The volt is the derived unit for electric potential, electric potential difference, electromotive force. It is named after the Italian physicist Alessandro Volta. One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points, it is equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, it is the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it, it can be expressed in terms of SI base units as V = potential energy charge = J C = kg ⋅ m 2 A ⋅ s 3. It can be expressed as amperes times ohms, watts per ampere, or joules per coulomb, equivalent to electronvolts per elementary charge: V = A ⋅ Ω = W A = J C = eV e; the "conventional" volt, V90, defined in 1987 by the 18th General Conference on Weights and Measures and in use from 1990, is implemented using the Josephson effect for exact frequency-to-voltage conversion, combined with the caesium frequency standard.
For the Josephson constant, KJ = 2e/h, the "conventional" value KJ-90 is used: K J-90 = 0.4835979 GHz μ V. This standard is realized using a series-connected array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz. Empirically, several experiments have shown that the method is independent of device design, measurement setup, etc. and no correction terms are required in a practical implementation. In the water-flow analogy, sometimes used to explain electric circuits by comparing them with water-filled pipes, voltage is likened to difference in water pressure. Current is proportional to the amount of water flowing at that pressure. A resistor would be a reduced diameter somewhere in the piping and a capacitor/inductor could be likened to a "U" shaped pipe where a higher water level on one side could store energy temporarily; the relationship between voltage and current is defined by Ohm's law. Ohm's Law is analogous to the Hagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems.
The voltage produced by each electrochemical cell in a battery is determined by the chemistry of that cell. See Galvanic cell § Cell voltage. Cells can be combined in series for multiples of that voltage, or additional circuitry added to adjust the voltage to a different level. Mechanical generators can be constructed to any voltage in a range of feasibility. Nominal voltages of familiar sources: Nerve cell resting potential: ~75 mV Single-cell, rechargeable NiMH or NiCd battery: 1.2 V Single-cell, non-rechargeable: alkaline battery: 1.5 V. Some antique vehicles use 6.3 volts. Electric vehicle battery: 400 V when charged Household mains electricity AC: 100 V in Japan 120 V in North America, 230 V in Europe, Asia and Australia Rapid transit third rail: 600–750 V High-speed train overhead power lines: 25 kV at 50 Hz, but see the List of railway electrification systems and 25 kV at 60 Hz for exceptions. High-voltage electric power transmission lines: 110 kV and up Lightning: Varies often around 100 MV.
In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In 1861, Latimer Clark and Sir Charles Bright coined the name "volt" for the unit of resistance. By 1873, the British Association for the Advancement of Science had defined the volt and farad. In 1881, the International Electrical Congress, now the International Electrotechnical Commission, approved the volt as the unit for electromotive force, they made the volt equal to 108 cgs units of voltage
The kilogram or kilogramme is the base unit of mass in the International System of Units. Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram, manufactured in 1889, stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants; the kilogram was defined as the mass of a litre of water. That was an inconvenient quantity to replicate, so in 1799 a platinum artefact was fashioned to define the kilogram; that artefact, the IPK, have been the standard of the unit of mass for the metric system since. In spite of best efforts to maintain it, the IPK has diverged from its replicas by 50 micrograms since their manufacture late in the 19th century; this led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena, now scheduled to take place in 2019. The new definition is based on invariant constants of nature, in particular the Planck constant, which will change to being defined rather than measured, thereby fixing the value of the kilogram in terms of the second and the metre, eliminating the need for the IPK.
The new definition was approved by the General Conference on Weights and Measures on 16 November 2018. The Planck constant relates a light particle's energy, hence mass, to its frequency; the new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram. The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice; the final kilogram, manufactured as a prototype in 1799 and from which the International Prototype Kilogram was derived in 1875, had a mass equal to the mass of 1 dm3 of water under atmospheric pressure and at the temperature of its maximum density, 4 °C. The kilogram is the only named SI unit with an SI prefix as part of its name; until the 2019 redefinition of SI base units, it was the last SI unit, still directly defined by an artefact rather than a fundamental physical property that could be independently reproduced in different laboratories.
Three other base units and 17 derived units in the SI system are defined in relation to the kilogram, thus its stability is important. The definitions of only eight other named SI units do not depend on the kilogram: those of temperature and frequency, angle; the IPK is used or handled. Copies of the IPK kept by national metrology laboratories around the world were compared with the IPK in 1889, 1948, 1989 to provide traceability of measurements of mass anywhere in the world back to the IPK; the International Prototype Kilogram was commissioned by the General Conference on Weights and Measures under the authority of the Metre Convention, in the custody of the International Bureau of Weights and Measures who hold it on behalf of the CGPM. After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature.
