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3-3
OPERATING INSTRUCTIONS
3.2
2, 3, and 4-Wire Operation
See Section 2.2 for connection instructions.
Operating an ohmmeter with two wires results in the lead
resistance, which is in series with the resistor under test,
being added to the unknown resistance. Two leads of
relatively heavy 18 gauge wire, each 8 feet long, will add 0.1
Ω to the measured value. Obviously, 2-wire measurement
should not be used with low-value resistors, without
subtracting the resistance of the test leads. Accuracy is
reduced with 2 or 3-wire operation.
3-wire measurement balances the resistance of one lead
against a second lead, compensating for the lead resistance
if the leads are identical. It is better than 2-wire
measurement, but not as good as 4-wire.
4-wire measurement uses two leads to deliver (and return)
the test current to the resistor under test. The two voltage
leads are then used to measure the I x R drop directly at the
unknown resistor. This is the most accurate method of
measuring small values of resistance.
