How could I calculate the internal resistance of a cell?

Firstly, we must set-up an experiment and would need the following apparatus: The cell, a voltmeter, an ammeter and a variable resistor (a rheostat for this example). Setup the experiment so that the cell, the ammeter and the rheostat are all in series and have the voltmeter in parallel with the cell. Now vary the resistance and take readings of the current using the ammeter and potential difference using the voltmeter. To get a good value for the error on the internal resistance it is important to take repeat readings.Now once you have a decent set of data we must plot it on a graph. The relevant equation for this experiment is E.M.F = V + Ir, where E.M.F is the electromotive force of the cell, V is the recorded voltages, I is the recorded currents and r is the internal resistance of the cell. We can rearrange this equation such that we can model our data to be represented by a linear graph. The re-arranged form of the equation would be V = -rI + E.M.F. Therefore, by plotting our recorded values for potential difference on the y-axis and current on the x-axis we should now be able to simply draw a line such that it fits all of our data points. Now since we know that straight line graphs follow the equation y = mx + c, we know the y-intercept represents the total E.M.F of the cell and the gradient represents the internal resistance.