7/31/2019 OHMMETER(English Version)

1/2

OHM-METERS

Part function of ohmmeters table:

no Parts name Function

1 Ohm sclale To show measures number

in Ohm unit.

2 Volt scale To show measures number

in Volt unit.

3 Current scale To show measures number

in Ampere unit4 Tumb adjusment To adjust a measurement

frequency

5 Out(+) or In(-) To plug and unplug of

cable to give ameasurement point object

6 scale To Show measurementpoint

7 neddle To show measures numberin multiple unit.

(Volt,Ohm,Ampere)

8 screw To return a screw to zeroadjustment

9 Range switch To show measuring limitpoint

10 switch To determine measuring oflimit point.

Ohm scale

Volt scale

scale

Tumb adjusment

neddle

screw

Range switch

switch

Current scale

OUT(+)

7/31/2019 OHMMETER(English Version)

2/2

n ohmmeter is an electrical instrument that measures electrical resistance, the opposition

to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make lowresistance 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 (

).The original design of an ohmmeter provided a small battery to apply a voltage to a resistance. Ituses 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 asresistance 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:

For high-precision measurements the above types of meter are inadequate. This is because themeter'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 dropdue 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. TheFour-terminal sensing method can also be utilized to conduct accurate measurements of low resistances.

A

http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Measuring_instrumenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Meggerhttp://en.wikipedia.org/wiki/%CE%A9http://en.wikipedia.org/wiki/%CE%A9http://en.wikipedia.org/wiki/%CE%A9http://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Galvanometerhttp://en.wikipedia.org/wiki/Ohm%27s_Lawhttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/Kelvin_bridgehttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Kelvin_bridgehttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Ohm%27s_Lawhttp://en.wikipedia.org/wiki/Galvanometerhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/%CE%A9http://en.wikipedia.org/wiki/Meggerhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Measuring_instrumenthttp://en.wikipedia.org/wiki/Electricity