A Love Meter at a Framingham, Massachusetts Rest Stop.
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the measurement results in a given number for the relationship between the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define their use, are the means by which this translation is made. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty.
Physicists use a vast range of instruments to perform their measurements. These range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators. Virtual instrumentation is widely used in the development of modern measuring instruments.
Time-points in the past can be measured with respect to the present (using radiometric dating, stop-watch ...) of an observer. Time-points in the future can be fixed (egg timer, alarm-clock) too. But there seems to exist no device that can set time to a predetermined value (time machine), unlike it is possible with other physical quantities (for example: distance or volume). And the time-point called present seems to move in one direction only. Entropy production and cause-and-effect observations of events correlate to this observation.
For more information on time, especially standards, also consult the time portal.
Timeline of time measurement technology
For the ranges of time-values see: Orders of magnitude (time)
Changing energy carriers, linear momentum to angular momentum. No measurement primarily intended.
Example: In a plant that furnishes pumped-storage hydroelectricity, mechanical work and electrical work is done by machines like electrically operated pumps and generators. The pumped water stores mechanical work. The amounts of work do balance, only if one includes losses due to friction (mechanical and hydrodynamical). Nobody seems to have reproducably observed that more work could be extracted from that system, than the amount of work that entered the system.
Such examples suggested the derivation of some unifying concepts: Instead of discerning (transferred) forms of work or stored work, there has been introduced one single quantity called energy. Energy is assumed to have substance-like qualities; energy can be apportioned and transferred. Energy is supposed to be never producable from nothing, or to be annihilated to nothing, thus energy becomes a conserved quantity, when properly balanced.
For the transfer of energy two dictions are used, sometimes somewhat interchangingly:
(energy carriers exchanging energy) Physical interactions occur by carriers (linear momentum, electric charge, entropy) exchanging energy. For example, a generator transfers energy from angular momentum to electric charge. [1]
(energy forms transforming energy) Energy forms are transformed; for example mechanical energy into electrical energy by a generator. [2]
Often the energy value results from multiplying two related quantities: (a generalized) potential (relative velocity, voltage, temperature difference) times some substance-like quantity (linear momentum, electrical charge, entropy). — Thus energy has to be measured by first choosing a carrier/form. The measurement usually happens indirectly, by obtaining two values (potential and substance-like quantity) and by multiplying their values.
- (see any measurement device for energy below)
For the ranges of energy-values see: Orders of magnitude (energy)
Power decribes energy exchanged by a system at a point in time (current of energy).
- (see any measurement device for power below)
For the ranges of power-values see: Orders of magnitude (power).
Action describes energy summed up over the time a process lasts (time integral over energy).
This includes basic quantities found in Classical- and continuum mechanics; but strives to exclude temperature-related dependences or quantities.
A steel ruler – an instrument for measuring length.
see also Distance measuring equipment
For the ranges of length-values see: Orders of magnitude (length)
For the ranges of area-values see: Orders of magnitude (area)
A measuring cup, a common instrument used to measure volume.
(if the mass density of a solid is known, weighing allows to calculate the volume)
For the ranges of volume-values see: Orders of magnitude (volume)
For the ranges of speed-values see: Orders of magnitude (speed)
A pair of scales: An instrument for measuring mass in a force field — actually balancing forces!
For the ranges of mass-values see: Orders of magnitude (mass)
Current density is also called flux.
For the ranges of pressure-values see: Orders of magnitude (pressure)
Timeline of temperature and pressure measurement technology
An instrument for measuring angle: The sextant.
For the value-ranges of angular velocity see: Orders of magnitude (angular velocity)
For the ranges of frequency see: Orders of magnitude (frequency)
Orientation in three dimensional space
See also the section about navigation below.
Considerations related to electric charge dominate Electricity and Electronics. Electrical charges interact via a field. That field is called electric if the charge doesn't move. If the charge moves, thus realizing an electric current, that field is called magnetic. Electricity can be given a quality — a potential. And electricity has a substance-like property, the electric charge. Energy (or power) in electrodynamics is calculated by multipying the potential by the amount of charge (or current) found at that potential: potential times charge (or current). (See Classical electromagnetism and its Covariant formulation of classical electromagnetism)
An instrument for detecting net charges, the eletroscope.
For the ranges of charge values see: Orders of magnitude (charge)
- These are instruments used for measuring electrical properties. Also see meter (electronics).
See also the relevant section in the article about the magnetic field.
For the ranges of magnetic flux see: Orders of magnitude (magnetic flux density)
Temperature-related considerations dominate Thermodynamics. There are two distinct thermal properties: A thermal potential — the temperature. For example: A glowing coal has a different thermal quality than a non-glowing one.
And a substance-like property, — the entropy; for example: One glowing coal won't heat a pot of water, but a hundred will.
Energy in thermodynamics is calculated by multipying the thermal potential by the amount of entropy found at that potential: temperature times entropy.
Entropy can be created by friction but not annihilated.
