Ohm
The ohm is the standard unit of resistance for direct current, and the standard unit of impedance for alternating current. According to Ohm's law, one ohm is the resistance between two points that requires one volt of potential difference to produce one ampere of current. In the process, one watt of energy is converted into heat.
History of the Ohm
The ohm was formally defined at an international conference in 1881, and is named after Georg Simon Ohm (1787 – 1854), a French physicist. The symbol is the Greek capital letter Omega.
Impedance
Impedance is the total amount of resistance and reactance. Reactance occurs when a component that has inductance or capacitance causes an additional restriction to the alternating current. For example, a speaker has resistance due to the coil's wire, but also has reactance caused by the coil's inductance when powered by alternating current.
Measuring Ohms
Resistance is measured using an ohmmeter or multimeter. Impedance is usually measured by a another meter, but some multimeters can also measure it. The resistance of components should be measured outside of the circuit, so that other components don't influence the value. Insulators are tested using a megohmmeter, which can measure millions of ohms.
Milliohm, Kilohm and Megohm
A milliohm is one thousandth of an ohm, a kilohm is one thousand ohms, and a megohm is one million ohms. Most resistors are in the kilohm range, and anything above several megohms is considered an insulator.
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