
A magnetic field is the magnetic effect of electric currents and magnetic materials. The magnetic field at any given point is specified by both a ''direction'' and a ''magnitude'' (or strength); as such it is a vector field.〔Strictly speaking, a magnetic field is a pseudo vector; pseudovectors, which also include torque and rotational velocity, are similar to vectors except that they remain unchanged when the coordinates are inverted.〕 The term is used for two distinct but closely related fields denoted by the symbols and , where is measured in units of amperes per meter (symbol: A·m^{−1} or A/m) in the SI. is measured in teslas (symbol:T) and newtons per meter per ampere (symbol: N·m^{−1}·A^{−1} or N/(m·A)) in the SI. is most commonly defined in terms of the Lorentz force it exerts on moving electric charges. Magnetic fields can be produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons. In everyday life, magnetic fields are most often encountered as a force created by permanent magnets, which pull on ferromagnetic materials such as iron, cobalt, or nickel, and attract or repel other magnets. Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. The Earth produces its own magnetic field, which is important in navigation, and it shields the Earth's atmosphere from solar wind. Rotating magnetic fields are used in both electric motors and generators. Magnetic forces give information about the charge carriers in a material through the Hall effect. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. ==History== (詳細はPetrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles.〔His ''Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt Militem de Magnete'', which is often shortened to ''Epistola de magnete'', is dated 1269 C.E.〕 Noting that the resulting field lines crossed at two points he named those points 'poles' in analogy to Earth's poles. He also clearly articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them. Almost three centuries later, William Gilbert of Colchester replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet. Published in 1600, Gilbert's work, ''De Magnete'', helped to establish magnetism as a science. In 1750, John Michell stated that magnetic poles attract and repel in accordance with an inverse square law. CharlesAugustin de Coulomb experimentally verified this in 1785 and stated explicitly that the north and south poles cannot be separated. Building on this force between poles, Siméon Denis Poisson (1781–1840) created the first successful model of the magnetic field, which he presented in 1824. In this model, a magnetic field is produced by 'magnetic poles' and magnetism is due to small pairs of north/south magnetic poles. Three discoveries challenged this foundation of magnetism, though. First, in 1819, Hans Christian Oersted discovered that an electric current generates a magnetic field encircling it. Then in 1820, AndréMarie Ampère showed that parallel wires having currents in the same direction attract one another. Finally, JeanBaptiste Biot and Félix Savart discovered the Biot–Savart law in 1820, which correctly predicts the magnetic field around any currentcarrying wire. Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model.〔 From the outside, the field of a dipole of magnetic charge has exactly the same form as a current loop when both are sufficiently small. Therefore, the two models differ only for magnetism inside magnetic material.〕 This has the additional benefit of explaining why magnetic charge can not be isolated. Further, Ampère derived both Ampère's force law describing the force between two currents and Ampère's law, which, like the Biot–Savart law, correctly described the magnetic field generated by a steady current. Also in this work, Ampère introduced the term electrodynamics to describe the relationship between electricity and magnetism. In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field. He described this phenomenon in what is known as Faraday's law of induction. Later, Franz Ernst Neumann proved that, for a moving conductor in a magnetic field, induction is a consequence of Ampère's force law. In the process he introduced the magnetic vector potential, which was later shown to be equivalent to the underlying mechanism proposed by Faraday. In 1850, Lord Kelvin, then known as William Thomson, distinguished between two magnetic fields now denoted and . The former applied to Poisson's model and the latter to Ampère's model and induction. Further, he derived how and relate to each other. The reason and are used for the two magnetic fields has been a source of some debate among science historians. Most agree that Kelvin avoided to prevent confusion with the SI fundamental unit of length, the Metre, abbreviated "m." However, some believe that Kelvin chose to honor his mentor Humphry Davy and chose because he and his wife never had children, and Kelvin strongly desired that his wife have a baby, and the word "baby" begins with "B." Others believe the choices were purely random.〔Kelvin (1900). "Kabinett physikalischer Raritäten." Page 200〕〔(Lord Kelvin of Largs ). physik.uniaugsburg.de. 26 June 1824〕 Between 1861 and 1865, James Clerk Maxwell developed and published Maxwell's equations, which explained and united all of classical electricity and magnetism. The first set of these equations was published in a paper entitled ''On Physical Lines of Force'' in 1861. These equations were valid although incomplete. Maxwell completed his set of equations in his later 1865 paper ''A Dynamical Theory of the Electromagnetic Field'' and demonstrated the fact that light is an electromagnetic wave. Heinrich Hertz experimentally confirmed this fact in 1887. The twentieth century extended electrodynamics to include relativity and quantum mechanics. Albert Einstein, in his paper of 1905 that established relativity, showed that both the electric and magnetic fields are part of the same phenomena viewed from different reference frames. (See moving magnet and conductor problem for details about the thought experiment that eventually helped Albert Einstein to develop special relativity.) Finally, the emergent field of quantum mechanics was merged with electrodynamics to form quantum electrodynamics (QED). ==Definitions, units, and measurement== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「magnetic field」の詳細全文を読む スポンサード リンク
