To decrease the effect on conductivity readings due to variations in test part thickness, one should:

Increasing the test frequency can effectively reduce the effect of variations in test part thickness on conductivity readings. This is primarily because higher frequencies tend to be less affected by the skin effect, which is the phenomenon where alternating current (AC) tends to flow near the surface of conductors. At higher frequencies, the current distribution within the conductor becomes more uniform throughout its cross-section, thus minimizing the influence of thickness variations on the conductivity measurement.

In contrast, lower frequencies may exacerbate the impact of thickness variations, as the currents penetrate deeper into the material, and the measurement becomes more sensitive to those variations. The fill factor, which represents the ratio of the contact area to the overall measurement area, can further complicate the readings and is typically not adjusted solely for the purpose of addressing thickness variations. Therefore, increasing the test frequency is a precise strategy to enhance the accuracy of conductivity measurements despite thickness discrepancies.