The electrolyte through which a current flows has a finite
resistance r, called the internal resistance Consider first the
situation when R is infinite so that I = V/R = 0, where V is the
potential difference between P and N Now,
V = Potential difference between P and A
       + Potential difference between A and B
       + Potential  difference between B and N
   = e
(3 37)
Thus, emf e is the potential difference between the positive and
negative electrodes in an open circuit, i