A data storage system includes a recording head and a compensating resistor. The recording head has a heating element. The compensating resistor is in electrical series with the heating element and is external to the recording head. A method includes applying an alternating current at a first angular frequency to a recording head. A voltage drop across the recording head heating element is measured. A component of the voltage drop is extracted. The component has a frequency that is three times the frequency of the first angular frequency.