Electrocardiography is a technique for analysing the condition of the heart.As each muscle cell in the heart contracts positive and negative charges are separated as shown.

These contractions happen maybe once per second. At any instant of time the electric charges on the heart exert forces on ions in the body's surface tissue that cause the ions to move – in the diagram below, sodium and chlorine atoms move as shown.

Electrodes placed on the arms absorb charges of different signs. An electrocardiogram or ECG is a recording of the charge separation on the body's surface causing by the charge separation on the heart. Abnormalities in the timing or size of the charge separation indicating many heart abnormalities are easily detected.