B&K Precision 310 - Manual - Manual (Page 13)

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THERMAL EFFECTS
Temperature can have a significant effect on the performance of a
digital milli-ohmmeter due to the temperature coefficient of the
resistance under test and thermal EMF's across the dissimilar
conductors.
Most conductors have a large temperature coefficient of resistance.
For example
0.4%/°C for copper. A copper conductor that has a
resistance of 10.00m ohm at 20°C will increase to 10.40m ohm at
30°C. This change should be taken into account when making
measurements.
A current going through a resistance will also elevate its temperature.
So duration of the test can change the resistance.
When measuring the resistance of item, such as current shunts,
which have joints of dissimilar conductors, thermal EMF can affect
the accuracy of the measurement. This condition can be detected if
the reading alters when the leads are reversed. To compensate for
this effect, the average of the two readings should be taken as the
true measurement.