One of the lessons from my chemistry classes in high school was that “hot glass looks like cold glass.” This was a lesson like “don’t touch the stove or you will get burned.” I never felt a need to verify this.
However, hot solder looks like cold solder. I was working to test my “ARC Generator.” I have a bench power supply. I was trying to protect the LEDs in the ARC generator from accidental shorts as I probed them by limiting the current & voltage on the power supply to a safe level.
I was getting pretty frustrated. Nothing was happening. I even tried powering up my little 5V regulator on a proto board and nothing happened there either. I decided to test the test equipment and I found that the red power lead was open. Then it became really obvious because the connector fell off onto the floor–cold solder joint.
I pulled out my soldering iron and solder and tried fixing the cable. The first try was a failure because when I picked the (supposedly) repaired wire, the tip fell off again. This is where the hot metal looks like cold metal applies: I tried to tap the loose part on the floor to gauge if it was cool enough. Well, it wasn’t. I picked it up and burned my fingers and tossed the piece of metal across the room.
I was more cautious on the second try and I think I’ve got the connection solid.
So, testing the testing equipment was a win. Picking up a piece of metal that was just soldered was a fail.
…and I still don’t know if the LEDs in the ARC generator (á la Iron Man) are wired correctly, but I can check that tomorrow.