- Usually determined indirectly. If mass and substance type of the sample are known, then atomic- or molecular masses (taken from a periodic table, masses measured by mass spectroscopy) give direct access to the value of the amount of substance. See also the article about molar masses. If specific molar values are given, then the amount of substance of a given sample may be determined by measuring volume, mass or concentration.
| unit |
overall range |
approximate precision |
| kelvin |
0.01-2,000 |
row 1, cell 3 |
| celsius |
-273.14-1,700 |
row 2, cell 3 |
See also Temperature measurement and Category:Thermometers. More technically related may be seen thermal analysis methods in materials science.
For the ranges of temperature-values see: Orders of magnitude (temperature)
An active Calorimeter lacking a temperature measurement device.
This includes Thermal capacitance or temperature coefficient of energy, reaction energy, heat flow ... Calorimeters are called passive if gauged to measure emerging energy carried by entropy, for example from chemical reactions. Calorimeters are called active or heated if they heat the sample, or reformulated: if they are gauged to fill the sample with a defined amount of entropy.
- see also Calorimeter or Calorimetry
Accessible indirectly by measurement of energy and temperature.
Phase change calorimeter's energy value divided by absolute temperature give the entropy exchanged. Phase changes produce no entropy and therefore offer themselves as an entropy measurement concept. Thus entropy values occur indirectly by processing energy measurements at defined temperatures, without producing entropy.
The given sample is cooled down to (almost) absolute zero (for example by submerging the sample in liquid helium). At absolute zero temperature any sample is assumed to contain no entropy (see Third law of thermodynamics for further information). Then the following two active calorimeter types can be used to fill the sample with entropy until the desired temperature has been reached: (see also Thermodynamic databases for pure substances)
Processes transferring energy from a non-thermal carrier to heat as a carrier do produce entropy (Example: mechanical/electrical friction, established by Count Rumford). Either the produced entropy or heat are measured (calorimetry) or the transferred energy of the non-thermal carrier may be measured.
- calorimeter
- (any device for measuring the work which will or would eventually be converted to heat and the ambient temperature)
Entropy lowering its temperature - without loosing energy - produces entropy (Example: Heat conduction in an isolated rod; "thermal friction").
Concerning a given sample, a proportionality factor relating temperature change and energy carried by heat. If the sample is a gas, then this coefficient depends significantly on being measured at constant volume or at constant pressure. (The terminiology preference in the heading indicates that the classical use of heat bars it from having substance-like properties.)
The temperature coefficient of energy divided by a substance-like quantity (amount of substance, mass, volume) describing the sample. Usually calculated from measurements by a division or could be measured directly using a unit amount of that sample.
For the ranges of specific heat capacities see: Orders of magnitude (specific heat capacity)
See also thermal analysis, Heat.
This includes mostly instruments which measure macroscopic properties of matter: In the fields of solid state physics; in condensed matter physics which considers solids, liquids and in-betweens exhibiting for example viscoelastic behavior. Furthermore fluid mechanics, where liquids, gases, plasmas and in-betweens like supercritical fluids are studied.
This refers to particle density of fluids and compact(ed) solids like crystals, in contrast to bulk density of grainy or porous solids.
For the ranges of density-values see: Orders of magnitude (density)
Shape and surface of a solid
Measurement results (a) brittle (b) ductile with breaking point (c) ductile without breaking point.
- Wind tunnel
- Tomograph, device and method for non-destructive analysis of multiple measurements done on a geometric object, for producing 2- or 3-dimensional images, representing the inner structure of that geometric object.
This section and the following sections include instruments from the wide field of Category:Materials science, Materials science.
The electrochemical cell: A device for measuring substance potentials.
Such measurements also allow to access values of molecular dipoles.
For other methods see the section in the article about magnetic susceptibility.
See also the Category:Electric and magnetic fields in matter
A reaction transmuting substances, from reactants to products, has an overall energy balance which consists of two parts: A balance that accounts for the changed entropy content of the substances. And another one that accounts for the energy freed or taken by that reaction itself, the Gibbs energy change. The sum of reaction energy and energy associated to the change of entropy content is also called enthalpy. Often the whole enthalpy is carried by entropy and thus measurable calorimetrically. For standard conditions in chemical reactions either molar entropy content and molar Gibbs energy with respect to some chosen zero point are tabulated. Or molar entropy content and molar enthalpy with respect to some chosen zero are tabulated. (See Standard enthalpy change of formation and Standard molar entropy)
The substance potential of a redox reaction is usually determined electrochemically using reversible cells.
See also the article on electrochemistry.
Sound, compression waves in matter
A device for unmixing sun-light: the prism.
(for lux meter see the section about human senses and human body)
See also Category:Optical devices
The measure of the total power of light emitted.
Ionizing radiation includes rays of "particles" as well as rays of "waves". Especially X-rays and Gamma rays transfer enough energy in non-thermal, (single) collision processes to separate electron(s) from an atom.
A cloud chamber detecting alpha-rays.
Identification and Content
This could include chemical substances, rays of any kind, elementary particles, quasiparticles. Many measurement devices outside this section may be used or at least become part of an identification process.
For identification and content concerning chemical substances see also analytical chemistry especially its List of materials analysis methods.
Substance content in mixtures, Substance identification
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