Thermal-buble initiated breakdown characteristics of liquid helium and nitrogen at atmospheric pressure
Experiments show that electric field promotes to form a vapor locking in a gap with a hot spot and in turn the formation of locked vapor produces the serious reduction of breakdown voltage. Especially it is argued that the dielectric strength of liquid helium under serious hot spot conditions decreases below that for saturated gas.
Dielectric breakdown of helium at low temperatures
Abstract Helium is of particular interest as a basic insulating agent in the development of superconducting electrical power equipment. This is due to the fact that contrary to the low electrical breakdown strength under ordinary conditions helium shows satisfactory breakdown strength at very low temperatures where its density is high. In fact the breakdown strength of helium in suitable states is superior to that of good transformer oil. In the paper it is shown that in agreement with breakdown theories of gases the breakdown strength of gaseous helium is determined mainly by the gas density. On the other hand the breakdown strength of liquid helium at pressures above 1 bar is determined mainly by the helium pressure. The phenomenon may be explained by the formation of gas bubbles produced by moving ions in the liquid leading to electrical breakdown.
Influence of electric fields on friction electrification between metals and borosilicate glass
Abstract In a cylindrical geometry, electric fields were applied through the entire contact area between the sliding metal and the glass surface. The maximum electric charges developed on the metal were measured and plotted against the applied voltage producing the field. The metals were high-purity nickel and gold. The borosilicate glass was Pyrex, brand 7740. The experiments with nickel, which has a work function greater than that of the glass, were performed in 10 −7 mm Hg vacuum. The experiments with gold, with a work function smaller than that of glass, were carried out in air at atmospheric pressure. The results show that: 1. 1) There is both asymmetry and a reversed slope portion in the charge vs. voltage plot. 2. 2) For a certain range of reversing fields, the electric charge developed on the nickel is always negative and the charge on gold is always positive.