How Does Electro Cardio Gram work?

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Since my father is a heart patient, he has gone through numerous Electro Cardio Gram tests.Electro Cardio Gram or ECG in short is a technology to determine the pattern of the heart beat. The result of the test comes as a print in a graph paper. The pulse is shown in high, low and curvy lines. Can anyone explain how this technology works and what are the patterns of healthy/weak heart in ECG report?