Comments, with reply, on "Fractal character of DC trees in polymethylmethacrylate" by M. Fujii et al

In the above-named work by M. Fujii et al. (ibid., vol.26, no.6, pp.1159-1162, Dec. 1991), the differing fractal descriptions of DC trees generated in polymethylmethacrylate (PMMA) are attributed to the local electric fields generated by space charges. The commenter argues that, while a fractal dimension approach to DC tree characterization is certainly useful, a knowledge of the value of the exponent would be much more informative from the point of view of understanding the physical mechanism involved in the stochastic development of trees. This is also important because it is eta which controls the dimension of the tree structure. In their reply, Fujii et al. points out that, since the mechanisms of the streamers in solids are not understood well at present, it is difficult to discuss the difference of the patterns of the negative and the positive point electrodes. Therefore, the difference of the patterns for both polarities cannot be discussed using only the field dependence parameter eta .<>