Okay...

I need you guys opinion here.


" IS THERE SUCH THING AS NEGATIVE RESISTENSE?"

Since high school and 4 years in University, the basic thing I learned about resistence is that there is NO MATERIAL in this world that can give -ve resistence i.e you get -ve ohm in multimeter.

If you get such reading in first place, it is either that you have not calibrated your multimeter or the multimeter is faulty...


But in technical explaination, if you get -ve reading it means that the material you are testing is GIVING OUT energy, i.e generating its own energy, and Im not referring to dry cells or any other type of batteries.

A friend of mine was taking a reading of a wire (basically a grounding wire), and he got -ve reading. When I told him that his reading was wrong, he said that the reading is correct and it read -ve because the material is very good. The reading although only read -0.4ohm, it still makes a lot of difference in my line of job.

So I need you guys professional opinion whether the reading taken by my friend is correct and Im wrong or vice versa?


Note that this is not to prove who is right or wrong, but it is important that an accurate reading is taken due to the nature of job...

hello???????????????????

Resistence? RESISTENSE? The correct spelling is resistance.

You are right that negative resistance means the power flow is out of the material/device.

You are also correct to say that the measurement is wrong. I don't think you need professional opinion. The reason is quite simple. The probable case is calibration error. Also, the conductor measuring might be conducting current from an external source. In resistance measurement mode, a multimeter injects a small constant current say 0.1mA, then measure the voltage across the probe. If the measured voltage is 1.3mV, then the display shows 13 ohm. When the measured conductor is carrying current (caused by sources other than the multimeter itself), the voltage measured can be anything and can be negative.

A third explanation is similar to the second one. It is due to the measured object acting as a chemical cell. Set multimeter to millivolt scale. Put the two probes into a cup of water, I think you have done this. You'll see that it has voltage. If switch to resistance mode, you can't measure correctly. The readings obtained are very far from the readings you get from an analog ohmmeter, which use a higher test current. Same case for measuring the resistance from your left hand fingers to right hand fingers. Analog meter gives about 200kOhm, but digital gives rubbish (up and down over wide range). Now try measuring voltage on a piece of oxidized metal, if you touch the probes lightly, you are likely to get something more than 1mV. However, I've never success in getting negative resistance by this way.

Xenon, I would take your first explaination as it is more relevent to my question...

Since I wasnt there when the measurement was taken, and only rely on the data showed, I would assume there is a leak of current somewhere causing the -ve reading.

Furthermore, even if it is not a leak, it may be different due to different grounding source..thus giving erroneous reading...

There will be another round of measurement next week, and I will be there to see how the reading is done..

thanx for your opinion...

really appreciate it