1. R. N. Hollyer, then at the Applied Physics Laboratory of The Johns Hopkins University, seems to be the first one to describe electric precursor signals in front of shock waves (May 1957, unpublished report). He used different probes inside steel and glass shock tubes. Although he made very ingenious experiments and even considered electron diffusion from the plasma behind the shock front he concluded erroneously that photoemission from the walls was the dominant mechanism. Hollyer also noticed that in a steel tube precursor signals are limited to much shorter distances from the shock front than is the case in glass tubes. In the light of the present paper this is understandable. In a steel tube electrons diffuse to the walls and get lost for the diffusion process. In a glass shock tube the diffusion of electrons to the wall is stopped by the potential which quickly builds up as electrons accumulate on the wall.