This quote from a Scientific American article on the ampullae of Lorenzini articulates just how powerful the shark's electrical sense is:
"This effect is equivalent to the intensity of the voltage gradient that would be produced in the sea by connecting up a 1.5-volt AA battery with one pole dipped in the Long Island Sound and the other pole in the waters off Jacksonville, Fla. Theoretically, a shark swimming between these points could easily tell when the battery was switched on or off. (Later measurements of brain response indicate that sharks can discern 15 billionths of a volt.) No other tissue, organ or animal exhibits such extreme sensitivity to electricity. Indeed, engineers have difficulty measuring such weak fields in sea-water using modern equipment."
For more info: