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Mule:
By definition an ohmmeter is an instrument that measures resistance. In its simplest form it will be a DC instrument that supplies excitation to the resistance being measured. The ability to measure the resistance of a cable has nothing to do with the length of the wire in a long cable other than maybe the time you have to wait for the circuit to reach steady state.
To measure resistance with what is called an ohmmeter a DC current is applied to the resistance being measured and the voltage across the resistance is measured. From the current and voltage the resistance is determined.
In a simple instrument like a Simpson 260 there is essentially a battery of known voltage, a fixed series resistance, and a series microamp meter. When the meter leads are shorted together the resistance or source voltage is adjusted a minor amount for full scale deflection of the meter. This correspond to zero external resistance. With infinite external resistance the meter current is zero. In between these two limits the current is nonlinear vs resistance and thus the nonlinear resistance scale on the meter. Analysis of the series resistors provides an equation of Runknown = Vinternal/Ireading - Rinternal. For my newer, but still old, 270 half scale is 12 on the resistance range. The internal voltage for the 10,000 times range is 6 V. Meter full scale is 50 microamps. So at 25 microamps the total series resistance across the 6 V is Rtotal = 6/(25*10^6) = 240,000 ohms. Thus, Rinternal is 120,000 ohms. At 1/5 of meter full scale current what should the ohms scale read? See if your calculation agrees with the Simpson scale marking.
A Fluke 27 works in some different fashion. At a 10 megohm load the voltage is about 0.761 V, at 100K about 0.130 V, at 10K 0.114 V, at 1K 0.114 V, at 100 ohms 0.0478 V, and at 22.1 ohms about 0.0108 V. I suspect this is approximately a constant current source below a terminal voltage of about 0.114 V and some other characteristic above this inflection point. In turn it implies a ratioing measurement of voltage to current to calculate resistance.
You can measure any length cable you want with an ohmmeter. Knowing the resistance per foot of the wire you can estimate the cable length. However, for wire like #12 copper and a 100 ft circuit, 200 ft of wire, the resistance is about 1.6/5 = 0.3 ohms and hard to read on either a Simpson 260 or Fluke 27 with adequate accuracy to make a good estimate of length.
However, with 10 amps thru 10 ft of #12 I can estimate its length to better than a fraction of a foot with two Fluke 27s. Probably estimate to 1 inch.
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