MR heads rely on the fact that the resistance of a conductor changes slightly when an external magnetic field is present. Rather than put out a voltage by passing through a magnetic-field flux reversal—as a normal head would—the MR head senses the flux reversal and changes resistance. A small current flows through the heads, and this sense current measures the change in resistance. This design provides an output that is three or more times more powerful than a TF head during a read. In effect, MR heads are power-read heads, acting more like sensors than generators.