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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: : where is the current through the conductor in units of amperes, ''V'' is the voltage measured ''across'' the conductor in units of volts, and ''R'' is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the ''R'' in this relation is constant, independent of the current.〔 〕 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. He presented a slightly more complex equation than the one above (see History section below) to explain his experimental results. The above equation is the modern form of Ohm's law. In physics, the term ''Ohm's law'' is also used to refer to various generalizations of the law originally formulated by Ohm. The simplest example of this is: : where ''J'' is the current density at a given location in a resistive material, ''E'' is the electric field at that location, and ''σ'' (Sigma) is a material-dependent parameter called the conductivity. This reformulation of Ohm's law is due to Gustav Kirchhoff.〔Olivier Darrigol, (''Electrodynamics from Ampère to Einstein'' ), p.70, Oxford University Press, 2000 ISBN 0-19-850594-9.〕 ==History== 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" (current) varied directly as the "degree of electrification" (voltage). He did not communicate his results to other scientists at the time,〔(【引用サイトリンク】title=Electricity )〕 and his results were unknown until Maxwell published them in 1879.〔Sanford P. Bordeau (1982) ''Volts to Hertz...the Rise of Electricity.'' Burgess Publishing Company, Minneapolis, MN. pp.86–107, ISBN 0-8087-4908-0〕 Ohm did his work on resistance in the years 1825 and 1826, and published his results in 1827 as the book ''Die galvanische Kette, mathematisch bearbeitet'' ("The galvanic circuit investigated mathematically"). He drew considerable inspiration from Fourier's work on heat conduction in the theoretical explanation of his work. For experiments, he initially used voltaic piles, but later 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, and knew that the voltage between the thermocouple terminals was proportional to the junction temperature. He then added test wires of varying length, diameter, and material to complete the circuit. He found that his data could be modeled through the equation : where ''x'' was the reading from the galvanometer, ''l'' was the length of the test conductor, ''a'' depended only on the thermocouple junction temperature, and ''b'' was a constant of the entire setup. From this, Ohm determined his law of proportionality and published his results. Ohm's law was probably the most important of the early quantitative descriptions of the physics of electricity. We consider it almost obvious today. When Ohm first published his work, this was not the case; critics reacted to his treatment of the subject with hostility. They called his work a "web of naked fancies"〔 Davies, B, "A web of naked fancies?", ''Physics Education'' 15 57–61, Institute of Physics, Issue 1, Jan 1980 () 〕 and the German Minister of Education proclaimed that "a professor who preached such heresies was unworthy to teach science."〔Hart, IB, ''Makers of Science'', London, Oxford University Press, 1923. p. 243. ()〕 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, and that scientific truths may be deduced through reasoning alone.〔Herbert Schnädelbach, ''Philosophy in Germany 1831-1933'', pages 78-79, Cambridge University Press, 1984 ISBN 0521296463.〕 Also, Ohm's brother Martin, a mathematician, was battling the German educational system. These factors hindered the acceptance of Ohm's work, and his work did not become widely accepted until the 1840s. Fortunately, Ohm received recognition for his contributions to science well before he died. In the 1850s, Ohm's law was known as such and was widely considered proved, and alternatives, such as "Barlow's law", were discredited, in terms of real applications to telegraph system design, as discussed by Samuel F. B. Morse in 1855. While the old term for electrical conductance, the mho (the inverse of the resistance unit ohm), is still used, a new name, the siemens, was adopted in 1971, honoring Ernst Werner von Siemens. The siemens is preferred in formal papers. In the 1920s, it was discovered that the current through a practical resistor actually has statistical fluctuations, which depend on temperature, even when voltage and resistance are exactly constant; this fluctuation, now known as Johnson–Nyquist noise, is due to the discrete nature of charge. This thermal effect implies that measurements of current and voltage that are taken over sufficiently short periods of time will yield ratios of V/I that fluctuate from the value of R implied by the time average or ensemble average of the measured current; Ohm's law remains correct for the average current, in the case of ordinary resistive materials. Ohm's work long preceded Maxwell's equations and any understanding of frequency-dependent effects in AC circuits. Modern developments in electromagnetic theory and circuit theory do not contradict Ohm's law when they are evaluated within the appropriate limits. 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「Ohm's law」の詳細全文を読む スポンサード リンク
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