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Originally Posted by DrMemory
The body is conductive. The (dry) skin has the highest resistance. Inside the body there is fairly low resistance due to the amount of (salty) fluids. However, as in all cases, the amount of voltage needed to produce a given current flow in any conductor (including the human body, through the dry skin) is given by Ohms law. With enough voltage, a current will flow. No cuts, punctures, Aztec rituals, or other silly suggestions are required.
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Yes all of that is correct, but think about the problem more. All the conductance in your body does not lead directly through the heart. So if you jab an electrode in each hand, most of the current will not be going through your heart. Some will pass through your lungs, others through your muscle tissue, some through the fat around your ribs, some will dip down into your lower body cavity and then come back up. In short its spread out all over your body with only a tiny fraction actually passing through the few inches of cross section represented by your heart.
In terms of circuit theory, your whole body is a big n-way current divider. So you need to put in a substantially higher amount of current at the outside to direct a significatant amount through the heart.
Though during heart surgery, shock is a much bigger problem since the heart is directly connected to instruments, some of which are actually electrified.
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If I recall correctly, there was some idiot that was trying to measure his internal resistance was. So, he punctured his skin to let the ohmmeter get past skin resistance. 9V battery going through internal resistance caused a mild heart attack. Unfortunately, he was alone and without means to reach medical assistance. Result? Darwin award. The story goes that this is why the Navy has safety precautions (associated with their ohmmeters I think).
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One of my textbooks has an example of someone who died licking a 9v battery. I wish it had details, but evidently he managed to fribrillate his heart doing it.