Metrology is the science of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France, when a length standard taken from a natural source was proposed; this led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; this has evolved into the International System of Units as a result of a resolution at the 11th Conference Generale des Poids et Mesures in 1960. Metrology is divided into three basic overlapping activities; the first being the definition of units of measurement, second the realisation of these units of measurement in practice, last traceability, linking measurements made in practice to the reference standards. These overlapping activities are used in varying degrees by the three basic sub-fields of Metrology.
The sub-fields are scientific or fundamental metrology, concerned with the establishment of units of measurement, technical or industrial metrology, the application of measurement to manufacturing and other processes in society, Legal metrology, which covers the regulation and statutory requirements for measuring instruments and the methods of measurement. In each country, a national measurement system exists as a network of laboratories, calibration facilities and accreditation bodies which implement and maintain its metrology infrastructure; the NMS affects how measurements are made in a country and their recognition by the international community, which has a wide-ranging impact in its society. The effects of metrology on trade and economy are some of the easiest-observed societal impacts. To facilitate fair trade, there must be an agreed-upon system of measurement; the ability to measure alone is insufficient. The first record of a permanent standard was in 2900 BC, when the royal Egyptian cubit was carved from black granite.
The cubit was decreed to be the length of the Pharaoh's forearm plus the width of his hand, replica standards were given to builders. The success of a standardised length for the building of the pyramids is indicated by the lengths of their bases differing by no more than 0.05 percent. Other civilizations produced accepted measurement standards, with Roman and Greek architecture based on distinct systems of measurement; the collapse of the empires and the Dark Ages which followed them lost much measurement knowledge and standardisation. Although local systems of measurement were common, comparability was difficult since many local systems were incompatible. England established the Assize of Measures to create standards for length measurements in 1196, the 1215 Magna Carta included a section for the measurement of wine and beer. Modern metrology has its roots in the French Revolution. With a political motivation to harmonise units throughout France, a length standard based on a natural source was proposed.
In March 1791, the metre was defined. This led to the creation of the decimal-based metric system in 1795, establishing standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875. Although the BIPM's original mission was to create international standards for units of measurement and relate them to national standards to ensure conformity, its scope has broadened to include electrical and photometric units and ionizing radiation measurement standards; the metric system was modernised in 1960 with the creation of the International System of Units as a result of a resolution at the 11th General Conference on Weights and Measures. Metrology is defined by the International Bureau of Weights and Measures as "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology", it establishes a common understanding of units, crucial to human activity. Metrology is a wide reaching field, but can be summarized through three basic activities: the definition of internationally accepted units of measurement, the realisation of these units of measurement in practice, the application of chains of traceability.
These concepts apply in different degrees to metrology's three main fields: scientific metrology. Scientific metrology is concerned with the establishment of units of measurement, the development of new measurement methods, the realisation of measurement standards, the transfer of traceability from these standards to users in a society; this type of metrology is considered the top level of metrology which strives for the highest degree of accuracy. BIPM maintains a database of the metrological calibration and measurement capabilities of institutes around the world; these institutes, whose activities are peer-reviewed, provide the fundamental reference points for metrological traceability. In the area of measurement, BIPM has identified nine metrology areas, which are acoustics and magnetism, length and related quantities and radiometry, ionizing radiation, time a
A gravimeter is an instrument used to measure gravitational acceleration. Every mass has an associated gravitational potential; the gradient of this potential is an acceleration. A gravimeter measures this gravitational acceleration; the first gravimeters were vertical accelerometers, specialized for measuring the constant downward acceleration of gravity on the earth's surface. The earth's vertical gravity varies from place to place over the surface of the Earth by about +/- 0.5%. It varies by about +/- 1000 nm/s^2 at any location because of the changing positions of the sun and moon relative to the earth; the change from calling a device an "accelerometer" to calling it a "gravimeter" occurs at the point where it has to make corrections for earth tides. Though similar in design to other accelerometers, gravimeters are designed to be much more sensitive, their first uses were to measure the changes in gravity from the varying densities and distribution of masses inside the earth, from temporal "tidal" variations in the shape and distribution of mass in the oceans and earth.
