## 1 Introduction  
Systematic nondestructive inspection of safety-critical structural components of power plants, bridges, pipelines, vehicles, etc., is essential in various industrial sectors. Many techniques exist for this purpose: among them, magnetic measurements are most frequently and successfully applied in electromagnetic non-destructive evaluation [1; 2; 3]. Usually, magnetic methods are technically simple and not expensive. They can be applied via electrical cables and coils. One of them, magneto-acoustic emission, can be used for monitoring of residual stresses [4]. Different versions of magnetic hysteresis measurements [5] are also useful in Non-Destructive Testing (NDT). In novel versions of magnetic hysteresis techniques, it is not the major magnetic hysteresis loop that is measured, as that is very difficult or even impossible in some situations, but rather the series of minor magnetization loops are recorded [6]. A recently developed version of the minor magnetization loop technique is the so-called magnetic adaptive testing (MAT) [7]. The advantages (large sensitivity, good reproducibility) of this method have been proven in the past [8]. Another well-known and frequently used magnetic method is the Barkhausen noise (MBN) technique, which is a useful technique for investigation of defects in the surface layer, microstructure changes, and residual stresses [9; 10; 11; 12; 13]. The principle behind Barkhausen noise is when a continuously changing electro-magnetic field (\(H\)) is applied to a material, a response of magnetization (\(M\)) in the form of discontinuous changes in magnetization occurs due to when the domain walls encounter pinning sites. Such pinning sites are in the form of dislocations, precipitates in the matrix, grain boundaries, or small volumes of second-phase material. During the electro-magnetic hysteresis cycle, such pinning sites prevent the movement of the domain walls within the material, which is the premise behind Barkhausen noise phenomena [14]. Although these methods can be successfully used in NDT, there exists a serious drawback in magnetic measurements: the influence of the surface condition on the measured quantities. Usually, samples to be measured are magnetized by a magnetizing yoke, attached directly to the surface. If the surface is not sufficiently smooth, the air gap between magnetizing yoke and sample surface results in undermining the magnetic coupling between the yoke and the investigated specimen: a part of the magnetic flux, pumped into the sample is dissipated to the surrounding space. If the quality of sample surface is the same for all the measured samples, there is less concern for the comparative methods, because the relevant modification of magnetic parameters can still be determined in a reliable way. However, if the surface condition varies among the measured samples to be compared, the uncertainty of measurements increases considerably. By applying a non-magnetic spacer, this problem can be managed, as shown in [15]. In other work [16], the influence of surface roughness was measured on a series of samples having the same magnetic properties. By using MAT measurement, direct influence of surface roughness on the measured magnetic permeability was studied. A well-measurable, direct quantitative correlation was found between surface roughness and the recorded magnetic parameters. The goal of the presented work was to be able to identify the differences in surface quality based on the recorded electro-magneto response. To address the uncertainty of measurements, and specifically conditions encountered in service, it was noticed that the surface quality of the material has a considerable influence on the extracted sensor response. In order to mitigate these effects, a design of experiments should be used to study the sensor response differences based on different material surface quality (the same base material will remain constant throughout the tests).