deviation
The square root of the sum of the squares of the deviations from the arithmetic mean of a frequency distribution. The deviations from the arithmetic mean are squared and added, and the square root of this sum is the standard deviation.
standard pressure
The pressure exerted by a column of mercury exactly 760 mm high.
standard temperature
The temperature of melting ice.
One-fourth of the solid angle around a point.
Stoke’s Law
the basis of kinematic viscosity which states that the terminal velocity of a sphere (or any object) falling freely through a fluid is controlled by the density of the sphere and the absolute viscosity of the fluid.
Deformation of a material body under the action of applied forces (stress).
This is the uniformity of direction throughout the extent of that feature, such as the freedom from bend, warp, or twist of a shaft.
Mutual force between contacting surfaces of bodies caused by an external force, such as tension or shear.
stress testing
Introducing mechanical, electrical, or thermal stress on electrical devices so as to modify their operation and allow intermittent problems to be observed.
An instrument used to determine the speed of a rotating body. It creates the optical illusion of slowing down or stopping the motion of an object by illuminating it with flashes of intense light at regular intervals.
The change of state from a solid to a vapor or gas without going through the liquid state.
summer solstice
Longest day of the year. It usually falls on June 21st in the northern hemisphere. The sun casts its shortest shadows in the summer solstice.
surface tension
The tendency of the surface of a liquid to contract.
The universal term applied to any of the various synchronous devices such as the "selsyn", "autosyn", "motor-torque generator", "magslip" and "siemens." The standard signal and control synchro today has two-pole single-phase rotor field and a delta or Y-wound single-phase variable-voltage stator.
systematic error
Systematic errors tend to bias all the measurements in one direction. The same error is occurring in measurement after measurement. Systematic errors can usually be blamed for trends, jumps, or drifts in a reading. They are also called persistent errors.
Collection of data in a form suitable for ready reference, frequently stored in sequential memory locations.
table look-up
Obtaining a value from a table of values stored in the computer.
An instrument for measuring rotational speed in revolutions per minute (rpm).
An instrument for making objects appear nearer and larger. The telescope forms the basis upon which physi-optical instruments are designed, such as the transit and Theodolite.
The quantitative measure of the relative hotness or coldness of an object.
temperature coefficient
A numerical value that indicates the relation between a temperature change and the resulting change in another property. The numerical value can be either negative or positive.
tensile strength
The force required to break a rod or wire of unit cross-sectional area.
terminal Linearity
Ratio of the actual error voltage in the output to the total input voltage. This will vary with the setting of the ratio voltage divider.
Relating to earthly matters. A terrestrial telescope is one in which the image appears normal, not reversed or inverted.
The load connected to the output end of a circuit or transmission line.
testing machine
A machine for applying forces to specimens of steel and other material to determine the applied force which the test specimen will withstand.
test instrument
The device which is being compared with the calibration standard. The test instrument is the instrument whose accuracy is being tested.
test set
A combination of instruments needed for making a particular combination of tests, or for servicing a particular type of equipment.
An optical instrument used for measuring horizontal or vertical angles.
thermal agitation
Random movement of free electrons in a circuit due to the presence of heat.
thermal energy
The potential and kinetic energy of the particles of a body which can be evolved as heat.
thermal runaway
A result of a regenerative increase in collector current and junction temperature.
thermal capacity
The amount of heat required to produce a unit temperature change. Water has the highest thermal capacity of any common substance.
A resistor whose value varies with temperature in a definite desired manner, used in circuits to compensate for temperature variations in other parts. It may have either a negative or a positive temperature coefficient. One type is made from a semiconducting material such as uranium oxide or silver sulfide, having a relatively large negative temperature coefficient of resistance. The name is a contraction of thermal resistor.
Two dissimilar metals joined at one end. When a difference of temperature exists between the ends, and EMF is generated across the thermocouple. This DC voltage is proportional to the heat applied to the thermocouple junction.
threshold sensitivity
Refers to the smallest fractional load which will cause a pressure system to indicate that a load is starting to be applied.
tilt graticule
A graduate reticule used in Collimators for measuring vertical and horizontal tilt, or angular deviation.
