# Ohmmeter

Ohmmeter
An ohmmeter

An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or in the case of a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms (Ω).

The original design of an ohmmeter provided a small battery to apply a voltage to a resistance. It uses 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 is decreased, the current through the meter would increase. Ohmmeters form circuits by themselves, therefore they cannot be used within an assembled circuit.

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:

$R = \frac{V}{I}$

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 Four-terminal sensing method can also be utilized to conduct accurate measurements of low resistances.

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### Look at other dictionaries:

• Ohmmeter — Ohm me ter, n. [Ohm + meter.] (Elec.) An instrument for indicating directly the electrical resistance of a circuit in ohms. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

• Ohmmeter — Ohmmeter, s. Meßinstrumente, elektrotechnische …   Lexikon der gesamten Technik

• ohmmeter — [ō′mēt΄ər] n. [ OHM + METER] an instrument for measuring electrical resistance in ohms or megohms …   English World dictionary

• Ohmmeter — Ohm|me|ter 〈n. 13〉 1. Gerät zum Messen des elektr. Widerstandes in Ohm 2. 〈〉 SI Einheit, Maßeinheit für den spezifischen elektrischen Widerstand * * * Ohm|me|ter, das; s, [aus ↑ 2Ohm u. ↑ meter] (Physik): Gerät zum Messen elektrischer… …   Universal-Lexikon

• Ohmmeter — Einheit Norm SI Einheitensystem Einheitenname Ohmmeter Einheitenzeichen Ωm Beschriebene Größe(n) spezifischer elektrischer Widerstand Größensymbol(e) …   Deutsch Wikipedia

• ohmmeter — /ohm mee teuhr/, n. Elect. an instrument for measuring electric resistance in ohms. [1895 1900; OHM + METER] * * * ▪ instrument       instrument for measuring electrical resistance, which is expressed in ohms. In the simplest ohmmeters, the… …   Universalium

• Ohmmeter — ommetras statusas T sritis automatika atitikmenys: angl. ohmmeter vok. Ohmmeter, n rus. омметр, m pranc. ohmmètre, m …   Automatikos terminų žodynas

• ohmmeter — ommetras statusas T sritis automatika atitikmenys: angl. ohmmeter vok. Ohmmeter, n rus. омметр, m pranc. ohmmètre, m …   Automatikos terminų žodynas

• Ohmmeter — ommetras statusas T sritis Standartizacija ir metrologija apibrėžtis Elektrinės varžos, išreikštos omais, matuoklis. atitikmenys: angl. ohmmeter vok. Ohmmeter, n; Widerstandsmesser, n rus. омметр, m pranc. ohmmètre, m …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• ohmmeter — ommetras statusas T sritis Standartizacija ir metrologija apibrėžtis Elektrinės varžos, išreikštos omais, matuoklis. atitikmenys: angl. ohmmeter vok. Ohmmeter, n; Widerstandsmesser, n rus. омметр, m pranc. ohmmètre, m …   Penkiakalbis aiškinamasis metrologijos terminų žodynas