Relation to measure cell potential 

                        The Galvani potential difference is not measurable. The measured potential difference between two metal electrodes assembled into a cell does not equal the difference of the Galvani potentials of the two metals (or their combination with the solution Galvani potential) because the cell needs to contains another metal-metal interface, as in the following schematic of a galvanic cell:

M⁽¹⁾ | S | M⁽²⁾ | M^(1)'

where:

• M⁽¹⁾ and M⁽²⁾ are the two different metals,
• S denotes the electrolyte,
• M^(1)' is the additional metal (here assumed to be the metal (1)) that must be inserted into the circuit to close it,
• the vertical bar, |, denotes a phase boundary.

Instead, the measured cell potential can be written as:

where:

• E is the potential of a single electrode,
• (S) denotes the electrolyte solution.

From the above equation, two metals in electronic contact (i.e., under electronic equilibrium) must have the same electrode potential.Also, the electrochemical potentials of the electrons within the two metals will be the same. However, their Galvani potentials will be different (unless the metals are identical).