Electric shock torture methods haven't been around as long as many other widely used methods -- humans didn't figure out how to harness electricity until the late 19th century. Once established, however, electricity soon came into use as a method of torture.
"Americans didn't just develop electric power," writes torture expert Darius Rejali in The Boston Globe, "they invented the first electrotorture devices and used them in police stations from Arkansas to Seattle." Electrical shocks can be delivered using stun guns, cattle prods and electroconvulsive therapy (ECT) devices.
This type of torture can be as crude as introducing a current to a victim via a cattle prod or other device designed to deliver a shock attached to a car battery. Shocks are used as a torture method because they're cheap and effective. What's more, shocks tend to leave behind little obvious physical trace of the agony they produce.