Summary
The ohm (symbol: Ω, the uppercase Greek letter omega) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Simon Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861. Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined as an exact value in terms of these constants. The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), the conductor not being the seat of any electromotive force. in which the following additional units appear: siemens (S), watt (W), second (s), farad (F), henry (H), joule (J), coulomb (C), kilogram (kg), and meter (m). In many cases the resistance of a conductor is approximately constant within a certain range of voltages, temperatures, and other parameters. These are called linear resistors. In other cases resistance varies, such as in the case of the thermistor, which exhibits a strong dependence of its resistance with temperature. In the US, a double vowel in the prefixed units "kiloohm" and "megaohm" is commonly simplified, producing "kilohm" and "megohm". In alternating current circuits, electrical impedance is also measured in ohms. The siemens (S) is the SI derived unit of electric conductance and admittance, historically known as the "mho" (ohm spelled backwards, symbol is ℧); it is the reciprocal of the ohm: S=Ω-1. The power dissipated by a resistor may be calculated from its resistance, and the voltage or current involved.
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