VONNEGUT AND MOORE 
Fic. 1—Lightning-like pattern produced by dielectric breakdown in charged methylmethacrylate 
block 
spark pattern is a treelike system that extends 
throughout the region of high charge density. 
There is no question but that the electric dis- 
charge has greatly reduced the high potential 
within the plastic and that from a gross point 
of view the charge within the block was neu- 
tralized almost instantaneously by the discharge. 
However, if we look at the fine structure of 
the pattern with a microscope we see that it is 
not infinitely subdivided. The successive branches 
become smaller and smaller and finally termi- 
nate. We see that between the tendrils of the 
discharge there is transparent unaffected plastic, 
whose volume is considerably larger than the 
volume affected by the spark. Since a large frac- 
tion of the charged plastic is unaffected by the 
discharge it is reasonable to conclude that the 
spark did not neutralize a good part of the 
charge within the plastic but merely balanced 
this negative charge with an equal and opposite 
positive charge that was distributed in the dis- 
charge pattern. 
Our picture of the nature of the lightning dis- 
charge within a cloud is based on the assumption 
that it is similar to what we believe takes place 
in the plastic. If this is true, the lightning does 
not neutralize the charged cloud particles but 
merely introduces a treelike pattern of approxi- 
