4.5. Operating principle
Suppose we wished to measure the resistance of some component
located a significant distance away from our ohmmeter. Such a
scenario would be problematic, because an ohmmeter measures all
resistance in the circuit loop, which includes the resistance of the
wires (R
wire
) connecting the ohmmeter to the component being
measured (R
subject
):
Usually, wire resistance is very small (only a few ohms per
hundreds of feet, depending primarily on the gauge (size) of the
wire), but if the connecting wires are very long, and/or the
component to be measured has a very low resistance anyway, the
measurement error introduced by wire resistance will be substantial.
An ingenious method of measuring the subject resistance in a
situation like this involves the use of both an ammeter and a
voltmeter. We know from Ohm's Law that resistance is equal to
voltage divided by current (R = E/I). Thus, we should be able to
determine the resistance of the subject component if we measure
the current going through it and the voltage dropped across it:
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