Gravimeters can detect vibrations and gravity changes from human activities. Depending on the interests of the researcher or operator, this might be counteracted by integral vibration isolation and signal processing; the resolution of the gravimeters can be increased by averaging samples over longer periods. Fundamental characteristic of gravimeters are the accuracy of a single measurement, the sampling rate. Resolution = SingleMeasurementResolution NumberOfSamples for example: Resolution per minute = Resolution per second 60 Gravimeters display their measurements in units of gals, nanometers per second squared, parts per million, parts per billion, or parts per trillion of the average vertical acceleration with respect to the earth; some newer units are pm/s2, fm/s2, am/s2 for sensitive instruments. Gravimeters are used for petroleum and mineral prospecting, geodesy, geophysical surveys and other geophysical research, for metrology, their fundamental purpose is to map the gravity field in time.
Most current work is earth-based, with a few satellites around earth, but gravimeters are applicable to the moon, planets, stars and other bodies. Gravitational wave experiments monitor the changes with time in the gravitational potential itself, rather than the gradient of the potential which the gravimeter is tracking; this distinction is somewhat arbitrary. The subsystems of the gravitational radiation experiments are sensitive to changes in the gradient of the potential; the local gravity signals on earth that interfere with gravitational wave experiments are disparagingly referred to as "Newtonian noise", since Newtonian gravity calculations are sufficient to characterize many of the local signals. The term "absolute gravimeter" has most been used to label gravimeters which report the local vertical acceleration due to the earth. "Relative gravimeter" refer to differential comparisons of gravity from one place to another. They are designed to subtract the average vertical gravity automatically.
They can be calibrated at a location where the gravity is known and transported to the location where the gravity is to be measured. Or they can calibrated in absolute units at their operating location. There are many methods for displaying acceleration fields called "gravity fields"; this includes traditional 2D maps, but 3D video. Since gravity and acceleration are the same, "acceleration field" might be preferable, since "gravity" is an oft misused prefix. Gravimeters for measuring the earth's gravity as as possible, are getting smaller and more portable. A common type measures the acceleration of small masses free falling in a vacuum, when the accelerometer is attached to the ground; the mass terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the acceleration of the mass can be measured. A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion; this allows cancellation of some measurement errors, however "rise and fall" gravimeters are not yet in common use.
Absolute gravimeters are used in the calibration of relative gravimeters, surveying for gravity anomalies, for establishing the vertical control network. Atom interferometric and atomic fountain methods are used for precise measurement of the earth's gravity, atomic clocks and purpose-built instruments can use time dilation measurements to track changes in the gravitational potential and gravitational acceleration on the earth; the term "absolute" does not convey the instrument's stability, accuracy, ease of use, bandwidth. So it and "relative" should not be used; the most common gravimeters are spring-based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas, they are a weight on a spring, by measuring the amount by which the weight stretches the spring, local gravity can be measured. However, the strength of the spring must be calibrated by placing the instrument in a locati
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists are interested in the root or ultimate causes of phenomena, frame their understanding in mathematical terms. Physicists work across a wide range of research fields, spanning all length scales: from sub-atomic and particle physics, through biological physics, to cosmological length scales encompassing the universe as a whole; the field includes two types of physicists: experimental physicists who specialize in the observation of physical phenomena and the analysis of experiments, theoretical physicists who specialize in mathematical modeling of physical systems to rationalize and predict natural phenomena. Physicists can apply their knowledge towards solving practical problems or to developing new technologies; the study and practice of physics is based on an intellectual ladder of discoveries and insights from ancient times to the present.
Many mathematical and physical ideas used today found their earliest expression in ancient Greek culture, for example in the work of Euclid, Thales of Miletus and Aristarchus. Roots emerged in ancient Asian culture and in the Islamic medieval period, for example the work of Alhazen in the 11th century; the modern scientific worldview and the bulk of physics education can be said to flow from the scientific revolution in Europe, starting with the work of Galileo Galilei and Johannes Kepler in the early 1600s. Newton's laws of motion and Newton's law of universal gravitation were formulated in the 17th century; the experimental discoveries of Faraday and the theory of Maxwell's equations of electromagnetism were developmental high points during the 19th century. Many physicists contributed to the development of quantum mechanics in the early-to-mid 20th century. New knowledge in the early 21st century includes a large increase in understanding physical cosmology; the broad and general study of nature, natural philosophy, was divided into several fields in the 19th century, when the concept of "science" received its modern shape.
