Final answer:
An electrocardiogram (ECG) is a record of the voltages created by the wave of depolarization and subsequent repolarization in the heart. Standard ECGs have 12 or more electrodes, including the lead II potential, placed on the chest.
Step-by-step explanation:
An electrocardiogram (ECG) is a record of the voltages created by the wave of depolarization and subsequent repolarization in the heart. Voltages between pairs of electrodes placed on the chest are vector components of the voltage wave on the heart.
Standard ECGs have 12 or more electrodes, but only three are shown in Figure 20.30 for clarity. The voltage between the right arm and the left leg is called the lead II potential and is the most often graphed.