Electronic – Why is resistance increasing while I’m measuring using a multimeter


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?

Best Answer

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.