Difference Between E.M.F and Potential Difference

The e.m.f is the work done by a source in driving one coulomb of charge around a complete circuit. No device is a perfect energy converter. Some of the energy produced by the chemical reaction inside the battery is lost when it is converted to electrical energy. The useful energy produced by the battery is the potential difference between its terminal.

The electromotive force of a battery is equal to the potential difference between its terminals in an "open circuit", when there is no current being drawn. The potential difference between the terminals generally drops when the current is being drawn.

You cannot measure the e.m.f of a battery directly because whenever you use a voltmeter to measure the potential difference between the terminals of a battery, a small current will flow from the battery through the voltmeter.

This means that a small potential drop has already occurred inside the battery before the current reaches the external circuit. The reading you get on the voltmeter is therefore the potential difference between the terminals  of the battery and not the e.m.f of the battery.

However, for practical purposes, the difference between the terminal potential difference and e.m.f of battery is negligible. Thus the reading of a voltmeter across the terminals of the battery is normally taken as the e.m.f of the battery.

The other way to measure the e.m.f of the battery is to get a series of values for the potential difference between its terminals for different magnitudes of current and then plot a graph of potential difference against current. E.m.f can be determined buy extrapolating the graph. When the current is zero, the potential difference is equal to the e.m.f of the battery.