The 4 conductor resistance metering system is being used for long in the
industry, mainly in RTD devices, and low value resistance.

When measuring resistance, the meter needs to supply a small or large
current, enough to generate a voltage drop on the resistance under
measurement, enough to be read by the device voltmeter and indicate the
representative resistance values.   You know that wires and connections
have their own resistance, so the current through the circuit would also
generate voltage drops over those points too.  If the voltmeter measures
the voltage drop internally, it would be measuring not only the voltage
generated over the resistance under measurement, but adding all the
small voltages generated everywhere, so the real voltage measured is
higher than the real one over the resistance under measurement. It means
that the voltmeter would be indicating a wrong "relative" resistance.

For example, suppose the meter supply a loop of 1 ma across the
resistance under measurement.  If the resistance value is 20 Ohms, the
voltage generated across it would be 20 x 0.001 = 0.02 Volts, now,
suppose the connections and probe wires offer a total of 3 Ohms, the
same 1 ma current would generate a voltage on wires and connections
equal to 3 x 0.001 = 0.003 Volts.  Now, if the voltmeter measures the
voltage right at the internal posts where the 1 ma goes out the
instrument, it would be measuring all the external voltage, that would
be equal to the sum of 0.02 plus 0.003 Volts, so 0.023 Volts.  When the
unit "calculates" the resistance based on Voltage/Current, it would
indicate 0.023/0.001 = 23 Ohms, that is wrong (resistance under
measurement is 20 Ohms).

To avoid this problem, the 4 wires device use two wires to supply the
current, and other 2 just to "measure" the voltage directly over the
resistance under measurement.  It means that the supply current wires
can have several ohms and generate voltage on wires and connections, but
the voltage generated over the resistance under measurement would be
always the same, just 0.02 Volts.  It works the same as if you have a
voltmeter connected directly over the resistance.

There are the possibility to use only 3 wires instead 4. It works this
way: Suppose wires #1 and #2 supply the current, while the pair #1 and
#3 is used to measure the voltage.  The unit also measure the voltage on
posts #1 and #2 (will be a higher voltage than #1 and #3), so it
subtract one from another and this difference would be the voltage drop
over the wire #2 and its connections.  Considering that probably the
wire #1 and its connections would have the same kind of resistance from
wire #2, that difference (V#1#2 - V#1#3) is reduced from the voltage at
#1 and #3 again (2 x V#1#3 - V#1#2) , so it goes to be very close to the
real voltage over the resistance.

Wagner.

Richard Martin wrote:
>
> I don't know of a "site", but the principles were pretty simple.
> Two terminals supplied a <high> current to the object under
> test D.U.T.  The magnitude of the current was the controlled variable
> so contact resistances (and the induced voltages)didn't perturb that
> value. The other two terminals measured (with high impedance)
> the voltage resulting from the applied currents. Since this
> 'took' negligible current from the D.U.T. it did not
> steal current from that applied, and (more importantly) didn't
> add in its own (ohmic at least) voltage errors. I don't recall
> there being much 'magic' to all this, other than some attention
> to thermal  (dissimilar metal thermocouple effects if not
> balanced) and in extreme cases chemical effects on the voltage
> probes.