Mad Phil forced me to take this little gem:

Dayton 4X221 Snap-Around Ammeter

Judging from the much-folded Dayton 4X221 Snap-Around Ammeter Operating Instructions (a scanned copy that I folded around the original and tucked inside the case), the ammeter dates back to 1979, which says Mad Phil probably used it in the early 80s, when he was repairing AV equipment. Unlike most vintage clamp-on ammeters, this one can also measure voltage and resistance:

Dayton 4X221 Snap-Around Ammeter – Specifications

The resistance function requires a single AAA alkaline cell in the bulky probe, so this should come as no surprise:

Dayton 4X221 Resistance Probe – battery contact corrosion

The probe housing contains a 1 A fast-blow fuse, which blocked the corrosion from getting deeper into the probe tip:

Dayton 4X221 Resistance Probe – battery and fuse

The AAA cell was “Best if installed by Jan 1999”, which I’m sure was true. Somehow, you never recognize the last time you use something; I suppose old instruments get used to not seeing the light over the workbench after a while.

Anyhow.

Douse the corrosion with vinegar to neutralize the potassium hydroxide, rinse out the probe body, polish the top of the fuse, buff up the battery contact on the test lead, and it’s all good again.