## 4 Discussion  
The results of the uniaxial loading experiments clearly demonstrated that the changes in the self-potential were related to the stress changes that occurred during the damage and fracturing of the anisotropic bituminous coal. There was also a correlation between the stress turning point and the acoustic emission count. The effects of anisotropy on the self-potential could be described as follows. During the elastic deformation stage, as the stress gradually increased, the self-potential of samples A1 to A6 showed marked fluctuations whereas the curves for samples B1 to B6 were relatively stable. The variations in the self-potential and stress of the bedding-perpendicular samples were greater than those of the bedding-parallel samples. In addition, the compressive strength of the samples obtained perpendicular to the bedding was markedly higher than that of the samples obtained parallel to the bedding. The variation amplitude of the axial and radial self-potential during uniaxial loading was greater for the samples that were cut perpendicular to the bedding. During the initial stage of uniaxial compression, as the uniaxial stress gradually increased, the self-potential of the bituminous coal exhibited a linear increase or linear decrease. Taking sample A3 as an example, the relationship between the self-potential and the stress of the sample (Figure 11) indicated that there was a marked relationship between the self-potential measured from electrode CH25 to electrode CH32 and the stress during the elastic deformation stage. The correlation coefficients (R\({}^{2}\)) of the linear fitting between the self-potential and the stress of bituminous coal samples A1 to A6 and B1 to B6 at different loading rates are listed in Table 2. Statistical analyses demonstrated that the probability of R\({}^{2}\) > 0.5 was 58.3% and the probability of R\({}^{2}\) > 0.8 was 55.3%. Of the 12 bituminous coal samples tested, only sample A5 exhibited a correlation coefficient between the stress and the self-potential of electrodes CH25 to CH32 of less than 0.5. From these results, we obtained the constitutive relationship between the self-potential and stress at the elastic deformation stage, which was Sp = a\(\cdot\)S + b, where S denotes the stress (MPa) and Sp denotes the self-potential (mV). The samples perpendicular to the bedding (B1 to B6) exhibited a stronger linear relationship with the stress during initial loading than the bedding-parallel samples (A1 to A6). Anisotropy can lead to different self-potential response characteristics during the deformation and fracture of a rock mass. The main mechanisms giving rise to the self-potential in coal rock are the redox electric field, filter electrical field, diffusion-adsorption electrical field, piezoelectric effect, triboelectric effect, grain boundary/dislocation charge separation and crack tip discharge. The piezoelectric effect, tribological electrification, grain boundary/dislocation charge separation and fissure-tip discharge are the main causes for self-potential changes induced by the deformation and fracturing of a coal rock mass under loading [7; 25; 26; 27], not considering gas-bearing and water-bearing rock masses. The tensile fracturing of coal rock leads to a separation of the charge at the tip of the expanding crack. Due to differing charges on the new surfaces on either side of a propagating crack, the formation and expansion of a large number of cracks lead to the accumulation of the charge and finally generate an electrical field. There is a positive association between higher potential amplitude increases and a faster crack formation rate. This mechanism is the main reason why a coal rock mass produces a higher potential and a sudden change of potential during the rapid instability stage of fracturing.