Resistance was often expressed as a multiple of the resistance of a standard length of telegraph wires different agencies used different bases for a standard, so units were not readily interchangeable. Telegraphers and other early users of electricity in the 19th century needed a practical standard unit of measurement for resistance. The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent, consistent, and international system of units for electrical quantities. Since the ohm belongs to a coherent system of units, when each of these quantities has its corresponding SI unit ( watt for P, ohm for R, volt for V and ampere for I, which are related as in § Definition) this formula remains valid numerically when these units are used (and thought of as being cancelled or omitted). Where alternating current is applied to the circuit (or where the resistance value is a function of time), the relation above is true at any instant, but calculation of average power over an interval of time requires integration of "instantaneous" power over that interval. Non-linear resistors have a value that may vary depending on the applied voltage (or current). Where P is the power, R is the resistance, V is the voltage across the resistor, and I is the current through the resistor.Ī linear resistor has a constant resistance value over all applied voltages or currents many practical resistors are linear over a useful range of currents. Ω = V A = 1 S = W A 2 = V 2 W = s F = H s = J ⋅ s C 2 = kg ⋅ m 2 s ⋅ C 2 = J s ⋅ A 2 = kg ⋅ m 2 s 3 ⋅ A 2 The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the conductor not being the seat of any electromotive force. One of the functions of many types of multimeters is the measurement of resistance in ohms.
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