Potential difference (PD) is measured taking into consideration the resistance (i.e. resistance of external circuit) other than the resistance provided by the cell. EMF or electromotive force is measured taking into consideration the resistance of external circuit as well as resistance offered by cell itself.
Internal resistance can be defined as the resistance offered by the material contained inside the cell, to the flow of electric current.
Formula for determination of internal resistance of a cell is
r= Internal resistance of the cell
E= Electro-motive force of the cell
R= Resistance of external circuit
I= Electric current in the circuit
PS:- Answered within the context of question.