User`s guide

86 34450A User’s Guide
3 Measurement Tutorial
Thermal EMF caused by dissimilar metals can create a
parasitic voltage in the measurement circuit (VEMF). The
thermal EMF can be caused by the input lead connections or
internally in resistor R. In general, this voltage will not
change with the current applied to the resistor.
The voltage measured, and so the resistance calculated, is in
error by V
EMF. Using offset compensation can reduce the
errors caused by V
EMF. To make an offset compensated
measurement, the meter makes two voltage measurements,
one with the current source on and one with the current
source off, and subtracts the two measurements. The actual
voltage drop across the resistor, and the calculated
resistance are obtained by:
First Reading - Second Reading = (I*R+V
EMF) - VEMF = I*R
Offset compensation can be used in 2-Wire or 4-Wire ohms
measurements (only available on 100 W, 1 kW and 10 kW).
Ideal
Meter
Ideal
Meter
I=0
R
V
EMF
E=IR+
V
EMF
I=0
R
V
EMF
E=(0*R)+
V
EMF
V
EMF
=