Two signal electrodes and a reference electrode are attached to dielectric members, from which their front faces project for engaging a person's skin. Embedded in the dielectric members that enclose the two signal electrodes are electrostatic shields that are behind the electrodes and spaced from them....http://www.google.de/patents/US4122843?utm_source=gb-gplus-sharePatent US4122843 - Electrode system for a heart rate monitor
