In that case the potential difference
between P and N is
V = V++ V– – I r
   = e – I r
(3 38)
Note the negative sign in the expression (I r) for the potential difference
between A and B This is because the current I flows from B to A in the
electrolyte In  practical calculations, internal resistances of cells in the circuit
may be neglected when the  current I is such that e >> I r