[Matthias] bought cheap clip leads online and, wisely, decided to check them. We’ve had the same experience that he’s had. Sometimes, these cheap leads are crimped and don’t make good contact. However, you can usually solder them and completely fix them. Not this time, however, as you can see in the video below.

The resistance for the leads was a bit on the high side, which is usually a sure sign of this problem. But soldering didn’t really make a big difference. A homemade clip lead, for example, read under 20 milliohms, but a test lead from the new batch read about 260 milliohms even after being soldered.

A thermal camera indicated the problem was actually the wire. At first, he thought the wire was just very thin. While it was thin, that wasn’t the real problem. The wire looked normal enough, but sanding the wire showed that it might be only copper-coated. Turns out, a magnet would grip the clip leads meaning they were iron wires coated with copper.

We were amazed at how many leads he was able to find with iron in them, primarily those with clips on at least one end. Oddly, mouse cables were also magnetic.

So, the lesson is to test the resistance and pass a magnet over those wires. Depending on your application, a few hundred milliohms might not matter. But you should at least know that some of your clip leads may have an order-of-magnitude difference in conductivity.

If you need an easy milliohmeter, there are plenty of options. You can even just haywire something up on a breadboard, or — like in the video — use a 1A current and measure millivolts.