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Appendix 2 Four-Terminal (Voltage-Drop) Method
A2
The resistance of the wiring connecting the measuring instrument and probes and the con-
tact resistance that occurs between probes and the measurement target may prevent low
resistance values from being measured at a high level of precision.
Wiring resistance varies greatly depending on the thickness and length of the wire. Cables
used in resistance measurement may, for example, exhibit resistance of 90 m
Ω
/m (for No.
24 AWG [0.2 sq] wiring) or 24 m
Ω
/m (for No. 18 AWG [0.75 sq] wiring).
Contact resistance varies with probe wear, contact voltage, and measurement current. With
good contact, resistance values are generally on the order of several milliohms but may
reach as high as several ohms on occasion.
The four-terminal method is used to facilitate reliable measurement of low resistance val-
ues.
With two-terminal measurements (Fig. 1), the resistance of the test leads is included in the
measurement target’s resistance, resulting in measurement errors.
The four-terminal method (Fig. 2) consists of current source terminals (SOURCE A,
SOURCE B) to provide constant current, and voltage detection terminals (SENSE A,
SENSE B) to detect voltage drop.
Little current flows to the voltage detection terminal lead lines that are connected to the
measurement target due to the voltmeter’s high input impedance. Consequently, measure-
ment can be performed accurately without being affected by the measurement lead resis-
tance or contact resistance.
*RM3545 voltmeter’s input impedance: 10 G
Ω
or more (reference value)
Appendix 2 Four-Terminal (Voltage-Drop)
Method
Two-Terminal Measurement Method Four-Terminal Measurement Method
Measurement current I flows through mea-
surement target resistance R
0
as well as lead
resistances r
1
and r
2
.
The voltage to be measured is obtained by
E = I (r
1
+ R
0
+ r
2
), which includes lead resis-
tances r
1
and r
2
.
Current I flows from r
2
through measurement
target resistance R
0
and through r
1
. The high
input impedance of the voltmeter allows only
negligible current flow through r
3
and r
4
.
So the voltage drop across r
3
and r
4
is prac-
tically nil, and voltage E across the measure-
ment terminals and voltage E
0
across
measurement target resistance R
0
are es-
sentially equal, allowing measurement target
resistance to be measured without being af-
fected by r
1
to r
4
.
Figure 1.
r
1
r
2
E
I
Resistance R
0
Voltmeter
Constant current source
Figure 2.
r
1
r
2
E
I
Resistance R
0
Voltmeter
Constant current source
r
3
r
4
E
0