instance_of henry 
In ___MASK39___ and ___MASK98___ , inductance is the ___MASK71___ of an electrical conductor by which a change in current through it induces an electromotive force in both the conductor itself and in any nearby conductors by mutual inductance . Magnetism is a ___MASK67___ of physical phenomena that are mediated by magnetic fields . Electric currents and the magnetic moments of elementary particles give rise to a magnetic ___MASK97___ , which acts on other currents and magnetic moments . The most familiar effects occur in ferromagnetic materials , which are strongly attracted by magnetic fields and can be magnetized to become permanent magnets , producing magnetic fields themselves . Only a few substances are ferromagnetic ; the most common ones are iron , nickel and cobalt and their alloys . The prefix "" refers to iron , because permanent magnetism was first observed in lodestone , a form of natural iron ore called magnetite , FeO . Electrochemistry is the ___MASK17___ of physical chemistry that studies the ___MASK93___ between electricity , as a measurable and quantitative ___MASK78___ , and identifiable ___MASK72___ change , with either electricity considered an outcome of a particular ___MASK72___ change or vice versa . These reactions involve electric charges moving between electrodes and an electrolyte ( or ionic ___MASK21___ in a solution ). Thus electrochemistry deals with the interaction between electrical ___MASK87___ and ___MASK72___ change . In ___MASK26___ and electrical engineering , a conductor is an object or type of material that allows the flow of an electrical current in one or more directions . A metal ___MASK20___ is a common electrical conductor . Electrical current is generated by the flow of negatively charged electrons , positively charged holes , and positive or negative ions in some cases . Electromagnetic or Magnetic induction is the ___MASK28___ of an electromotive ___MASK14___ ( i . e ., voltage ) across an electrical ___MASK41___ due to its dynamic interaction with a magnetic ___MASK97___ . ___MASK98___ is the science of controlling electrical energy electrically , in which the electrons have a fundamental role . ___MASK98___ deals with electrical circuits that involve active electrical components such as vacuum tubes , transistors , diodes , integrated circuits , associated passive electrical components , and interconnection technologies . Commonly , electronic devices contain circuitry consisting primarily or exclusively of active semiconductors supplemented with passive elements ; such a circuit is described as an ___MASK81___ . The henry ( ___MASK70___ H ) is the unit of electrical inductance in the International System of Units . The unit is named after Joseph Henry ( 1797 -- 1878 ) , the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday ( 1791 -- 1867 ) in England . The magnetic permeability of vacuum is 4 × 10  7 H m - ___MASK4___ ( henry per metre ) . The United States National Institute of Standards and Technology recommends English - speaking users of SI to write the plural as henries . The National ___MASK86___ for the ___MASK65___ of Science organization was established in Washington , D . C . in May , 1840 , and was heir to the mantle of the earlier " Columbian Institute for the ___MASK65___ of Arts and Sciences ". The National ___MASK86___ for the ___MASK65___ of Science was later renamed the " National Institute " and eventually became a part of the Smithsonian ___MASK86___ . An ___MASK75___ is a type of magnet in which the magnetic ___MASK97___ is produced by an electric current . The magnetic ___MASK97___ disappears when the current is turned off . Electromagnets usually consist of insulated wire wound into a coil . A current through the wire creates a magnetic ___MASK97___ which is concentrated in the hole in the center of the coil . The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron ; the magnetic core concentrates the magnetic flux and makes a more powerful magnet . The farad ( ___MASK70___ : F ) is the ___MASK19___ of electrical capacitance , the ability of a body to store an electrical charge . It is named after the English physicist Michael Faraday . Michael Faraday FRS ( 22 ___MASK63___ 1791  ___MASK34___ August 1867 ) was an English scientist who contributed to the study of electromagnetism and electrochemistry . His main discoveries include the principles underlying electromagnetic induction , diamagnetism and electrolysis . A relay is an electrically operated switch . Many relays use an ___MASK75___ to mechanically operate a switch , but other operating principles are also used , such as solid - state relays . Relays are used where it is necessary to control a circuit by a separate low - power ___MASK11___ , or where several circuits must be controlled by one ___MASK11___ . The first relays were used in long distance telegraph circuits as amplifiers : they repeated the ___MASK11___ coming in from one circuit and re - transmitted it on another circuit . Relays were used extensively in telephone exchanges and early computers to perform logical operations . Joseph Henry ( ___MASK99___ 17 , 1797  May 13 , 1878 ) was an American scientist who served as the first Secretary of the Smithsonian Institution . He was the secretary for the