10.4 Effects of Current Density
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Appendix
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 W 300 X L 370 X t 0.4, mea-
sured values taken from the same location may differ markedly
as shown below:
When using a 0.2-mm-pitch pin lead: 1.1 mΩ
When using a 0.5-mm-pitch pin lead: 0.92 - 0.97 mΩ
When using a 9287-10 CLIP TYPE LEAD: 0.85 - 0.95 Ω
This is caused not by contact resistance between the probe and
the subject of measurement but by current distribution in the
subject of measurement.
Illustration 1 is an example of a plot of the equipotential lines of a
metal plate. Just like the relationship between wind and the
barometric charts used in weather forecasts, current density is
higher where the equipotential lines are close to each other and
lower where the lines are farther apart. The illustration shows
that the potential gradient is greater near the sources of current.
This is because these points are where the electric current is in
the middle of spreading across the metal plate, leading to higher
current density. For this reason, when a terminal for detecting
voltage is placed near one of these sources of current, just the
slightest change in contact position can lead to major variation in
measured values. Use of Hioki's 9453 four-terminal lead or a
similar lead to detect voltage on the inner side of the sources of
current is desirable to avoid these effects. In other words, if mea-
surement is conducted within the width (W) or thickness (t) of the
subject of measurement, current distribution is likely to be sta-
ble.
10.4 Effects of Current Density
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