Appx.
6
Causes of Error in Voltage Measurement
Thermal electromotive force relative to copper
Metal
Thermal electromotive
force (μV/°C)
Nickel
-22.4
Platinum
-7.6
Aluminum
-3.4
Lead
-3.2
Brass
-1.6
Carbon
-0.6
Silver
-0.2
Zinc
0
Copper
0
Gold
0.2
Iron
12.2
Use a metal with a positive value for contacts facing copper and a metal with a negative value for the opposite
side. (Chronological Scientific Table, 2006 Edition)
Effects of input resistance
When the measurement target has a large output resistance, measured values will be attenuated
by the instrument’s input resistance. Caution is particularly warranted when selecting the 100 V
range or the 1000 V range, or when fixing the input resistance to
10 M
Ω
for the 100 mV range to
the 10 V range.
Example: Measuring a coin battery with an open voltage of 3 V with the input resistance set to
10 M
Ω
and a measurement target output resistance of 1 k
Ω
V
9997
.
2
3
k
1
M
10
M
10
=
×
Ω
+
Ω
Ω
V
R
OUT
R
IN
V
V
R
R
R
IN
OUT
IN
+
Output resistance
Voltage
Input resistance
Figure. Effects of Input Resistance
Effects of bias current
A miniscule current flows to the instrument’s input terminal. This current, which is needed in order
to drive the instrument’s measurement circuit, is known as a bias current. When the measurement
target has a large output resistance, the measurement error caused by the bias current will
increase in magnitude.
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