In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, infrared, ultraviolet, X-rays, gamma rays. Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light, which, in a vacuum, is denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave; the wavefront of electromagnetic waves emitted from a point source is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them electromagnetic induction and electrostatic induction phenomena. In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic force, responsible for all electromagnetic interactions.
Quantum electrodynamics is the theory of. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation; the energy of an individual photon is greater for photons of higher frequency. This relationship is given by Planck's equation E = hν, where E is the energy per photon, ν is the frequency of the photon, h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light; the effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules or break chemical bonds; the effects of these radiations on chemical systems and living tissue are caused by heating effects from the combined energy transfer of many photons. In contrast, high frequency ultraviolet, X-rays and gamma rays are called ionizing radiation, since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds.
These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, can be a health hazard. James Clerk Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry; because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. Maxwell's equations were confirmed by Heinrich Hertz through experiments with radio waves. According to Maxwell's equations, a spatially varying electric field is always associated with a magnetic field that changes over time. A spatially varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, vice versa; this relationship between the two occurs without either type of field causing the other.
In fact, magnetic fields can be viewed as electric fields in another frame of reference, electric fields can be viewed as magnetic fields in another frame of reference, but they have equal significance as physics is the same in all frames of reference, so the close relationship between space and time changes here is more than an analogy. Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again interact with the source; the distant EM field formed in this way by the acceleration of a charge carries energy with it that "radiates" away through space, hence the term. Maxwell's equations established that some charges and currents produce a local type of electromagnetic field near them that does not have the behaviour of EMR. Currents directly produce a magnetic field, but it is of a magnetic dipole type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential produce an electric dipole type electric
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
International System of Units
The International System of Units is the modern form of the metric system, is the most used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, second, kilogram, mole, a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units; the system specifies names for 22 derived units, such as lumen and watt, for other common physical quantities. The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, one physical artefact; the artefact is the international prototype kilogram, certified in 1889, consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units in terms of constants of nature, scheduled to be put into effect on 20 May 2019.
Derived units may be defined in terms of other derived units. They are adopted to facilitate measurement of diverse quantities; the SI is intended to be an evolving system. The most recent derived unit, the katal, was defined in 1999; the reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but on precise definition of those constants. The set of underlying constants is modified as more stable constants are found, or may be more measured. For example, in 1983 the metre was redefined as the distance that light propagates in vacuum in a given fraction of a second, thus making the value of the speed of light in terms of the defined units exact; the motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second systems and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures, established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and standardise the rules for writing and presenting measurements.
The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units rather than any variant of the CGS. Since the SI has been adopted by all countries except the United States and Myanmar; the International System of Units consists of a set of base units, derived units, a set of decimal-based multipliers that are used as prefixes. The units, excluding prefixed units, form a coherent system of units, based on a system of quantities in such a way that the equations between the numerical values expressed in coherent units have the same form, including numerical factors, as the corresponding equations between the quantities. For example, 1 N = 1 kg × 1 m/s2 says that one newton is the force required to accelerate a mass of one kilogram at one metre per second squared, as related through the principle of coherence to the equation relating the corresponding quantities: F = m × a. Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, thus are not independent.
Other useful derived quantities can be specified in terms of the SI base and derived units that have no named units in the SI system, such as acceleration, defined in SI units as m/s2. The SI base units are the building blocks of the system and all the other units are derived from them; when Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass and time. Giorgi identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units were added later; the early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are interchangeable, but in scientific contexts the difference matters. Mass the inertial mass, represents a quantity of matter, it relates the acceleration of a body to the applied force via Newton's law, F = m × a: force equals mass times acceleration. A force of 1 N applied to a mass of 1 kg will accelerate it at 1 m/s2.
