An unconventional use for a multimeter
So I have this device that takes AA batteries. The battery holder got bombed out by alkalines some time back, and I cleaned it out last year; I got most of the corrosion out with Naval Jelly (an acidic goop that dissolves rust and other corrosion) and elbow grease, but it sometimes takes a bit of rolling the batteries to get them to make contact.
(I use NiMHs now. Less waste and less chance of suddenly finding the device damaged by corrosive leakage!)
Today, having freshly charged the NiMH batteries, I put them in and turned the thing on and… nothing. Doesn’t light up. I might as well have left them in the charger. Normally some rolling the batteries fixes it, but this time, no dice.
So I got out the multimeter and started poking around. I don’t really know what I’m doing so I was kind of just poking randomly, but at one point I had put the multimeter probes on either end of one battery and… the device lit up!
Multimeter: Works.
No multimeter: Doesn’t work.
Well that’s not convenient. Lemme just take this battery out so I can try other random nonsense and… wait, it lit up! Without the multimeter!
Battery’s positive terminal resting on top of the corresponding contact: Works.
Battery properly seated: Doesn’t work.
By this point I’d figured out the problem. There was some thin coating of gunk on that contact that wasn’t getting rubbed off just by rolling the battery. (Maybe it used to be a less even coating, with gaps that enabled connection, but the rolling gradually spread it out.) The multimeter probe bridged the contact’s top surface (which doesn’t normally touch the battery) to the battery terminal, bypassing the gunk and closing the circuit. Likewise, pulling the battery up let it rest on that same clean surface, again bypassing the problem.
I got out an alcohol wipe and rubbed off the gunk and that fixed it properly.