Reference Electrode. The potential of a chemical half-cell cannot be measured directly but requires to be used with a reference cell whose potential is known and remains sensibly constant. Thus, the potential of a measuring cell can be obtained as the difference between the total potential across the cell (comprised of the two half cells) and the reference potential of the reference half cell. There are certain criteria required for a satisfactory reference electrode; the chemical system must be reversible and predictable and it’s potential should remain constant over significant period of time and should exhibit no thermal hysterisis.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links