Specific categories emerged, such as "biology" and "biologist", "physics" and "physicist", "chemistry" and "chemist", among other technical fields and titles. The term physicist was coined by William Whewell in his 1840 book The Philosophy of the Inductive Sciences. A standard undergraduate physics curriculum consists of classical mechanics and magnetism, non-relativistic quantum mechanics, statistical mechanics and thermodynamics, laboratory experience. Physics students need training in mathematics, in computer science. Any physics-oriented career position requires at least an undergraduate degree in physics or applied physics, while career options widen with a Master's degree like MSc, MPhil, MPhys or MSci. For research-oriented careers, students work toward a doctoral degree specializing in a particular field. Fields of specialization include experimental and theoretical astrophysics, atomic physics, biological physics, chemical physics, condensed matter physics, geophysics, gravitational physics, material science, medical physics, molecular physics, nuclear physics, radiophysics, electromagnetic field and microwave physics, particle physics, plasma physics.
The highest honor awarded to physicists is the Nobel Prize in Physics, awarded since 1901 by the Royal Swedish Academy of Sciences. National physics professional societies have many awards for professional recognition. In the case of the American Physical Society, as of 2017, there are 33 separate prizes and 38 separate awards in the field; the three major employers of career physicists are academic institutions and private industries, with the largest employer being the last. Physicists in academia or government labs tend to have titles such as Assistants, Professors, Sr./Jr. Scientist, or postdocs; as per the American Institute of Physics, some 20% of new physics Ph. D.s holds jobs in engineering development programs, while 14% turn to computer software and about 11% are in business/education. A majority of physicists employed apply their skills and training to interdisciplinary sectors. Job titles for graduate physicists include Agricultural Scientist, Air Traffic Controller, Computer Programmer, Electrical Engineer, Environmental Analyst, Medical Physicist, Oceanographer, Physics Teacher/Professor/Researcher, Research Scientist, Reactor Physicist, Engineering Physicist, Satellite Missions Analyst, Science Writer, Software Engineer, Systems Engineer, Microelectronics Engineer, Radar Developer, Technical Consultant, etc.
A majority of Physics terminal bachelor's degree holders are employed in the private sector. Other fields are academia and military service, nonprofit entities and teaching. Typical duties of physicists with master's and doctoral degrees working in their domain involve research and analysis, data preparation, instrumentation and development of industrial or medical equipment and software development, etc. Chartered Physicist is a chartered status and a professional qualification awarded by the Institute of Physics, it is denoted by the postnominals "CPhys". Achieving chartered status in any profession denotes to the wider community a high level of specialised subject knowledge and professional competence. According to the Institute of Physics, holders of the award of the Chartered Physicist demonst
A spring is an elastic object that stores mechanical energy. Springs are made of spring steel. There are many spring designs. In everyday use, the term refers to coil springs; when a conventional spring, without stiffness variability features, is compressed or stretched from its resting position, it exerts an opposing force proportional to its change in length. The rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring; that is, it is the gradient of the force versus deflection curve. An extension or compression spring's rate is expressed in units of force divided by distance, for example or N/m or lbf/in. A torsion spring is a spring. A torsion spring's rate is in units of torque divided by angle, such as N · ft · lbf/degree; the inverse of spring rate is compliance, that is: if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N. The stiffness of springs in parallel is additive. Springs are made from a variety of the most common being spring steel.
Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication. Some non-ferrous metals are used including phosphor bronze and titanium for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current. Simple non-coiled springs were used throughout human history. In the Bronze Age more sophisticated spring devices were used, as shown by the spread of tweezers in many cultures. Ctesibius of Alexandria developed a method for making bronze with spring-like characteristics by producing an alloy of bronze with an increased proportion of tin, hardening it by hammering after it was cast. Coiled springs appeared early in door locks; the first spring powered-clocks appeared in that century and evolved into the first large watches by the 16th century. In 1676 British physicist Robert Hooke postulated Hooke's law, which states that the force a spring exerts is proportional to its extension. Springs can be classified depending on how the load force is applied to them: Tension/extension spring – the spring is designed to operate with a tension load, so the spring stretches as the load is applied to it.
