Define Electric Potentials in the Cell?

The electric potential - at a point in space is de?ned as the change in the electrical potential energy of an in?nitesimal test charge when it is brought to this point from a position infinitely far from other charges, divided by the charge. We are concerned with the electric potential within a phase-the inner electric potential, or Galvani potential. We can measure the difference between the values of this electric potential in the two terminals of a galvanic cell, provided the terminals have the same chemical composition. If the terminals were of different metals, at least one of them would have an unknown metal-metal contact potential in its connection to the external measuring circuit. Since we will be applying the concept of electric potential to macroscopic phases, the value of the Galvani potential at a point in a phase should be interpreted as the average value in a small volume element at this point that is large enough to contain many molecules.