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Ohmmeter
 
 
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An ohmmeter is an electrical
Electricity

Electricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena such as lightning and static electricity, but in addition, less familiar concepts such as the electromagnetic field and electromagnetic induction....
 instrument
Measuring instrument

In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantity of real-world object and phenomenon....
 that measures electrical resistance
Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
, the opposition to an electric current
Electric current

Electric current is the flow of electric charge. The electric charge may be either electrons or ions.The International System of Units unit of electric current intensity is the ampere....
. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or megger) measure large values of resistance. Measured in ohm (O
O

O is the fifteenth letter of the modern Latin alphabet. Its name in English language is spelled o , plural oes ....
)

The original design of an ohmmeter provided a small battery to apply a voltage
Voltage

Electrical tension is the potential difference between two points of an electrical or electronic circuit, expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor....
 to a resistance. It used a galvanometer
Galvanometer

A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an Analogue electronics electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil....
 to measure the electric current through the resistance.






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At which end of the ohmmeter scale is the resistance reading more reliable,the crowded or the uncrow...
At which end of the ohmmeter scale is the resistance reading more reliable,the crowded or the uncrow...








Encyclopedia


An ohmmeter is an electrical
Electricity

Electricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena such as lightning and static electricity, but in addition, less familiar concepts such as the electromagnetic field and electromagnetic induction....
 instrument
Measuring instrument

In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantity of real-world object and phenomenon....
 that measures electrical resistance
Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
, the opposition to an electric current
Electric current

Electric current is the flow of electric charge. The electric charge may be either electrons or ions.The International System of Units unit of electric current intensity is the ampere....
. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or megger) measure large values of resistance. Measured in ohm (O
O

O is the fifteenth letter of the modern Latin alphabet. Its name in English language is spelled o , plural oes ....
)

The original design of an ohmmeter provided a small battery to apply a voltage
Voltage

Electrical tension is the potential difference between two points of an electrical or electronic circuit, expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor....
 to a resistance. It used a galvanometer
Galvanometer

A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an Analogue electronics electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil....
 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.

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
Ohm's law

Ohm's law applies to electrical circuits; it states that the electric current through a conductor between two points is directly Proportionality to the potential difference or voltage across the two points, and inversely proportional to the Electrical resistance between them....
, the value of the resistance (R) is given by:



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 measurement
Four-terminal sensing

Four-terminal sensing or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of electric current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal sensing....
 technique is called Kelvin sensing, after William Thomson, Lord Kelvin
William Thomson, 1st Baron Kelvin

William Thomson, 1st Baron Kelvin , Order of Merit , Royal Victorian Order, Privy Council of the United Kingdom, Presidents of the Royal Society, Royal Society of Edinburgh, was an Ireland-born United Kingdom of Great Britain and Ireland Mathematical physics and engineer....
, who invented the Kelvin bridge
Kelvin bridge

A Kelvin bridge is a measuring instrument invented by William Thomson, 1st Baron Kelvin. It is used to measure an unknown electrical resistance below 1 Ohm....
 in 1861 to measure very low resistances. Four-point-probe can also be ultilized to conduct accurate measurements of low resistances.

See also

  • Ammeter
    Ammeter

    An ammeter is a measuring instrument used to measure the electric current in a Electrical circuit. Electric currents are measured in amperes , hence the name....
  • Voltmeter
    Voltmeter

    A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter....
  • Multimeter
    Multimeter

    A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an Electronics measuring instrument that combines several functions in one unit....
  • Meter (electronics)
    Meter (electronics)

    In electronics, a Measuring instrument is an instrument for displaying the magnitude of one of a wide variety of quantities in electrical circuits. The term can refer to either an electronic component that is part of a larger device, or a free-standing electronic test equipment....
  • Measuring instrument
    Measuring instrument

    In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantity of real-world object and phenomenon....


  • Electronic test equipment
    Electronic test equipment

    Electronic test equipment is used to create signals and capture responses from electronic Devices Under Test . In this way, the proper operation of the DUT can be proven or faults in the device can be traced and repaired....
  • Electronics
    Electronics

    Electronics refers to the flow of charge through nonmetal electrical conductor , whereas electrical refers to the flow of charge through metal electrical conductor....
  • Electric circuit
  • List of electronics topics
    List of electronics topics

    This is a list of communications, computers, electronic circuits, fiberoptics, microelectronics, medical electronics, reliability, and semiconductors....


  • Series and parallel circuits
    Series and parallel circuits

    In electronics, components of an electronic circuit can be connected in series or in parallel. Components connected in series are connected along a single path, so the same electric current flows through all of the components....


  • Galvanometer
    Galvanometer

    A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an Analogue electronics electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil....
  • Wheatstone bridge
    Wheatstone bridge

    A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843....