Why is resistance increasing while I'm measuring using a multimeter?

Question

I have two copper strips with a one-megaohm resistor between them. That makes the resistance between the two strips 1 M ohm. I am measuring resistance between two copper strips using a multimeter.

With this arrangement, the reading on the multimeter shows 985 K ohm.

Now I put one water drop in between the strips, then as it is in parallel with the one-megaohm, and since water has some resistance, the equivalent resistance between those two copper strips will reduce.

That is what is happening, and the multimeter reading shows 473 ohm; that is correct. But the problem is, it starts increasing slowly like 475, 478, 482.... to 736 K ohm after some 5 to 10 minutes before I turn the multimeter off.

I thought it may be that the water drop might not be at the same position and there may be a little bit of spreading so that might be the cause of resistance change, but the question is why it is always increasing and not decreasing, and why so much of change like from 475 to 736 is a huge change of approx 200 K ohm. I believe there is some other reason. Can someone give a solution to this problem?

Answer 1

After you run this test for 10 minutes, wipe off the water and take a close look at the copper where the water drop was. You will see some discoleration. Essentially the copper was corroded a bit where the water drop was. As you probably notice, bare copper left in the elements is no longer that bright copper color after a while. The same thing happened to your copper electrodes, except that the electric current speeded up the process.

The reason the resistance goes up is because the corrosion layer has significantly more resistivity than copper. This is one reason the mating surfaces of electrical connectors aren't made from copper. They are usually made from material that doesn't oxidize, like gold or nickel, or forms a conductive oxide, like tin.

Answer 2

A number of things may be happening here.

The first is that there may be evaporation of the the drop.

The second is related to Olin's assertion of "corrosion", but more specific. Electrons aren't conducted through the drop of water -- Ions are (which is why distilled water is a poor conductor). The ionic reactions at the water/copper junction are non-reversable, so you have what's called a "polarizing" electrode. Thus your water drop is slowly reaching a state where it will stop conducting electricity. The "corrosion" is a byproduct of these electrochemical processes.

I don't believe you're seeing much electrolysis of the water -- which is another possibility. Is the water bubbling?

Answer 3

My personal interpretation.

Clean water, e.g. distilled, is a poor conductor, so I assume that you have used tap or mineral water which is a much better conductor. The salts dissolved in normal water provide the charge carriers in the form of free ions that allow a current flow.

With that in mind, I can think of two possibilities why the resistance rises:

• Electrolysis takes place and elements of the salts are set free, for example chlorine. This would reduce the amount of available charge carriers.
• In the electric field between the two copper strips, the ions will become separated and can no longer float freely in the water. That would also limit the current flow capability.

Eventually, someone with an (electro)chemical background can say more.

Answer 4

I think Olin's answer is correct, but there's another factor that may account for at least part of the effect you're seeing.

I'm pretty sure water has higher adhesion to clean copper than to epoxy (which forms the surface of most PC boards). When you first drop the water on the board, it'll form a relatively round drop on the surface because of its cohesion. Over a short time, however, the higher adhesion to the copper will cause it to "pull" into two drops over the copper. As the water migrates toward the copper traces, you'll end up with less and less water between the traces, so you have a thinner connection, leading to increased resistance (and eventually, all the water will migrate to the traces, and you'll be left with the original resistor as the only connection between the traces).

Answer 5

The question which led me here was in regard to the increasing resistance of an agarose gel petri dish over time. As suggested by Olin, I tried changing my electrodes from copper to another material, but saw no difference.

After some more research, and taking into consideration that agarose gel contains salt ions which carry the current, it is likely that the Ohmmeter is in a way depleting the ions available for conductance since they have been drawn to their respective poles.

Answer 6

I did your experiment with copper strips with adhesive side. and got the same effect. After watching for a bit, I noticed that water was flowing down under the copper strips. However, the adhesive side does not allow electric current to pass through, and gradually the resistance increases.

I turned the strips over, sticky side up, and added water. Now the resistance has become more stable. It still decreases as the water dries, and water tends to flow under the strips. My water resistance is approximately 40 kΩ.