This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is m × g. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision
A laser pointer or laser pen is a small handheld device with a power source and a laser diode emitting a narrow coherent low-powered laser beam of visible light, intended to be used to highlight something of interest by illuminating it with a small bright spot of colored light. Power is restricted in most jurisdictions not to exceed 5 mW; the small width of the beam and low power of typical laser pointers make the beam itself invisible in a reasonably clean atmosphere, only showing a point of light when striking an opaque surface. Some higher-powered laser pointers project a visible beam via scattering from dust particles or water droplets along the beam path. Higher-power and higher-frequency green or blue lasers may produce a beam visible in clean air because of Rayleigh scattering from air molecules when viewed in moderately-to-dimly lit conditions; the intensity of such scattering increases when these beams are viewed from angles near the beam axis. Such pointers in the green-light output range, are used as astronomical-object pointers for teaching purposes.
The low-cost availability of infrared diode laser modules of up to 1000 mW output has created a generation of IR-pumped, frequency doubled, green and violet diode-pumped solid-state laser pointers with visible power up to 300 mW. Because the invisible IR component in the beams of these visible lasers is difficult to filter out, because filtering it contributes extra heat, difficult to dissipate in a small pocket "laser pointer" package, it is left as a beam component in cheaper high-power pointers; this invisible IR component causes a degree of extra potential hazard in these devices when pointed at nearby objects and people. Laser pointers make a potent signaling tool in daylight, are able to produce a bright signal for potential search and rescue vehicles using an inexpensive and lightweight device of the type that could be carried in an emergency kit. If aimed at a person's eyes, laser pointers can cause temporary disturbances to vision. There is some evidence of rare minor permanent harm, but low-powered laser pointers are not hazardous to health.
They may be a major annoyance in some circumstances. In rare cases a dot of light from a red laser pointer may be thought to be due to a laser gunsight; when pointed at aircraft at night, laser pointers may dazzle and distract pilots, strict laws have been passed to ban this. Early laser pointers were helium–neon gas lasers and generated laser radiation at 633 nanometers designed to produce a laser beam with an output power under 1 milliwatt; the least expensive laser pointers use a deep-red laser diode near the 650 nm wavelength. More expensive ones use a red-orange 635 nm diode, more visible because of the greater sensitivity of the human eye at 635 nm. Other colors are possible too, with the 532 nm green laser being the most common alternative. Yellow-orange laser pointers, at 593.5 nm became available. In September 2005 handheld blue laser pointers at 473 nm became available. In early 2010 "Blu-ray" laser pointers at 405 nm went on sale; the apparent brightness of a spot from a laser beam depends on the optical power of the laser, the reflectivity of the surface, the chromatic response of the human eye.
For the same optical power, green laser light will seem brighter than other colors because the human eye is most sensitive at low light levels in the green region of the spectrum. Sensitivity decreases for shorter wavelengths; the output power of a laser pointer is stated in milliwatts. In the U. S. lasers are classified by the American National Standards Institute and Food and Drug Administration —see Laser safety#Classification for details. Visible laser pointers operating at less than 1 mW power are Class 2 or II, visible laser pointers operating with 1–5 mW power are Class 3A or IIIa. Class 3B or IIIb lasers generate between 5 and 500 mW; the US FDA Code of Federal Regulations stipulates that "demonstration laser products" such as pointers must comply with applicable requirements for Class I, IIa, II, or IIIa devices. These are the simplest pointers; the pointer is nothing more than a battery-powered laser diode. The first red laser pointers released in the early 1980s were large, unwieldy devices that sold for hundreds of dollars.
Today, they are much smaller and cost little. In the 21st century, diode-pumped solid-state red laser pointers emitting at 671 nm became available. Although this wavelength can be obtained directly with an inexpensive laser diode, higher beam quality and narrower spectral bandwidth are achieved through DPSS versions. Yellow laser pointers emitting at 593.5 nm became available in the last few years. Although they are based on the DPSS process, in this case two lasing lines of the ND:YVO4, 1064 nm and 1342 nm, are summed together with a nonlinear crystal; the complexity of this process makes these laser pointers inherently unstable and inefficient, with their outputs ranging from 1 mW to about 10 mW varying with temperature and mode-hopping if they get too hot or too cold. That is because such a complex process may require temperature stabilizers and active cooling, which can't be mounted into a small-sized host. Most smaller 593.5 nm pointers work in pulsed mode, so they can use smaller and less powerful pumping diodes.
