For the internal resistance I got 3.6 ohms:
The emf was 6.0V, and the terminal pd was 5.0V (worked out from oscilloscope, work out displacement from P to both Q and R and multiply by two).
Resistance of the resistor was 18 ohms, so using V=RI you get I=V/R =5/18 =0.27777... A
Considering the internal resistance, the lost volts were 1V (as emf = terminal pd + lost volts, so lost volts = emf - terminal pd = 6-5 = 1V)
V=RI and R=V/I =1/0.2777777... =3.6 ohms.
You can check that this result is true using the emf equation:
emf = terminal pd + lost volts, so E=IR+Ir
E=(0.27777...)(18) + (0.2777777...)(3.6)
E=6.0V [QED]