I'm reading a book about electrocardiograms. In one page, the author says that when a wave of depolarization (positively charged sodium ions enter the muscle cells of the heart, causing contraction) moves through the heart toward an electrode placed on the skin, an upward deflection is registered on the ECG record. That got me wondering about the physics of it. A wave of depolarization is not exactly an electric current, since there's no net movement of charges, right? What exactly does this wave of depolarization do to the electrode? And what would a possible mechanism be by which that is recorded as an upward deflection?