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The ohm (symbol: Ω) 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, length 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. == Definition == The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.〔(BIPM SI Brochure: Appendix 1, p. 144 )〕 in which the following units appear: volt (V), ampere (A), siemens (S), watt (W), second (s), farad (F), joule (J), kilogram (kg), metre (m), and coulomb (C). In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters. These are called linear resistors. In other cases resistance varies (e.g., thermistors). Commonly used multiples and submultiples in electrical and electronic usage are the microohm, milliohm, kilohm, megohm, and gigaohm,〔The (''NIST Guide to the SI: 9.3 Spelling unit names with prefixes'' ) reports that IEEE/ASTM SI 10-2002 ''IEEE/ASTM Standard for Use of the International System of Units (SI): The Modern Metric System'' states that there are three cases in which the final vowel of an SI prefix is commonly omitted: megohm, kilohm, and hectare. "In all other cases in which the unit name begins with a vowel, both the final vowel of the prefix and the vowel of the unit name are retained and both are pronounced."〕 the term "gigohm", though not official, is in common use for the latter. In alternating current circuits, electrical impedance is also measured in ohms (Ω). 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」の詳細全文を読む スポンサード リンク
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