Magnetic Metals typically measures distortion using Hewlett-Packard HP339A Distortion Analyzer and the Audio Precision System One. Distortion is typically measured with a 1:1 turns ratio coil and the required load resistor (see Fig 3).

The analyzer supplies the test signal. (In special cases an external signal can be applied to the transformer primary. This external signal must be a pure sinewave of the desired frequency.) The distortion analyzer filters out the fundamental frequency of the output signal and measures the power of the remaining signal.

where:
Q = transformer Q factor
B = flux density
A = Area
= permeability

It has been observed that 1/Q traces distortion very closely for a single sample when varying frequency. However, this relationship does not hold when comparing Qs of different samples. Also, distortion is proportional to the change in permeability with induction. However, it appears to be more strongly dependent on the permeability than the change in permeability. Distortion is also dependent upon the core loss of a sample.