Performance of electrodes at the first voltage breakdown in liquid helium

A cryogenic system for the investigation of the dielectric properties of liquid helium is described. Results of first breakdown voltage measurements made with a great number of fresh (unsparked) sphere gaps are presented. Examples for typical conditioning effects resulting from repeated sparking of one gap are given and discussed. A main objective of this work has been the study of the influence of electrode material on the first voltage breakdown in freshly prepared sphere gaps.