Communication Some Observations on Tree Patterns in Pmma Under Alternating Voltages

Observations of the development of electrical trees in polymethy-lmethacrylate (PMMA) using a single needle plane ground electrode system and 50 Hz voltages are reported for both voidless samples and samples having an artificial void. The temporal changes in the tree pattern from branch-like to bush- or fan-like observed in the voidless specimens are intimately associated with the occurrence of localized cracks. No change in pattern and no cracks occur in samples with an artificial void. The results support the theory that a gaseous discharge occurs in the various hollow channels of the branch-like tree pattern.