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QUESTION: Measuring a resistor with a common multimeter (MM) if I heat only one of the terminals of the resistor, this happens:

-heating the terminal connected to the positive probe of the MM, the resistance increases
-heating the terminal connected to the negative probe of the MM, the resistance decreases

How to explain this situation?

ANSWER: Are you disconnecting the resistor from the circuit?  You must do that because leaving in the circuit provides parallel loading to the resistor.  The MM applies a voltage to the resistor under test and the polarity change can cause what you are seeing.

Therefore, disconnect the resistor and try again.  

PS: you will likely get the same results without heating the leads.

---------- FOLLOW-UP ----------

QUESTION: Hi, thanks for prompt reply...
The situation I described is with the resistor disconnected from the circuit. It happens with carbon, metal film or wirewound resistors.

ANSWER: How are you connecting the MM leads?  Are you holding or touching the connection area?  And, is it a really high resistance value?  Maybe very low?  If high there could be some leakage resistance involved. If low value the contact resistance between the probe lead and the resistor could become part of the circuit.

Why are you heating the resistor?  And how long do you leave the heat on?  As you know the change in temperature can cause a big change in the resistor value.  So, that may be a factor.

Carry on!

---------- FOLLOW-UP ----------

QUESTION: I found an explanation: it's a Seebeck effect
Thanks a lot for your attention

That could explain it only if the resistor is a thermocouple or thermistor type product under test.  Or, perhaps the junction of the connection to the leads of the resistor was large enough to produce the Seebeck effect.  In electronics, and this is an electronics forum, the connection joints are so small that the Seebeck effect is negligible and does not enter into practical electronic systems..... That is my thought.

Best to you.  


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IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

BSEE University of North Dakota

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