B&K Precision 879B - Manual - Manual (Page 87)

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provide an option for DC resistance measurements.
Therefore, when measuring a resistive component
that is designed to be measured with DC, readings will
be incorrect or inaccurate. Before using the meter to
measure resistance, please verify whether the DUT
(device under test) requires DC or AC resistance
measurement method. Depending on the method,
results will vary greatly.
Guard Terminal
One of the input sockets and terminals is labeled as
“ GUARD”. This terminal does not have to be used in
all instances for the meter to make measurements.
But in some instances, it is very useful. Guard
terminal generally serves two purposes.
If user is using test leads, the guard terminal can be
used to connect to the shielding of the test leads.
Doing so can be useful when making large resistive
component measurements. For example, when
measuring a 10 MΩ resistor with test leads, at the
high range the reading may seem to be unstable as a
few digits may continuously be changing. Having the
shield of the test leads connected to the guard
terminal will help stabilize the reading in some
instances.