Half Cells and Electrode Pairs

From electrochemistry, our discipline has borrowed important terminology. One electrode with electrolyte is called a half-cell to underline that one electrode is not enough. Two electrodes, an electrode pair, are needed to close the electric circuit so that electric current can flow (CC electrodes). A whole cell is two electrodes both submerged in the same electrolyte (e.g., in a glass dish). 

Also when a potential is to be measured, two PU electrodes must be used. The potential of each electrode is either measured individually with respect to a third electrode and then subtracted or the potential difference between the two PU electrodes are measured directly without the use of a third electrode. DC voltage measured is the sum of the equilibrium potentials of the two electrodes, plus the potential difference in the tissue between them. The tissue potential may be membrane potentials, for instance the skin DC potential. If the two electrodes are made of different metals (e.g., steel and platinum), a DC voltage of hundreds of millivolt may be generated by the electrode pair (battery effect). 

If impedance is to be measured the electrodes must be connected to an AC current source. The two CC electrodes, the membrane, and the electrolytes all contribute. As they are physically in series the contributions must be added. 

The behavior of an electrode pair depends on the application, is it to be CC or is it to be potential reading (PU) with negligible current flow If it is a PU pair the skin contact area is less important, but the skin site may be very important. If bofli electrodes are on skin sites that are under the same nervous control (same dermatome), the potential difference will be small or negligible and with no signal the bipolar lead is useless. But if one of the electrodes are on an indifferent (passive) skin site the signal may be large, the lead is monopolar and useful. 

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