Humans are fragile. Our bodies are easily mutilated by our own creations: Crushed, mulched, zipped. But physical force is weak and inefficient compared to good old electrocution, which, according to MythBusters' Adam Savage, doesn't kill you the way you think it does.
If you learned about how electrocution kills you from cartoons or Ernest P. Worrell—you get fried as your body flashes like fireworks and everybody can see your bones—well, you got learned wrong. Electricity doesn't actually fry you—that actually requires way more juice than it takes to kill you, which is a frighteningly minuscule amount.
But before we get to the scary part, let's get through the technical part, so we're on all the same page of scariness. You've got a few major units when it comes to electricity: Volts relate voltage, amperes (amps) describe the current, watts measure power and ohms refer to resistance. A pretty good analogy from HowStuffWorks relates the basic differences between them, plumbing style: Voltage is like water pressure, current (amps) is like the flow rate, and resistance (ohms) is like the size of the pipe. Increasing the voltage results in a greater current (more amps)—assuming a constant resistance-since increasing the pressure logically increases flow [Update: Clarified this sentence]. Power (wattage) is simply the voltage multiplied by the current (amps). One amp is equal to about 6.242 × 10^18 electrons per second moving through a point. And a single watt is equivalent to one joule of energy per second, but that doesn't matter so much for our purposes.
Alright, now let's get real. And who's more real and had more opportunity to get electrocuted than Adam Savage from MythBusters? So we called and asked him just how much electricity you need to kill a human. His reply? "I'm about to freak you out."
Seven milliamps. For three seconds. That's all it takes. Electricity kills you by interrupting your heart rhythm. If 7 milliamps reaches your heart continuously for three seconds, "your heart goes arrhythmic," he explained. Then everything else starts shutting down. "You could quite easily kill someone with a 9-volt or AAA battery directly to the heart."
The reason electricity isn't able to murder millions of people a day with ultra-tiny shocks is that our bodies have built-in resistance against electricity, so it doesn't shoot straight to our heart. The skin's resistance is about 5,000 to 15,000 ohms. Adam said that "it's super difficult to quantify" precisely how much juice you need to break through, since there's all kinds of variables in play, like the clothes you're wearing. Not to mention, "how do you quantify that someone's actually died?"
But if it's any consolation, Adam says that the kind of static shock that actually stings your skin is about 20,000 volts—high voltage, just a really tiny amperage.
So the trick is getting the proper amount of power to cut through our skin and clothes and rubber-soled shoes to zap our heart. There's a reliable way to do that: Lightning. With lightning, Adam said, "all bets are off." A lightning bolt can hit over a billion volts. Air's resistance, he explained, is about 10,000 volts per centimeter—so for electricity to move just 10cm through air requires 100,000 volts.
Machines could generate lightning artificially—this dude Charles Steinmetz built a lightning machine back in 1916 that generated over 10,000 amps and 100,000 volts. The reason some people survive is that they luck out with the path it takes through their body—so they might get scorched if it travels along the outside of their body, like if you're wet, but if their heart goes untouched, they could come out alive.
That's obviously wildly impractical—the sophistication and energy required for lightning-shooting machines would be more easily put toward acquiring nukes, a la every apocalyptic movie ever. Besides, there are far simpler machines that do a similar job when it comes to electrocuting people. Simple skin-penetrating Tasers already kill people occasionally. However, according to Adam, Tasers are designed with the 3-second-kill problem in mind—most pulse at much shorter intervals to avoid being fatal.
Still, we likely have little to fear from extinction by electrocution. With the exception of the admittedly clumsy electric chair, no one's ever systematically killed people with electricity. Machines, if they were to develop a murderous intent, would most likely use all of the other ways humans have designed to kill each other.
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