When the cell is supplying a current, the voltage is lower because the cell has internal resistance. Some the the EMF is used to drive the current against the internal resistance of the cell.
You can easily measure the EMF
\[E\]
of a cell by measuring the voltage across the terminals when the cell is not in a circuit, so not supplying a current. If the cell is in a circuit, and the current is \[I\]
, then the voltage across the terminals, measured with a voltmeter, is \[V=E-Ir\]
, where \[r\]
is the internal resistance of the cell.Potential difference is often confused with EMF, Both are measured in Volts. Potential difference is the voltage difference between any two points. For a cell not in a circuit, the EMF is equal to the potential difference between the terminals. We can also think of voltage with reference to the equations
\[W=QV \rightarrow V=\frac{W}{Q}\]
and \[V=IR\]
where \[W, \: Q, \: V, \: I, \: R\]
are work done, charge, potential difference, current and resistance.