Any device such as a cell or dynamo which is able to drive an electric current round a circuit is said to possess electromotive force (e.m.f.). Electromotive force which is measured in volts is defined on page 406.
For the time being, however, we may regard the e.m.f. of a cell as being equal to the potential difference across its terminals when it is not producing current in a circuit. For practical purposes the e.m.f. of a cell is measured by connecting a voltmeter (page 412) to its terminals.