At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, h. The decision was deferred until 2014. CIPM has proposed revised definitions of the SI base units, for consideration at the 26th CGPM; the formal vote, which took place on 16 November 2018, approved the change, with the new definitions coming into force on 20 May 2019. The accepted redefinition defines the Planck constant as 6.62607015×10−34 kg⋅m2⋅s−1, thereby defining the kilogram in terms of the second and the metre. Since the second and metre are defined in terms of physical constants, the kilogram is defined in terms of physical constants only; the avoirdupois pound, used in both the imperial and US customary systems, is now defined in terms of the kilogram. Other traditional units of weight and mass around the world are now defined in terms of the kilogram, making the kilogram the primary standard for all units of mass on Earth; the word kilogramme or kilogram is derived from the French kilogramme, which itself was a learned coinage, prefixing the Greek stem of χίλιοι khilioi "a thousand" to gramma, a Late Latin term for "a small weight", itself from Greek γράμμα.
The word kilogramme was written into French law in 1795, in the Decree of 18 Germinal, which revised the older system of units introduced by the French National Convention in 1793, where the gravet had been defined as weight of a cubic centimetre of water, equal to 1/1000 of a grave. In the decree of 1795, the term gramme thus replaced gravet, kilogramme replaced grave; the French spelling was adopted in Great Britain when the word was used for the first time in English in 1795, with the spelling kilogram being adopted in the United States. In the United Kingdom both spellings are used, with "kilogram" having become by far the more common. UK law regulating the units to be used when trading by weight or measure does not prevent the use of either spelling. In the 19th century the French word kilo, a shortening of kilogramme, was imported into the English language where it has been used to mean both kilogram and kilometre. While kilo is acceptable in many generalist texts
The ohm is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass and time and of a convenient size for practical work as early as 1861; the definition of the ohm was revised several times. Today, the definition of the ohm is expressed from the quantum Hall effect; the ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the conductor not being the seat of any electromotive force. Ω = V A = 1 S = W A 2 = V 2 W = s F = H s = J ⋅ s C 2 = kg ⋅ m 2 s ⋅ C 2 = J s ⋅ A 2 = kg ⋅ m 2 s 3 ⋅ A 2 in which the following units appear: volt, siemens, second, henry, kilogram and coulomb.
In many cases the resistance of a conductor in ohms is constant within a certain range of voltages and other parameters. These are called linear resistors. In other cases resistance varies. A vowel of the prefixed units kiloohm and megaohm is omitted, producing kilohm and megohm. In alternating current circuits, electrical impedance is measured in ohms; the siemens is the SI derived unit of electric conductance and admittance known as the mho. The power dissipated by a resistor may be calculated from its resistance, the voltage or current involved; the formula is a combination of Ohm's law and Joule's law: P = V ⋅ I = V 2 R = I 2 ⋅ R where: P is the power R is the resistance V is the voltage across the resistor I is the current through the resistorA linear resistor has a constant resistance value over all applied voltages or currents. Non-linear resistors have a value. Where alternating current is applied to the circuit, the relation above is true at any instant but calculation of average power over an interval of time requires integration of "instantaneous" power over that interval.
Since the ohm belongs to a coherent system of units, when each of these quantities has its corresponding SI unit (watt for P, ohm for R, volt for V and ampere for I, which are related as in § Definition, this formula remains valid numerically when these units are used. The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent and international system of units for electrical quantities. Telegraphers and other early users of electricity in the 19th century needed a practical standard unit of measurement for resistance. Resistance was expressed as a multiple of the resistance of a standard length of telegraph wires. Electrical units so defined were not a coherent system with the units for energy, mass and time, requiring conversion factors to be used in calculations relating energy or power to resistance. Two different methods of establishing a system of electrical units can be chosen. Various artifacts, such as a length of wire or a standard electrochemical cell, could be specified as producing defined quantities for resistance, so on.
Alternatively, the electrical units can be related to the mechanical units by defining, for example, a unit of current that gives a specified force between two wires, or a unit of charge that gives a unit of force between two unit charges. This latter method ensures coherence with the units of energy. Defining a unit for resistance, coherent with units of energy and time in effect requires defining units for potential and current, it is desirable that one unit of electrical potential will force one unit of electric current through one unit of electrical resistance, doing one unit of work in one unit of time, otherwi
International Exposition of Electricity
The first International Exposition of Electricity in Paris ran from August 15, 1881 through to November 15, 1881 at the Palais de l'Industrie on the Champs-Élysées. It served to display the advances in electrical technology since the small electrical display at the 1878 Universal Exposition. Exhibitors came from the United Kingdom, United States, Germany and the Netherlands, as well as from France; as part of the exhibition, the first International Congress of Electricians presented numerous scientific and technical papers, including definitions of the standard practical units volt and ampere. Adolphe Cochery, Minister of Posts and Telegraphs of the time, had suggested that an international exposition should be held; this show was a great stir. The public could admire the dynamo of Zénobe Gramme, the incandescent light, the Théâtrophone, the electric tramway of Werner von Siemens, the telephone of Alexander Graham Bell, an electrical distribution network by Marcel Deprez, an electric boat by Gustave Trouvé.