Compression spring – is designed to operate with a compression load, so the spring gets shorter as the load is applied to it. Torsion spring – unlike the above types in which the load is an axial force, the load applied to a torsion spring is a torque or twisting force, the end of the spring rotates through an angle as the load is applied. Constant spring – supported load remains the same throughout deflection cycle Variable spring – resistance of the coil to load varies during compression Variable stiffness spring – resistance of the coil to load can be dynamically varied for example by the control system,some types of these springs vary their length thereby providing actuation capability as well They can be classified based on their shape: Flat spring – this type is made of a flat spring steel. Machined spring – this type of spring is manufactured by machining bar stock with a lathe and/or milling operation rather than a coiling operation. Since it is machined, the spring may incorporate features in addition to the elastic element.
Machined springs can be made in the typical load cases of compression/extension, etc. Serpentine spring – a zig-zag of thick wire – used in modern upholstery/furniture. Garter spring - A coiled steel spring, connected at each end to create a circular shape; the most common types of spring are: Cantilever spring – a spring fixed only at one end. Coil spring or helical spring – a spring is of two types: Tension or extension springs are designed to become longer under load, their turns are touching in the unloaded position, they have a hook, eye or some other means of attachment at each end. Compression springs are designed to become shorter, their turns are not touching in the unloaded position, they need no attachment points. Hollow tubing springs can be either extension springs or compression springs. Hollow tubing is filled with oil and the means of changing hydrostatic pressure inside the tubing such as a membrane or miniature piston etc. to harden or relax the spring, much like it happens with water pressure inside a garden hose.
Alternatively tubing's cross-section is chosen of a shape that it changes its area when tubing is subjected to torsional deformation – change of the cross-section area translates into change of tubing's inside volume and the flow of oil in/out of the spring that can be controlled by valve thereby controlling stiffness. There are many other designs of springs of hollow tubing which can change stiffness with any desired frequency, change stiffness by a multiple or move like a linear actuator in addition to its spring qualities. Volute spring – a compression coil spring in the form of a cone so that under compression the coils are not forced against each other, thus permitting longer travel. Hairspring or balance spring – a delicate spiral spring used in watches and places where electricity must be carried to rotating devices such as steering wheels without hindering the rotation. Leaf spring – a flat spring used in vehicle suspensions, electrical switches, bows. V-spring – used in antique firearm mechanisms such as the wheellock and percussion cap locks.
Door-lock spring, as
Hooke's law is a law of physics that states that the force needed to extend or compress a spring by some distance x scales linearly with respect to that distance. That is: F s = k x, where k is a constant factor characteristic of the spring: its stiffness, x is small compared to the total possible deformation of the spring; the law is named after 17th-century British physicist Robert Hooke. He first stated the law in 1676 as a Latin anagram, he published the solution of his anagram in 1678 as: sic vis. Hooke states in the 1678 work that he was aware of the law in 1660. Hooke's equation holds in many other situations where an elastic body is deformed, such as wind blowing on a tall building, a musician plucking a string of a guitar, the filling of a party balloon. An elastic body or material for which this equation can be assumed is said to be linear-elastic or Hookean. Hooke's law is only a first-order linear approximation to the real response of springs and other elastic bodies to applied forces.
It must fail once the forces exceed some limit, since no material can be compressed beyond a certain minimum size, or stretched beyond a maximum size, without some permanent deformation or change of state. Many materials will noticeably deviate from Hooke's law well before those elastic limits are reached. On the other hand, Hooke's law is an accurate approximation for most solid bodies, as long as the forces and deformations are small enough. For this reason, Hooke's law is extensively used in all branches of science and engineering, is the foundation of many disciplines such as seismology, molecular mechanics and acoustics, it is the fundamental principle behind the spring scale, the manometer, the balance wheel of the mechanical clock. The modern theory of elasticity generalizes Hooke's law to say that the strain of an elastic object or material is proportional to the stress applied to it. However, since general stresses and strains may have multiple independent components, the "proportionality factor" may no longer be just a single real number, but rather a linear map that can be represented by a matrix of real numbers.
In this general form, Hooke's law makes it possible to deduce the relation between strain and stress for complex objects in terms of intrinsic properties of the materials it is made of. For example, one can deduce that a homogeneous rod with uniform cross section will behave like a simple spring when stretched, with a stiffness k directly proportional to its cross-section area and inversely proportional to its length. Consider a simple helical spring that has one end attached to some fixed object, while the free end is being pulled by a force whose magnitude is F s. Suppose that the spring has reached a state of equilibrium, where its length is not changing anymore. Let x be the amount by which the free end of the spring was displaced from its "relaxed" position. Hooke's law states that F s = k x or, equivalently, x = F s k where k is a positive real number, characteristic of the spring. Moreover, the same formula holds when the spring is compressed, with F s and x both negative in that case.
According to this formula, the graph of the applied force F s as a function of the displacement x will be a straight line passing through the origin, whose slope is k. Hooke's law for a spring is stated under the convention that F s is the restoring force exerted by the spring on whatever is pulling its free end. In that case, the equation becomes F s = − k x since the direction of the restoring force is opposite to that of the displacement. Hooke's spring law applies to any elastic object, of arbitrary complexity, as long as both the deformation and the stress can be expressed by a single number that can be both positive and negative. For example, when a block of rubber attached to two parallel plates is deformed by shearing, rather than stretching or compression, the shearing force F s and the sideways displacement of the plates x obey Hooke's law. Hooke's law applies when a straight steel bar or concrete beam, supported at both ends, is bent by a weight F placed at some intermediate point.
The displacement x in this case is the deviation of the beam, measured in the transversal direction, relative to its unloaded shape. The law applies when a stretched steel wire is twisted by pulling on a lever attached to one end. In this case the stress F s can be taken as the force applied to the lever, x as the distance traveled by it along its circular path. Or, one can let F s be the torque applied by the lever to the end of the wire, x be the angle by which that end turns. In either case F s is proportional to x In the case of a helical spring, stretched or compressed along its axis, the applied force and the resulting elongation or compression have the same direction (which is the directi
A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, explosions. Seismometers are combined with a timing device and a recording device to form a seismograph; the output of such a device — recorded on paper or film, now recorded and processed digitally — is a seismogram. Such data is used to locate and characterize earthquakes, to study the earth's internal structure. A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging from a spring, both suspended from a frame that moves along with any motion detected; the relative motion between the weight and the frame provides a measurement of the vertical ground motion. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a seismogram. Any movement of the ground moves the frame; the mass tends not to move because of its inertia, by measuring the movement between the frame and the mass, the motion of the ground can be determined.
Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop; the motion of the mass relative to the frame is measured, the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, recorded digitally. In other systems the weight is allowed to move, its motion produces an electrical charge in a coil attached to the mass which voltage moves through the magnetic field of a magnet attached to the frame; this design is used in a geophone, used in exploration for oil and gas. Seismic observatories have instruments measuring three axes: north-south, east-west, vertical. If only one axis is measured, it is the vertical because it is less noisy and gives better records of some seismic waves.
The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock; the best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. European seismographs were placed in a particular area after a destructive earthquake. Today, they are concentrated in high-risk regions; the word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake. Seismograph is another Greek term from γράφω, gráphō, to draw, it is used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined, than to modern systems, in which these functions are separated.
Both types provide a continuous record of ground motion. The technical discipline concerning such devices is called seismometry, a branch of seismology; the concept of measuring the "shaking" of something means that the word "seismograph" might be used in a more general sense. For example, a monitoring station that tracks changes in electromagnetic noise affecting amateur radio waves presents an rf seismograph, and Helioseismology studies the "quakes" on the Sun. The first seismometer was made in China during the 2nd Century; the first Western description of the device comes from the French physicist and priest Jean de Hautefeuille in 1703. The modern seismometer was developed in the 19th century. In December 2018, a seismometer was deployed on the planet Mars by the InSight lander, the first time a seismometer was placed onto the surface of another planet. In AD 132, Zhang Heng of China's Han dynasty invented the first seismoscope, called Houfeng Didong Yi; the description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter.
When there was an earthquake, one of the dragons' mouths would open and drop its ball into a bronze toad at the base, making a sound and showing the direction of the earthquake. On at least one occasion at the time of a large earthquake in Gansu in AD 143, the seismoscope indicated an earthquake though one was not felt; the available text says that inside the vessel was a central column that could move along eight tracks. The first earthquake recorded by this seismoscope was "somewhere in the east". Days a rider from the east reported this earthquake. By the 13th century, seismographic devices existed in the Maragheh observatory in Persia. French physicist and priest Jean de Hautefeuille built one in 1703. After 1880, most seismometers were descend