One time-honored but slightly unpleasant way in which a hobbyist determines how much energy is left in a 9-V battery is to touch both of its terminals to his tongue briefly. He experiences a pinching feeling that's mild when the battery is almost dead but one that's startingly sharp when the battery is fresh. (Don't try this technique yourself.) What is the circuit involved in this "taste test"? How is energy being transferred?
There is a flow of current from the battery's positive terminal, through the hobbyist's tongue to the battery's negative terminal. Energy is been transferred from the battery to the hobbyist's tongue by the current.