As part of the exhibition, the first International Congress of Electricians, which met in the halls of the Palais du Trocadero, presented numerous scientific and technical papers, including definitions of the standard practical units volt and ampere, the International System of Electrical and Magnetic Units. "The Exposition boasted a gallery of paintings illuminated by incandescent lamps, an engraving of which appeared in La Lumière électrique 4, 1881." George Berger was the Commissioner General. Aside from the provision of the building by the French government, the exhibition was financed. Organizers would donate profits to scientific works in the public interest; this congress was a decisive step in the building of the modern International System of Units, since ohm, ampere and farad were defined at this occasion. Main participants include Éleuthère Mascart, William Thomson, Hermann von Helmholtz, Rudolf Clausius, Gustav Kirchhoff, Gustav Heinrich Wiedemann, Carl Wilhelm Siemens and his brother the industrialist Werner von Siemens, who had to renounce to the siemens mercury as the resistance unit.
Among the exhibits were: Apparatus for production and transmission of electricity and artificial magnets, compasses, devices used in the study of electricity, many applications of electricity, old instruments in connection with electricity. Electric lighting with incandescent lamps was one of key developments on display at the exposition, with up to 2500 lamps used to light the venue; the lamps of Thomas Edison, St. George Lane-Fox, Hiram Maxim, Joseph Swan were compared in extensive tests by a committee, including exposition juror William Crookes, to establish the most efficient lamp design; the conclusion was the high resistance Edison lamp was the most efficient, followed by the Lane-Fox and Maxim lamps. Edison's agents had lobbied to set test conditions favorable to his lamp. CNAM. "Exposition internationale d'Électricité". Retrieved Aug 16, 2008. Gérard Borvon, Histoire de l'électricité, de l'ambre à l'électron, Vuibert, 2009, ISBN 978-2-7117-2492-5 Gérard Borvon. First International Exposition of Electricity, Paris, 1881 1881: first international congress of electricians, first international electrical units system
Silver nitrate is an inorganic compound with chemical formula AgNO3. This compound is a versatile precursor to many other silver compounds, such as those used in photography, it is far less sensitive to light than the halides. It was once called lunar caustic because silver was called luna by the ancient alchemists, who believed that silver was associated with the moon. In solid silver nitrate, the silver ions are three-coordinated in a trigonal planar arrangement. Albertus Magnus, in the 13th century, documented the ability of nitric acid to separate gold and silver by dissolving the silver. Magnus noted. Silver nitrate can be prepared by reacting silver, such as a silver bullion or silver foil, with nitric acid, resulting in silver nitrate and oxides of nitrogen. Reaction byproducts depend upon the concentration of nitric acid used. 3 Ag + 4 HNO3 → 3 AgNO3 + 2 H2O + NO Ag + 2 HNO3 → AgNO3 + H2O + NO2This is performed under a fume hood because of toxic nitrogen oxides evolved during the reaction.
A typical reaction with silver nitrate is to suspend a rod of copper in a solution of silver nitrate and leave it for a few hours. The silver nitrate reacts with copper to form hairlike crystals of silver metal and a blue solution of copper nitrate: 2 AgNO3 + Cu → Cu2 + 2 AgSilver nitrate decomposes when heated: 2 AgNO3 → 2 Ag + O2 + 2 NO2Qualitatively, decomposition is negligible below the melting point, but becomes appreciable around 250 °C and decompose at 440 °C. Most metal nitrates thermally decompose to the respective oxides, but silver oxide decomposes at a lower temperature than silver nitrate, so the decomposition of silver nitrate yields elemental silver instead. Silver nitrate is the least expensive salt of silver, it is non-hygroscopic, in contrast to silver silver perchlorate. It is stable to light, it dissolves in numerous solvents, including water. The nitrate can be replaced by other ligands, rendering AgNO3 versatile. Treatment with solutions of halide ions gives a precipitate of AgX.
