Shibuya, Zoledziowski and Calderwood8 observed, photographed, and commented on the following stages in treeing: appearance of light at an electrode tip, appearance of fine shadow lines around the electrode tip, formation of a void, void growth, tree initiation, and tree growth. Ieda and Nawata9 conducted treeing tests that indicate the presence of a homogeneous space charge in the material under test. This space charge acts to affect the breakdown voltage of the material, increasing for negative electrodes and decreasing for positive electrodes.