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Magnetic package leads can cause errors in the measurement of the thermal resistance of semiconductor devices. The errors are the result of distortions of the voltage waveforms apparently caused by an increase in the impedance of the leads at short times after switching. This is related to the skin effect, which is the tendency of currents to concentrate on the surface of and decay exponentially into the leads. The concentration increases as the magnetic permeability increases. The influence of the skin effect in the magnetic leads on the measured waveforms and on the measured thermal resistance is shown. A technique for correcting the measured thermal resistance is demonstrated.