Electrode Theory

        An interface is necessary between the body and the electronic measuring device when recording potentials and currents in the body.  Biopotential electrodes produce small voltages directly related to the changing electric field produced by a beating heart..  The Ag/AgCl electrode is a practical electrode that approaches the characteristics of a perfectly nonpolarizable electrode.  Perfectly nonpolarizable refers to the freedom of ions to pass through the electrode-electrolyte interface to be transduced into an electrical current.  The electrode converts the ionic current produced by the body into a voltage, and the ECG amplifies this voltage.  

The electrode-electrolyte interface is the junction where the ionic transfer occurs.  A temporary current is induced in the electrode from the changing electric field of the beating heart.  This current causes electrons and anions to move across the electrode-electrolyte interface in the direction opposite to the flow of the current, and for cations to migrate across this interface in the direction of the current.  This temporary separation of charge produces a temporary potential.  This potential is created from a current induced from the heart and is thus directly related to the changing electric field produced by a beating heart.  The ECG circuit hugely amplifies the potential, and the output gives the electric characteristics of a beating heart. 

Another sensor that was considered was the piezoelectric sensor.  Piezoelectric materials generate an electric potential when mechanically strained.  During a heart beat, the pressure in the blood vessels is higher than when the heart is in its resting stage.  This higher blood pressure causes a physical deformation in the skin, and thus a piezoelectric sensor can produce an electic potential during every heartbeat.  The principal reason why the piezeoelectric sensor is less than ideal is that it is pressure sensitive.  In order to pick up a signal the nurse or doctor would have to press the sensor hard against the patient which could cause a permanent deformation of the piezoelectric material  This information, combined with the fact that hospitals across the nation use Silver/Silver Chloride sensors,  made it obvious that the silver-silver chloride sensors were the best to use for this project.  :)