Being an electrical engineer I do know a bit about this.

As was said earlier the conductivity of the area is the key. Sweat makes the skin conduct better and thus one needs a higher voltage to pass the same current through as happens with a lower voltage on sweaty skin.

Also where the current enters and leaves is important as current through the heart or brain is much more critical than other parts of the body.

There is a documented case of a (sweaty) guy working on his car who reached down into the engine bay and had one ear touch the car battery terminal and the other ear touched part of the engine and he was killed instantly. Normally 12 volts would not be a problem but he had the double whammy of low skin resistance and current through the head.

There is also the fact that muscles contract when current passes through them and this frequently breaks the circuit. This has saved lives in many instances and has happened to me a couple of times during my life in the industry.

Just because a shock doesn't kill, it can still do damage. Nerves and muscles can be injured and it just happens that the 50 or 60 Hertz mains frequency is perfect to make a heart go into fibrulation.

So just a 9 volt battery accross the nipples of a sweaty person could possibly (but unlikely) stop the heart. Note that batteries are DC and are safer than AC mainly because you get a single muscle contraction with DC and you don't have the same potential for fibrulation

A bit of careful shocking around the genitals is probably OK but as with most BDSM activities one must be careful.

The violet wands (I think they are called) that have high voltage and give off corona and sparks are not capable of sourcing a high current and are pretty harmless from what I've read - but do be sure.

Interestingly the first electric chair was designed the same way and produced very high voltage but very low current. The poor guy strapped in had his hair stand on end but he didn't die. Folks then thought it was voltage which was the killer when in fact it is current (anything over 6 milliamps) which is a factor of voltage and resistance that kills.

Trust someone finds this interesting - and do be very careful