When making photographic film, silver nitrate is treated with halide salts of sodium or potassium to form insoluble silver halide in situ in photographic gelatin, applied to strips of tri-acetate or polyester. Silver nitrate is used to prepare some silver-based explosives, such as the fulminate, azide, or acetylide, through a precipitation reaction. Treatment of silver nitrate with base gives dark grey silver oxide: 2 AgNO3 + 2 NaOH → Ag2O + 2 NaNO3 + H2O The silver cation, Ag+, reacts with halide sources to produce the insoluble silver halide, a cream precipitate if Br- is used, a white precipitate if Cl− is used and a yellow precipitate if I− is used; this reaction is used in inorganic chemistry to abstract halides: Ag+ + X− → AgXwhere X− = Cl−, Br−, or I−. Other silver salts with non-coordinating anions, namely silver tetrafluoroborate and silver hexafluorophosphate are used for more demanding applications; this reaction is used in analytical chemistry to confirm the presence of chloride, bromide, or iodide ions.
Samples are acidified with dilute nitric acid to remove interfering ions, e.g. carbonate ions and sulfide ions. This step avoids confusion of silver sulfide or silver carbonate precipitates with that of silver halides; the color of precipitate varies with the halide: yellow. AgBr and AgI photo-decompose to the metal, as evidence by a grayish color on exposed samples; the same reaction was used on steamships in order to determine whether or not boiler feedwater had been contaminated with seawater. It is still used to determine if moisture on dry cargo is a result of condensation from humid air, or from seawater leaking through the hull. Silver nitrate is used in many ways in e.g. for deprotection and oxidations. Ag+ binds alkenes reversibly, silver nitrate has been used to separate mixtures of alkenes by selective absorption; the resulting adduct can be decomposed with ammonia to release the free alkene. Silver Nitrate is soluble in water but is poorly soluble in most organic solvents, except acetonitrile.
In histology, silver nitrate is used for silver staining, for demonstrating reticular fibers and nucleic acids. For this reason it is used to demonstrate proteins in PAGE gels, it can be used as a stain in scanning electron microscopy. Silver salts have antiseptic properties. In 1881 Credé introduced the use of dilute solutions of AgNO3 in newborn babies' eyes at birth to prevent contraction of gonorrhea from the mother, which could cause blindness. Fused silver nitrate, shaped into sticks, was traditionally called "lunar caustic", it is used as a cauterizing agent, for example to remove granulation tissue around a stoma. General Sir James Abbott noted in his journals that in India in 1827 it was infused by a British surgeon into wounds in his arm resulting from the bite of a mad dog to cauterize the wounds and prevent the onset of rabies. Silver nitrate is used to cauterize superficial blood vessels in the nose to help prevent nose bleeds. Dentists sometimes use silver nitrate-infused swabs to heal oral ulcers.
Silver nitrate is used by some podiatrists to kill cells located in the nail bed. The Canadian physician C. A. Douglas Ringrose researched the use of silver nitrate for sterilization procedures, believing that silver nitrate could be used to block and corrode the fallopian tubes; the technique was ineffective. Much research has been done in evaluating the
Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force exhibits electromagnetic fields such as electric fields, magnetic fields, light, is one of the four fundamental interactions in nature; the other three fundamental interactions are the strong interaction, the weak interaction, gravitation. At high energy the weak force and electromagnetic force are unified as a single electroweak force. Electromagnetic phenomena are defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as different manifestations of the same phenomenon; the electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life. Ordinary matter takes its form as a result of intermolecular forces between individual atoms and molecules in matter, is a manifestation of the electromagnetic force.
Electrons are bound by the electromagnetic force to atomic nuclei, their orbital shapes and their influence on nearby atoms with their electrons is described by quantum mechanics. The electromagnetic force governs all chemical processes, which arise from interactions between the electrons of neighboring atoms. There are numerous mathematical descriptions of the electromagnetic field. In classical electrodynamics, electric fields are described as electric potential and electric current. In Faraday's law, magnetic fields are associated with electromagnetic induction and magnetism, Maxwell's equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents; the theoretical implications of electromagnetism the establishment of the speed of light based on properties of the "medium" of propagation, led to the development of special relativity by Albert Einstein in 1905. Electricity and magnetism were considered to be two separate forces; this view changed, with the publication of James Clerk Maxwell's 1873 A Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be mediated by one force.
There are four main effects resulting from these interactions, all of which have been demonstrated by experiments: Electric charges attract or repel one another with a force inversely proportional to the square of the distance between them: unlike charges attract, like ones repel. Magnetic poles attract or repel one another in a manner similar to positive and negative charges and always exist as pairs: every north pole is yoked to a south pole. An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire, its direction depends on the direction of the current in the wire. A current is induced in a loop of wire when it is moved toward or away from a magnetic field, or a magnet is moved towards or away from it. While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation; as he was setting up his materials, he noticed a compass needle deflected away from magnetic north when the electric current from the battery he was using was switched on and off.
