This question comes up all the time in physical computing classes and workshops. Recently Zach Eveland posted a good informal but thorough answer to the ITP physical computing list. This isn’t an authoritative medical answer by any means, but it’s good enough to keep you safe if you follow his recommendations:
“…it’s the current that kills, in most cases by interfering with the normal operation of the heart. Be careful though - with the right conditions, shocks way below 1 Amp can be lethal. In designs where you are trying to prevent electrocution, 5 milliamps is considered the upper limit of safe operation. Notice that that’s 5 MILLIamps - barely enough to light an LED.
“The thing to measure is how much current is flowing through the heart and you have to assume the worst case; that someone is holding positive with one hand and negative with the other, letting the electricity go right through their chest and heart. Body and skin resistance, hand-to-hand, varies but 10 kohm is a safe figure to work with (though it can be even less with wet hands and can go much higher, up to hundreds of kohms).
“Here’s an example for figuring current: with a 9V battery and 10 kohm of body resistance, if I hold positive with one hand and negative with the other, the current through my heart is equal to 9V divided by 10 kohm or 0.9 mA - in the safe range.
“One more warning; be careful with this - keep your hands dry, don’t stand in water, have somebody around when you’re testing, don’t shock people with weak hearts, etc.
“In general, [to produce a safe shock, something I don't recommend - tigoe] you want a high-voltage, very low-current source of electricity, ideally DC since AC voltages are more likely to interfere with heart rhythm. A good value would be several hundred volts at way less than 5 mA. The best design would limit the amount of current to a safe level. Luckily, there are pre-built devices that do this (though I sure as hell won’t vouch for any of them): search eBay for “nerve stimulator” or “TENS”. Other options would be the peizo igniter from a cigarette or BBQ lighter or the flash unit from a disposable camera.
“Again, be safe and smart, and good luck.”
-Zach E