New 589 nm yellow laser pointers have been introduced using a more robust and secretive method of harmonic generation from a DPSS laser system. This "sodium" wavelength, although only 4
British Science Association
The British Science Association is a charity and learned society founded in 1831 to aid in the promotion and development of science. Until 2009 it was known as the British Association for the Advancement of Science; the Chief Executive is Katherine Mathieson. In the present, the British Science Association's mission is to get more people engaged in the field of science by coordinating and overseeing different projects that are suited to achieve these goals. To maintain this vision of a world that puts science in the heart of today's culture and society, the British Science Association partners with many national and local organizations that share their vision. Diversifying the people involved in science increases the potential of being able to solve some of the world's biggest challenges in science and to do this the British Science Association are putting together a strategy for 2018-2020 to help them achieve their goals; these key components include: 1. Championing diversity and inclusion, 2.
Improving science education, 3. Influencing and convening stakeholders. Located in the Wellcome Wolfson Building, the BSA's professional team of staff works on creating and delivering a range of projects and events that both recognize and encourage people involved in science; these include the British Science Festival, British Science Week, the CREST Awards, Huxley Summit, Youth Pannel, Media Fellowships Scheme, along with regional and local events. The Association was founded in 1831 and modelled on the German Gesellschaft Deutscher Naturforscher und Ärzte, it was founded during post-war reconstruction after the Peninsula war to improve the advancement of science in England. The prime mover was Reverend William Vernon Harcourt, following a suggestion by Sir David Brewster, disillusioned with the elitist and conservative attitude of the Royal Society. Charles Babbage, William Whewell and J. F. W. Johnston are considered to be founding members; the first meeting was held in York on Tuesday 27 September 1831 with various scientific papers being presented on the following days.
It was chaired by Viscount Milton, President of the Yorkshire Philosophical Society, "upwards of 300 gentlemen" attended the meeting. The Preston Mercury recorded that those gathered consisted of "persons of distinction from various parts of the kingdom, together with several of the gentry of Yorkshire and the members of philosopher societies in this country"; the newspaper published the names of over a hundred of those attending and these included, amongst others, eighteen clergymen, eleven doctors, four knights, two Viscounts and one Lord. From that date onwards a meeting was held annually at a place chosen at a previous meeting. In 1832, for example, the meeting was held in Oxford, chaired by Reverend Dr William Buckland. By this stage the Association had four sections: Physics, Chemistry and Natural History. During this second meeting, the first objects and rules of the Association were published. Objects included systematically directing the acquisition of scientific knowledge, spreading this knowledge as well as discussion between scientists across the world, to focus on furthering science by removing obstacles to progress.
The rules established included what constituted a member of the Association, the fee to remain a member, the process for future meetings. They include dividing the members into different committees; these committees separated members into their preferred subject matter, were to recommend investigations into areas of interest report on these findings, as well as progress in their science at the annual meetings. Additional sections were added throughout the years by either splitting off part of an original section, like making Geography and Ethnology its own section apart from Geology in 1851, or by defining a new subject area of discussion, such as Anthropology in 1869. A important decision in the Association's history was made in 1842 when it was resolved to create a “physical observatory”. A building that became well known as the Kew Observatory was taken on for the purpose and Francis Ronalds was chosen as the inaugural Honorary Director. Kew Observatory became one of the most renowned meteorological and geomagnetic observatories in the world.