This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, that it confirmed a direct relationship between electricity and magnetism. At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months he began more intensive investigations. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire; the CGS unit of magnetic induction is named in honor of his contributions to the field of electromagnetism. His findings resulted in intensive research throughout the scientific community in electrodynamics, they influenced French physicist André-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery represented a major step toward a unified concept of energy.
This unification, observed by Michael Faraday, extended by James Clerk Maxwell, reformulated by Oliver Heaviside and Heinrich Hertz, is one of the key accomplishments of 19th century mathematical physics. It has had far-reaching consequences, one of, the understanding of the nature of light. Unlike what was proposed by the electromagnetic theory of that time and other electromagnetic waves are at present seen as taking the form of quantized, self-propagating oscillatory electromagnetic field disturbances called photons. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies. Ørsted was not the only person to examine the relationship between magnetism. In 1802, Gian Domenico Romagnosi, an Italian legal scholar, deflected a magnetic needle using a Voltaic pile; the factual setup of the experiment is not clear, so if current flew across the needle or not.
An account of the discovery was published in 1802 in an Italian newspaper, but it was overlooked by the contemporary scientific community, because Romagnosi did not belong to this community. An earlier, neglected, connec
System of measurement
A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have been important and defined for the purposes of science and commerce. Systems of measurement in use include the International System of Units, the modern form of the metric system, the imperial system, United States customary units; the French Revolution gave rise to the metric system, this has spread around the world, replacing most customary units of measure. In most systems, length and time are base quantities. Science developments showed that either electric charge or electric current could be added to extend the set of base quantities by which many other metrological units could be defined. Other quantities, such as power and speed, are derived from the base set: for example, speed is distance per unit time. A wide range of units was used for the same type of quantity: in different contexts, length was measured in inches, yards, rods, furlongs, nautical miles, leagues, with conversion factors which were not powers of ten.
Such arrangements were satisfactory in their own contexts. The preference for a more universal and consistent system only spread with the growth of science. Changing a measurement system has substantial financial and cultural costs which must be offset against the advantages to be obtained from using a more rational system; however pressure built up, including from scientists and engineers for conversion to a more rational, internationally consistent, basis of measurement. In antiquity, systems of measurement were defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size itself defined in hands and knuckles; the unifying characteristic is. Cubits and strides gave way to "customary units" to meet the needs of merchants and scientists. In the metric system and other recent systems, a single basic unit is used for each base quantity.
Secondary units are derived from the basic units by multiplying by powers of ten, i.e. by moving the decimal point. Thus the basic metric unit of length is the metre. Metrication is complete or nearly complete in all countries. US customary units are used in the United States and to some degree in Liberia. Traditional Burmese units of measurement are used in Burma. U. S. units are used in limited contexts in Canada due to the large volume of trade. A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom – whose road signage legislation, for instance, only allows distance signs displaying imperial units – or Hong Kong. In the United States, metric units are used universally in science in the military, in industry, but customary units predominate in household use. At retail stores, the liter is a used unit for volume on bottles of beverages, milligrams, rather than grains, are used for medications; some other standard non-SI units are still in international use, such as nautical miles and knots in aviation and shipping.
Metric systems of units have evolved since the adoption of the first well-defined system in France in 1795. During this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, to English speaking countries. Multiples and submultiples of metric units are related by powers of ten and their names are formed with prefixes; this relationship is compatible with the decimal system of numbers and it contributes to the convenience of metric units. In the early metric system there were the metre for length and the gram for mass; the other units of length and mass, all units of area and derived units such as density were derived from these two base units. Mesures usuelles were a system of measurement introduced as a compromise between the metric system and traditional measurements, it was used in France from 1812 to 1839. A number of variations on the metric system have been in use; these include gravitational systems, the centimetre–gram–second systems useful in science, the metre–tonne–second system once used in the USSR and the metre–kilogram–second system.
The current international standard metric system is the International System of Units It is an mks system based on the metre and second as well as the kelvin, ampere and mole. The SI includes two classes of units which are agreed internationally; the first of these classes includes the seven SI base units for length, time, electric current, luminous intensity and amount of substance. The second class consists of the SI derived units; these derived. All other quantities are expressed in terms of SI derived units. Both imperial units and US customary units derive from earlier English units. Imperial units were used in the former British Empire and