Difference between revisions of "Ohmmeter"

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{{Unreferenced|date=December 2009}}
 
{{Unreferenced|date=December 2009}}
 
[[Image:Ohmmeter.jpg|thumb|An ohmmeter]]
 
[[Image:Ohmmeter.jpg|thumb|An ohmmeter]]
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An '''ohmmeter''' is an [[electricity|electrical]] [[measuring instrument|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]]).
+
An '''ohmmeter''' is an [[electricity|electrical]] [[measuring instrument|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.
+
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:
+
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:
  
 
: <math> R = \frac{V}{I} </math>
 
: <math> R = \frac{V}{I} </math>
  
−
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 current drop due to the resistance of the first pair of leads and their contact resistances is ignored by the meter. This [[Four-terminal sensing|four terminal measurement]] technique is called Kelvin sensing, after [[William Thomson, 1st Baron Kelvin|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.  
+
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 sensing|four terminal measurement]] technique is called Kelvin sensing, after [[William Thomson, 1st Baron Kelvin|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.
  
 
==See also==
 
==See also==
 
{{Commons category|Ohmmeters}}
 
{{Commons category|Ohmmeters}}
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*[[Electrical Chuck]]
 
 
*[[Ammeter]]
 
*[[Ammeter]]
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*[[Volocity]]
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*[[Multimeter]]
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*[[Multimedia]]
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−
*[[Meter (electronics)]]
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−
*[[E-mail]] [[Scientology]]
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*[[Measuring instrument]]
 
*[[Measuring instrument]]
 
*[[Electronic test equipment]]
 
*[[Electronic test equipment]]
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*[[Series and parallel circuits]]
 
*[[Series and parallel circuits]]
 
*[[Galvanometer]]
 
*[[Galvanometer]]
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*[[Bigstone bridge]]
 
  
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==External links== 
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*[http://www.wisegeek.com/what-is-an-ohmmeter.htm What is an Ohmmeter? Guide by WiseGeek]
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{{Electrical and electronic measuring equipment}}
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*[http://www.cropico.co.uk Cropico - UK Market leader in Ohmmeter manufacture]
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[[Category:Electrical safety]]
 
[[Category:Electrical safety]]
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[[Category:Occupational safety and health]]
 
 
[[Category:Safety engineering]]
 
[[Category:Safety engineering]]
 
[[Category:Electronic test equipment]]
 
[[Category:Electronic test equipment]]
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[[ar:مقياس مقاومة]]
 
[[ar:مقياس مقاومة]]
 
[[bg:Омметър]]
 
[[bg:Омметър]]
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[[ca:Òhmmetre]]
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[[ca:Ohmímetre]]
 
[[cs:Ohmmetr]]
 
[[cs:Ohmmetr]]
 
[[da:Ohmmeter]]
 
[[da:Ohmmeter]]
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[[it:Ohmetro]]
 
[[it:Ohmetro]]
 
[[he:מד התנגדות]]
 
[[he:מד התנגדות]]
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[[kk:Омметр]]
 
[[nl:Ohmmeter]]
 
[[nl:Ohmmeter]]
 
[[ja:絶縁抵抗計]]
 
[[ja:絶縁抵抗計]]
 
[[pl:Omomierz]]
 
[[pl:Omomierz]]
 
[[pt:Ohmímetro]]
 
[[pt:Ohmímetro]]
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[[ro:Ohmmetru]]
 
[[ru:Омметр]]
 
[[ru:Омметр]]
 
[[sr:Омметар]]
 
[[sr:Омметар]]
 
[[ta:ஓம்மானி]]
 
[[ta:ஓம்மானி]]
 
[[tr:Ohmmetre]]
 
[[tr:Ohmmetre]]
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[[uk:Омметр]]
 
[[vi:Ôm kế]]
 
[[vi:Ôm kế]]
 
[[zh:歐姆計]]
 
[[zh:歐姆計]]
 
[[Category:2Fix]]
 
[[Category:2Fix]]

Latest revision as of 13:52, 10 December 2011

File:Ohmmeter.jpg
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:

<math> R = \frac{V}{I} </math>

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.

See also


ar:مقياس مقاومة

bg:Омметър ca:Ohmímetre cs:Ohmmetr da:Ohmmeter de:Widerstandsmessgerät el:Ωμόμετρο es:Óhmetro eo:Omometro fa:اهم‌متر fr:Ohmmètre hi:ओममापी id:Ohm-meter it:Ohmetro he:מד התנגדות kk:Омметр nl:Ohmmeter ja:絶縁抵抗計 pl:Omomierz pt:Ohmímetro ro:Ohmmetru ru:Омметр sr:Омметар ta:ஓம்மானி tr:Ohmmetre uk:Омметр vi:Ôm kế zh:歐姆計

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