An Ohmmeter is an electrical measuring instrument that measures electrical resistance, the opposition to the flow of an electric current. For songs called Electricity, see Electricity (song title). ... Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea In physics and engineering, measurement is the activity of comparing physical quantities of real-world objects and events. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... In electricity, current refers to electric current, which is the flow of electric charge. ...

The original design of an ohmmeter provided a small battery to apply a voltage to a resistance. It used a galvanometer to measure the electric current through the resistance. The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance decreased, the current through the meter would increase. This article may be too technical for most readers to understand. ... It has been suggested that Tangent galvanometer be merged into this article or section. ... In electricity, current refers to electric current, which is the flow of electric charge. ...

A more accurate type of ohmmeter has an electronic circuit that passes a constant current I through the resistance, and another circuit that measures the voltage V across the resistance. According to the following equation, derived from Ohm's Law, the value of the resistance R is given by: Ohms law, named after its discoverer Georg Ohm [1], states that the potential difference between two points, or equivalently the voltage drop from one point to a second point, usually designated by U or V, of a device capable of conducting an electrical current and the current I flowing...

For high-precision measurements the above types of meter are inadequate. This is because the meter's reading is the sum of the resistance of the measuring leads, the contact resistances and the resistance being measured. To reduce this effect, a precision ohmmeter has four terminals, called Kelvin contacts. Two terminals carry the current from the meter, while the other two allow the meter to measure the voltage across the resistor. With this type of meter, any voltage drop due to the resistance of the first pair of leads and their contact resistances is ignored by the meter. This four terminal measurement technique is called Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances. The Right Honourable William Thomson, 1st Baron Kelvin, GCVO, OM, PC, PRS (26 June 1824–17 December 1907) was a Irish-Scottish mathematical physicist and engineer, an outstanding leader in the physical sciences of the 19th century. ...

See also

• Ammeter
• Voltmeter
• Multimeter
• Meter (electronics)
• Measuring instrument

• Electronic_test_equipment
• Electronics
• Electric circuit
• List of electronics topics

• Series and parallel circuits

• Galvanometer
• Wheatstone bridge