National Institute for the ___MASK65___ of Science , a precursor of the Smithsonian Institution . He was highly regarded during his lifetime . While building electromagnets , Henry discovered the electromagnetic phenomenon of self - inductance . He also discovered mutual inductance independently of Michael Faraday , ( 1791  1867 ), though Faraday was the first to make the discovery and publish his results . Henry developed the electromagnet into a practical device . He invented a precursor to the electric doorbell ( specifically a bell that could be rung at a distance via an electric wire , 1831 ) and electric relay ( 1835 ). The SI unit of inductance , the henry , is named in his honor . Henry ' s work on the electromagnetic relay was the basis of the practical electrical telegraph , invented by Samuel F . B . Morse and Sir Charles Wheatstone , separately . An electrical telegraph is a telegraph that uses electrical signals , usually conveyed via dedicated telecommunication lines or ___MASK80___ . ___MASK39___ is a branch of ___MASK26___ involving the study of the electromagnetic force , a type of physical interaction that occurs between electrically charged particles . The electromagnetic force usually exhibits electromagnetic fields such as electric fields , magnetic fields , and light and is one of the four fundamental interactions ( commonly called forces ) in nature . The other three fundamental interactions are the strong interaction , the weak interaction , and gravitation . An ___MASK29___ is an atom or a ___MASK10___ in which the total number of electrons is not equal to the total number of protons , giving the atom or ___MASK10___ a net positive or negative electrical charge . Ions can be created , by either chemical or physical means , via ionization . A doorbell is a signaling ___MASK50___ typically placed near an entry ___MASK94___ to a ___MASK88___ . When a visitor presses a button the ___MASK57___ rings inside the ___MASK88___ , alerting the occupant to the presence of the visitor . Although the first doorbells were mechanical , activated by pulling a cord , modern doorbells are electric  they are actuated by an electric switch . The Smithsonian ___MASK86___ , established in 1846 " for the increase and diffusion of knowledge ," is a group of museums and research centers administered by the ___MASK96___ of the United States . Originally organized as the " United States National Museum ," that name ceased to exist as an administrative entity in 1967 . Termed " the nation ' s attic " for its eclectic holdings of 138 million items , the ___MASK86___ ' s nineteen museums , nine research centers , and zoo include historical and architectural landmarks , mostly located in the District of Columbia . Additional facilities are located in Arizona , Maryland , Massachusetts , New York City , Virginia , Texas , and Panama . More than 200 institutions and museums in 45 states , Puerto Rico , and Panama are Smithsonian Affiliates The ___MASK86___ ' s thirty million annual visitors are admitted without charge . The institution ' s annual ___MASK30___ is around $ 1 . 2 billion with 2 / 3 coming from annual federal appropriations . Other funding comes from the ___MASK86___ ' s endowment , private and corporate contributions , membership dues , and earned retail , concession , and licensing revenue . ___MASK86___ publications include " Smithsonian " and " Air & Space " magazines . Electromotive ___MASK14___ , also called emf ( denoted formula_1 and measured in volts ), is the voltage developed by any source of electrical ___MASK87___ such as a ___MASK64___ or dynamo . It is generally defined as the electrical potential for a source in a circuit . A device that supplies electrical ___MASK87___ is called electromotive ___MASK14___ or emf . Emfs convert chemical , mechanical , and other forms of ___MASK87___ into electrical ___MASK87___ . The product of such a device is also known as emf . In chemistry and ___MASK5___ , electrolysis is a technique that uses a direct electric current ( DC ) to drive an otherwise non - spontaneous ___MASK72___ reaction . Electrolysis is commercially important as a ___MASK49___ in the separation of elements from naturally occurring sources such as ores using an electrolytic cell . The voltage that is needed for electrolysis to occur is called the decomposition potential . Diamagnetic materials are repelled by a magnetic ___MASK97___ ; an applied magnetic ___MASK97___ creates an induced magnetic ___MASK97___ in them in the opposite ___MASK24___ , causing a repulsive force . In contrast , paramagnetic and ferromagnetic materials are attracted by a magnetic ___MASK97___ . Diamagnetism is a quantum mechanical effect that occurs in all materials ; when it is the only contribution to the magnetism the material is called diamagnetic . In paramagnetic and ferromagnetic substances the weak diamagnetic force is overcome by the attractive force of magnetic dipoles in the material . The magnetic permeability of diamagnetic materials is less than  , the permeability of vacuum . In most materials diamagnetism is a weak effect which can only be detected by sensitive ___MASK69___ instruments , but a superconductor acts as a strong diamagnet because it repels a magnetic ___MASK97___ entirely from its interior .