The Association relinquished control of the Kew Observatory in 1871 to the management of the Royal Society, after a large donation to grant the observatory its independence. In 1872, the Association purchased its first central office in London, acquiring four rooms at 22 Albemarle Street; this office was intended to be a resource for members of the Association. One of the most famous events linked to the Association Meeting was an exchange between Thomas Henry Huxley and Bishop Samuel Wilberforce in 1860. Although it is described as a "debate", the exchange occurred after the presentation of a paper by Prof Draper of New York, on the intellectual development of Europe with relation to Darwin's theory and the subsequent discussion involved a number of other participants. Although a number of newspapers made passing references to the exchange, it was not until that it was accorded greater significance in the evolution debate. One of the most important contributions of the British Association was the establishment of standards for electrical usage: the ohm as the unit of electrical resistance, the volt as the unit of electrical potential, the ampere as the unit of electrical current.
A need for standards a
An engine or motor is a machine designed to convert one form of energy into mechanical energy. Heat engines, like the internal combustion engine, burn a fuel to create heat, used to do work. Electric motors convert electrical energy into mechanical motion, pneumatic motors use compressed air, clockwork motors in wind-up toys use elastic energy. In biological systems, molecular motors, like myosins in muscles, use chemical energy to create forces and motion; the word engine derives from Old French engin, from the Latin ingenium–the root of the word ingenious. Pre-industrial weapons of war, such as catapults and battering rams, were called siege engines, knowledge of how to construct them was treated as a military secret; the word gin, as in cotton gin, is short for engine. Most mechanical devices invented during the industrial revolution were described as engines—the steam engine being a notable example. However, the original steam engines, such as those by Thomas Savery, were not mechanical engines but pumps.
In this manner, a fire engine in its original form was a water pump, with the engine being transported to the fire by horses. In modern usage, the term engine describes devices, like steam engines and internal combustion engines, that burn or otherwise consume fuel to perform mechanical work by exerting a torque or linear force. Devices converting heat energy into motion are referred to as engines. Examples of engines which exert a torque include the familiar automobile gasoline and diesel engines, as well as turboshafts. Examples of engines which produce thrust include rockets; when the internal combustion engine was invented, the term motor was used to distinguish it from the steam engine—which was in wide use at the time, powering locomotives and other vehicles such as steam rollers. The term motor derives from the Latin verb moto which means to maintain motion, thus a motor is a device. Motor and engine are interchangeable in standard English. In some engineering jargons, the two words have different meanings, in which engine is a device that burns or otherwise consumes fuel, changing its chemical composition, a motor is a device driven by electricity, air, or hydraulic pressure, which does not change the chemical composition of its energy source.
However, rocketry uses the term rocket motor though they consume fuel. A heat engine may serve as a prime mover—a component that transforms the flow or changes in pressure of a fluid into mechanical energy. An automobile powered by an internal combustion engine may make use of various motors and pumps, but all such devices derive their power from the engine. Another way of looking at it is that a motor receives power from an external source, converts it into mechanical energy, while an engine creates power from pressure. Simple machines, such as the club and oar, are prehistoric. More complex engines using human power, animal power, water power, wind power and steam power date back to antiquity. Human power was focused by the use of simple engines, such as the capstan, windlass or treadmill, with ropes and block and tackle arrangements; these were used in cranes and aboard ships in Ancient Greece, as well as in mines, water pumps and siege engines in Ancient Rome. The writers of those times, including Vitruvius and Pliny the Elder, treat these engines as commonplace, so their invention may be more ancient.
By the 1st century AD, cattle and horses were used in mills, driving machines similar to those powered by humans in earlier times. According to Strabo, a water powered mill was built in Kaberia of the kingdom of Mithridates during the 1st century BC. Use of water wheels in mills spread throughout the Roman Empire over the next few centuries; some were quite complex, with aqueducts and sluices to maintain and channel the water, along with systems of gears, or toothed-wheels made of wood and metal to regulate the speed of rotation. More sophisticated small devices, such as the Antikythera Mechanism used complex trains of gears and dials to act as calendars or predict astronomical events. In a poem by Ausonius in the 4th century AD, he mentions a stone-cutting saw powered by water. Hero of Alexandria is credited with many such wind and steam powered machines in the 1st century AD, including the Aeolipile and the vending machine these machines were associated with worship, such as animated altars and automated temple doors.
Medieval Muslim engineers employed gears in mills and water-raising machines, used dams as a source of water power to provide additional power to watermills and water-raising machines. In the medieval Islamic world, such advances made it possible to mechanize many industrial tasks carried out by manual labour. In 1206, al-Jazari employed a crank-conrod system for two of his water-raising machines. A rudimentary steam turbine device was described by Taqi al-Din in 1551 and by Giovanni Branca in 1629. In the 13th century, the solid rocket motor was invented in China. Driven by gunpowder, this simplest form of internal combustion engine was unable to deliver sustained power, but was useful for propelling weaponry at high speeds towards enemies in battle and for fireworks. After invention, this innovation spread throughout Europe; the Watt steam engine was the first type of steam engine to make use of steam at a pressure just above atmospheric to drive the piston he
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: I = V R, where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, R is the resistance of the conductor in units of ohms. More Ohm's law states that the R in this relation is constant, independent of the current. Ohm's law is an empirical relation which describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current; however some materials do not obey Ohm's law, these are called non-ohmic. The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire.
Ohm explained his experimental results by a more complex equation than the modern form above. In physics, the term Ohm's law is used to refer to various generalizations of the law; this reformulation of Ohm's law is due to Gustav Kirchhoff. In January 1781, before Georg Ohm's work, Henry Cavendish experimented with Leyden jars and glass tubes of varying diameter and length filled with salt solution, he measured the current by noting how strong a shock he felt as he completed the circuit with his body. Cavendish wrote that the "velocity" varied directly as the "degree of electrification", he did not communicate his results to other scientists at the time, his results were unknown until Maxwell published them in 1879. Francis Ronalds delineated “intensity” and “quantity” for the dry pile – a high voltage source – in 1814 using a gold-leaf electrometer, he found for a dry pile that the relationship between the two parameters was not proportional under certain meteorological conditions. Ohm did his work on resistance in the years 1825 and 1826, published his results in 1827 as the book Die galvanische Kette, mathematisch bearbeitet.
He drew considerable inspiration from Fourier's work on heat conduction in the theoretical explanation of his work. For experiments, he used voltaic piles, but used a thermocouple as this provided a more stable voltage source in terms of internal resistance and constant voltage, he used a galvanometer to measure current, knew that the voltage between the thermocouple terminals was proportional to the junction temperature. He added test wires of varying length and material to complete the circuit, he found that his data could be modeled through the equation x = a b + l, where x was the reading from the galvanometer, l was the length of the test conductor, a depended on the thermocouple junction temperature, b was a constant of the entire setup. From this, Ohm published his results. In modern notation we would write, I = E r + R, where E is the open-circuit emf of the thermocouple, r is the internal resistance of the thermocouple and R is the resistance of the test wire. In terms of the length of the wire this becomes, I = E r + R l, where R is the resistance of the test wire per unit length.
Thus, Ohm's coefficients are, a = E R, b = r R. Ohm's law was the most important of the early quantitative descriptions of the physics of electricity. We consider it obvious today; when Ohm first published his work, this was not the case. They called his work a "web of naked fancies" and the German Minister of Education proclaimed that "a professor who preached such heresies was unworthy to teach science." The prevailing scientific philosophy in Germany at the time asserted that experiments need not be performed to develop an understanding of nature because nature is so well ordered, that scientific truths may be deduced through reasoning alone. Ohm's brother Martin, a mathematician, was battling the German educational system; these factors hindered the acceptance of Ohm's work, his work did not become accepted until the 1840s. However, Ohm received recognition for his contributions to science. In the 1850s, Ohm's law was known as such and was considered proved, alternatives, such as "Barlow's law", were discredited, in terms of real ap