The period during which an action or process continues; measurement of duration.
The cause of rotary motion. Torque is equal to the applied force multiplied by the distance from the center of rotation. (lb/ft, oz/in, etc..)
torque wrench
A wrench with which the mechanic can apply specific amounts of torque, usually as indicated by the setting of the handle.
1/760 of an atmosphere - 1 mm Hg.
total force
The force acting against the entire area of a particular surface.
The instantaneous surge of voltage or current that occurs as the result of a change from one steady-state condition to another.
transient vibration
Abrupt changes or shocks in the levels of other motion.
Similar to a Theodolite; can only make measurements with the use of accessories. Readings are linear deviation.
A change in the identity of a nucleus because of a change in its number of protons.
Having the property of transmitting light without appreciable scattering so that bodies lying beyond are entirely visible.
trivalent impurity
Any impure atom that has three electrons in its valence band.
To seek the cause of a malfunction or erroneous program behavior in order to remove the malfunction.
troubleshooting tree
Flow diagram consisting of tests and measurements used to diagnose and locate faults in a product.
tropical year
The time between two successive vernal equinoxes. Our calendar is based on the tropical year. It is equal to 365 days, 5 hours, 48 minutes, and 49.7 seconds.
true mass
Mass as measured in a vacuum.
true value
The value of a physical quantity that would be attributable to a material object or physical system if that value could be determined without error.
twisted pair
A cable composed of two insulated conductors twisted together either with or without a common covering.
A range of invisible radiation frequencies beyond the visible spectrum at the high frequency end, and extending into the region of low frequency X-rays.
A parameter (such as standard deviation), associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand.
Having or using one fiber, wire or thread.
A value, quantity, or magnitude in terms of which other values, quantities, or magnitudes are expressed. In general, a unit is fixed by definition and is independent of such physical conditions as temperature. Examples:  yard, pound, gallon, meter, liter, gram.
Any pressure below atmospheric. In gage pressure measurement, 5 psig vacuum means 5 psi below atmospheric pressure. In absolute pressure measurements, any pressure from zero psia (perfect vacuum) up to atmospheric pressure.
The number representing the combining or displacing power of an atom; number of electrons lost, gained, or shared by an atom in a compound; the number of hydrogen atoms with which an atom will combine, or the number it will displace.
valence band
The outermost orbit of an atom that will contain electrons at absolute zero.
valence electrons
Electrons which are gained, lost, or shared in chemical reactions.
The production of a vapor or gas from matter in another physical state.
The time rate of change of position.
velocity constant
The ratio of the velocity of propagation in a transmission line to the velocity of light.
vernal (spring) equinox
First day of spring in the northern hemisphere. It usually falls on March 21st in the northern hemisphere. There are about 12 hours of light and 12 hours of darkness every place on the Earth during an equinox.
An auxiliary scale made to work in conjunction with the divisions of a graduated instrument for indicating parts of a division.
vertically polarized wave
An electromagnetic wave in which the electric field (E) is perpendicular to the horizon and the magnetic field (H) is horizontal (parallel to the Earth's surface).
Perpendicular to the horizontal plane. The direction of gravity.
vertical axis
The axis about which the telescope rotates when sweeping a horizontal plane.
Mechanical oscillations or motion about a reference point or equilibrium.
virtual image
The impression of an object as viewed by the observer. Light rays do not pass through, but only appear to come from the image.
The internal friction of a fluid. Also a quantitative measure of a fluid's lubricity.
Very Large Scale Integration.
Readily vaporizable at a relatively low temperature.
The amount of space which matter occupies.
wave front
A surface composed at any instant of all the points just reached by a vibrational disturbance in its propagation through a medium.
A weak prism, used when very small deviations of a beam are required. The wedge is also used in conjunction with penta and other prisms for corrective purposes.
The force of gravity acting on an object.
winter solstice
Shortest day of the year. It usually falls on December 21st in the northern hemisphere. The sun casts its longest shadows in the winter solstice.
That which is accomplished when a force acts on matter and moves it. (ft/lb, in/oz, etc...)
The point of the celestial sphere that is directly opposite the nadir and vertically above the observer.

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