Measuring Amperes

Well, let's start from scratch. We can liken an electrical wire, a conductor, to a water pipe. When the switch is closed, it's like a faucet; nothing flows. When you open the faucet, or the switch, current or water flows. There are two variables that describe what happens: electric voltage (measured in Volts), which is equivalent to the water pressure in the pipe. The current (measured in Amperes) is the amount of water (expressed in liters/minute, for example) that flows through the pipe. Intuitively, we can feel that there is a relationship between the two: the higher the pressure, the more water flows; in electricity, this is Ohm's law: current (Amperes) is proportional to voltage (Volts).
In the case of a single battery, you can only measure the voltage (or potential difference) between the + and - terminals since it doesn't supply any current (thankfully). There's just pressure behind the tap; nothing flows in the pipe (because there is no pipe), so there's no current or amperes.