## 5 Experimental Setup and Results
### Results and Discussion  
In experiments, the pulse excitation had a 10 V amplitude, a 200 Hz frequency and a 50% duty cycle. The sampling frequency was 1 MHz and the sampling point was 2500. When the rising edge was triggered, the data acquisition card began to record the probe signals. The samples included 45# steel, 65 Mn steel, A3 steel and nodular cast iron plates. The sample had a length of 100 mm, width of 100 mm and height of 10 mm. The conductivities of the samples are listed in Table 1, which were measured using the four-wire Kelvin resistivity measurement method [(20; 21)] in a laboratory environment on samples with a polished top surface. Figure 14: Experimental setup. Figure 15: Sample and magnet. Figure 16 shows the difference PEC signals when the distance between the magnet and sample changed. It was observed that the difference PEC signals integrally increased or decreased when the magnet distance changed. In terms of 45\(\#\) steel, 65 Mn steel, A3 steel and nodular cast iron samples, the difference PEC signals did not exhibit intersection phenomenon and the amplitude of difference PEC signals was obviously affected. The results were consistent with the simulation results, which indicated that the difference PEC signals were affected when permeability changes and the PI feature did not appear. Based on the above differences PEC signals, the thrice subtraction method was used to obtain thrice subtraction signals and the PI feature. Figure 17 shows that a PI feature appeared in thrice subtraction signals in terms of 45\(\#\) steel, 65 Mn steel, A3 steel and nodular cast iron samples. At the PI feature, the thrice subtraction signals were almost independent of the permeability effect. Furthermore, the PI features were extracted from thrice subtraction signals, as shown in Figure 18. It was observed that the PI time and amplitude increased along a curve with increasing conductivity. The change rule of conductivity was consistent with the actual measurement. The result was in agreement with the simulation result under a certain condition, which demonstrated that by testing PI amplitude and time, the conductivity of ferromagnetic samples could be determined without the permeability effect. The \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **65 Mn Steel** & **45\(\#\) Steel** & **A3 Steel** & **Nodular Cast Iron** \\ \hline Conductivity & 4.47 & 3.76 & 3.41 & 1.56 \\ (MS/m) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Conductivity of experiment samples. Figure 16: The difference PEC signals. (**a**) A3 steel; (**b**) 45\(\#\) steel; (**c**) 65 Mn steel; (**d**) nodular cast iron. results pointed that the thrice subtraction method was a feasible program to obtain a PI feature for reducing the permeability effect in the conductivity measurement of ferromagnetic samples. In PEC testing, the formulation of the PEC model was established in a simplified condition. Thus, the simulation results fit one specific case in terms of a certain condition. The simulation results were available to verify the change law of the PEC response with different thickness, conductivity and permeability. The simulations may have led to the inconsistency with experimental results under the same conditions. On the other hand, due to the measurement error, there may have been deviation in the conductivity measured by the four-wire Kelvin resistivity measurement method. According to experiment and simulation results, it was feasible to verify the variation and trend of the conductivity  
Figure 17: Thrice subtraction signals. (**a**) A3 steel; (**b**) 45# steel; (**c**) 65 Mn steel; (**d**) nodular cast iron. Figure 18: PIs with samples of different materials. (**a**) PI time curve; (**b**) PI amplitude curve. in terms of real value. Although there were some shortcomings in the simulations and experiments, the change law of conductivity and variation of conductivity was consistent with the actual measurement, which demonstrated that the measurement results were feasible and the proposed method was effective.