The electromotive force of a cell is the energy supplied per unit charge by the cell.

This is easily measured with a voltmeter by taking the cell out of a circuit and putting a voltmeter across its terminals.

All cells have an effective internal resistance. We assume this is fixed and then use it to account for the maximum current a cell can deliver when short circuit and the voltage dropped across the internal resistance of the cell when a current flows.

$$ \mathcal{E} - Ir = IR $$

or

$$ \mathcal{E} - Ir = V $$

The gradient is -r and the y-intercept is \( \mathcal{E} \)