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Take an artificial pacemaker. This device transmits an electrical voltage to the biological pacemaker cells of the heart. In a healthy human, these pacemaker cells generate their own action potential, an electrical waveform of about 100 millivolts. This may not sound like much energy until we remember that this electrical potential is sustained across an insulating membrane only five nanometers thick. That is 5 billionths of a meter. So the energy of an action potential is almost 20,000,000 volts per meter. Compare this to the 12,000 volts per meter at a standard wall plug. Healthy pacemaker cells spark the electrical wave that drives heart muscle contraction. When these cells malfunction, an artificial pacemaker may be implanted to take over. Waves of electrical voltage generated at the metal lead of the artificial device cross over to living tissue and initiate normal muscle contraction.