Here is an exerpt from "Braunwald: Heart Disease: A Textbook of Cardiovascular Medicine, 6th ed"

The electrocardiogram (ECG), as used today, is the product of a series of technological and physiological advances pioneered over the past two centuries. Early demonstrations of the heart's electrical activity reported during the last half of the 19th century, for example, by Marchand and others, were closely followed by direct recordings of cardiac potentials by Waller in 1887. Invention of the string galvanometer by Willem Einthoven in 1901 provided a reliable and direct method for registering electrical activity of the heart. By 1910, use of the string galvanometer had emerged from the research laboratory into the clinic. Subsequent achievements built on the limited, but very solid foundation supplied by the early electrocardiographers. The result has become a widely used and invaluable clinical tool for the detection and diagnosis of a broad range of cardiac conditions, as well as a technique that has contributed to the understanding and treatment of virtually every type of heart disease. Furthermore, the ECG is essential in the management of major metabolic abnormalities such as hyperkalemia and certain other electrolyte disorders, as well as assessing drug effects and toxicities such as caused by digitalis, antiarrhythmic agents, and tricyclic antidepressants. Moreover, it has remained the most direct method for assessing abnormalities of cardiac rhythm.

Use of the ECG for any of these clinically important purposes is the final outcome of a complex series of physiological and technological processes. This sequence is depicted in the figure below. First, an extracellular cardiac electrical field is generated by ion fluxes across cell membranes and between adjacent cells. These ion currents are synchronized by cardiac activation and recovery sequences to generate a cardiac electrical field in and around the heart that varies with time during the cardiac cycle.

This electrical field passes through numerous other structures, including the lungs, blood, and skeletal muscle, before reaching the body surface. These structures—known as transmission factors—differ in their electrical properties and perturb the cardiac electrical field as it passes through them. The potentials reaching the skin are then detected by electrodes placed in specific locations on the extremities and torso and configured to produce leads. The outputs of these leads are amplified, filtered, and displayed by a variety of electronic devices to construct an ECG recording. Finally, diagnostic criteria are applied to these recordings to produce an interpretation. The criteria have statistical characteristics that determine the clinical utility of the findings.

for a more detailed description about the physics and priniciples of depolerization and the cardiovascular electrical conduction system please refer to our links and other resources.