10.3 AC Four-Terminal Method
The instrument uses the AC four-terminal method, so that resistance measurement can be carried
out with the resistance of the leads and the contact resistance between the leads and the object to
be measured cancelled out. The following figure shows the principle of the AC four-terminal
measurement method.
An AC current (I
s
) is supplied from the SOURCE terminals of the instrument across the tested
battery. The voltage drop across the internal impedance of the battery (V
ls
) is measured by the
SENSE terminals. At this point, since the SENSE terminals are connected to an internal voltmeter
with a high impedance, almost no current flows through the resistances R₂ and R₃ which represent
the lead resistances and contact resistances. As a result, there is almost no voltage drop across the
resistances R₂ and R₃. Thus, the voltage drop due to the lead resistances and contact resistances is
very small, and these can be cancelled out. In the instrument, a synchronised wave detection system
is used, whereby the internal impedance is separated into resistance and reactance, and the
resistive component only displayed.
If the lead resistance, the contact resistance between measured object and lead, or the contact
resistance between the lead and the instrument increases, the instrument can no longer supply
normal current to the measured object, resulting in an abnormal measurement status indicated by ‘-
- - -‘ within the measured resistance field.
10.4 Effects of Current Density
When the Subject of Measurement is Wide or Thick
When the subject of measurement has width or thickness, such as when it is in plate or block form, it
is difficult to get accurate measurements using clip-type or pin leads. Using such leads could result in
variation measured values ranging from single to double digits in percentage points, due to contact
pressure or angles. For example, when the subject of measurement is a metal sheet with the
dimensions 300 (W) x 370 (L) x 0.4 (t), measured values taken from the same location may differ
markedly as shown below:
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