The electromotive force is the work done by a battery on the charge carriers to transport them across the entire circuit, per unit charge. It can be thought of as the specific energy given to the charge carriers.

To measure the emf of a battery, one can connect it in a circuit with a resistor, a voltmeter and an ampere meter. The emf will be the sum of the voltage differences across the resistor and the battery itself: E = U_{ext} + U_{int}. Since U_{int} = I*r, where r is the internal resistance of the circuit, the equation can be rearranged as U _{ext} = E - I*r. Hence, by measuring U