One beaker (right) contains substance A with an equimolar mixture of the reduced (Ared) and oxidized (Aox) members of its redox couple.

   The other beaker contains the hydrogen reference standard ( H
2 <-->2H+ + 2e-), whose redox potential is arbitrarily set to ZERO by international agreement. A salt bridge is formed from a concentrated KCl solution allows the ions K+ & Cl- to move between the two beakers to neutralize the charges between them. The metal wire (red) provides a resistance-free path for electrons, and a voltmeter measures the redox potential of substance A.

   If electrons move from Ared to H
+, as shown, the redox couple has a negative redox potential. If they flow from H2 to Aox, this couple has a positive redox potential.