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Figure 2: By joining the wires from the right arm, left arm, and left foot with 5000 Ohm resistors Frank Wilson defines an “indifferent electrode” later called the “Wilson Central Terminal.” The combined lead acts as an earth and is attached to the negative terminal of the ECG. An electrode attached to the positive terminal then becomes “unipolar” and can be placed anywhere on the body. Wilson defines the unipolar limb leads VR, VL, and VF where “V” stands for voltage (the voltage seen at the site of the unipolar electrode).