A galvanic electrochemical cell can act as a source of electrical energy due to a spontaneous redox reaction taking place within the cell between the oxidant of one redox couple and the reductant of another. 

If the redox couples involved in the reaction are separated from one another as shown in the galvanic cell at the right, the electron transfer occurs through the wire connecting them.
 
The polarity of the electrodes can be deduced if a voltmeter is inserted in the circuit.  The voltmeter is a resistance which stops the current flow and measures potential of the electrodes relative to one another.  When a galvanic cell is constructed using a pair of redox couples, if the difference in potential shown on the meter is positive, the couple with the more positive potential will be the positive electrode.

Because both members of each couple (the oxidant and the reductant) are present in each electrode, in principle, either of these species could be the reactant in the electrode reaction. One can deduce the spontaneous reaction at each electrode from its polarity (negative or positive).

 

• The spontaneous reaction is the reaction that would occur if current were allowed to flow.
   
• Reaction occurring within the cell is the source of current in a galvanic cell; therefore
  • electrons would be released by the reductant at the negative electrode:
    reductant oxidant + ze^–
  • electrons are consumed by the oxidant at the positive electrode:
    oxidant + ze^– reductant
• The spontaneous cell reaction is the sum of the electrode reactions.

The cell potential is the difference in the potentials of the two couples:

E(cell) = E(more positive couple) - E(more negative couple)