Your heart is a complicated electrochemical machine which produces time-varying voltages as it beats. For each heart beat a wave of electrical potential spreads through the heart. This causes the heart to have net positive charge on one part and net negative charge on another part. The sizes, positions and orientations of these charges change in a regular way during each heart beat cycle. These electric fields produce small voltage differences between points on the skin which can be measured and used to diagnose the condition of the heart. Voltage differences on the skin are about 1 mV (1 millivolt = 10-3 V).