I bought this capacitor for an experiment, but I'm not 100% sure of it's polarity. One lead is long, while the other is rather short. The "NEG" marking on the cap doesn't tell me that much because it's an axial cap.

I posted a picture below:

 

Your pic ap007 shows the long lead is connected to the can. That is the negative lead.

 

The small indented ring at the end with the rubber cap is usually positive, the case is negative. In most cases if it were non-polarized there would be a ring at each end. If it's non-polarized no problem, if it were designed by a psychotic and the polarity is reversed from standard it will fail. Since the lead is cut it's either pulled from a circuit or it was cut with the intention of installing in a circuit board.

IMHO it is incompetently marked, modern parts would be more explicit. That appears to be a surplus military part and it's common for some parts to be badly marked.

 

Excellent, thank you both for the valuable information... and correct information, as I was able to use this cap in my test circuit and it didn't blow up in my face -- I had a big plastic cup over the cap and safety glasses on, just in case.

 

Isn't it safe to test such a capacitor with a multi-meter? It should show good conduction if the capacitor is reverse-biased. Would those low currents degrade it at all?

 

If it's leaky enough to show low ohms in reverse, it's a goner.

Be aware that the seal may be imperfect. If so, the electrolyte has dried, and the cap will fail. Make sure you know the voltage rating and stay well below it if experimenting.

 

Well, it should show some conduction because electrolysis occurs in reverse-biased polarized capacitors. On the other hand, if it's forward-biased, it's resistance should be too high to measure. My question/doubt was whether or not any significant and/or irreversible damage occurs to the capacitor (electrolysis is somewhat reversible AFAIK).